Transcript
UNIT 304 Understand and apply domestic central heating system installation, commissioning, service and maintenance techniques
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his unit provides learning in installation, maintenance and application of design techniques to include heat and ventilation loss through the building fabric, diagnostics and rectification of faults and commissioning procedures. In addition, this unit also provides in-depth learning of system types, components, controls, and servicing requirements, in systems up to large domestic dwellings and/or systems of equal size in commercial and industrial premises. There are nine Learning Outcomes for this unit. Central heating is a complex subject with many diverse systems and controls so, the unit will be separated into subject headings. The learning outcomes and the assessment criteria will be listed at the head of each section. There are 98 Guided Learning Hours recommended for this unit. The learner will: 1 2 3 4 5 6 7 8 9
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Know the types of central heating system and their layout requirements. Know the design techniques for central heating systems. Be able to apply design techniques for central heating systems. Know the installation requirements of central heating systems and components. Be able to install central heating systems and components. Know the fault diagnosis and rectification procedures for central heating systems and components. Be able to diagnose and rectify faults in central heating systems and components. Know the commissioning requirements of central heating systems and components. Be able to commission central heating systems and components.
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System types and layouts of central heating systems (LO1, LO2, LO4) SmartScreen Unit 304 PowerPoints and Handout 1
There are seven assessment criteria for these Learning Outcomes: 1 Define the factors, which affect the selection of central heating systems for dwellings. 2 Define the space heating zoning requirements under statutory legislation for larger single occupancy dwellings. 3 State the criteria used when selecting heating system and component types: a. customer’s needs b. building layout and features c. suitability of system d. energy efficiency e. environmental impact. 4 Analyse the operating principles of environmental heat sources used in conjunction with central heating systems: a. heat pumps: ground source air source b. micro-combined heat and power. 5 Identify the system layout features for multiple boiler installations incorporating low loss headers. 6 Specify the positioning and fixing requirements of multiple boiler installations with low loss headers. 7 Specify the positioning, fixing and connection requirements of new central heating components for sealed central heating systems: a. connections to a boiler b. fully pumped central heating control components – mid position or 2x2-port valve arrangement c. sealed system components d. connections to panel radiators or underfloor heating manifold e. connections to hot water cylinder. Central heating is a vast and complex subject. There are now more options with regard to sources of heat, pipe materials and heat emitters than ever before. Environmentally friendly technology and the re-emergence of underfloor heating has meant that the customer can now afford to be selective about the system they have installed into their property. The advent of heat pumps and solar systems, with the savings on fuel and running costs, has dramatically lowered the carbon footprint of domestic properties. No longer does the
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customer have to rely on appliances that burn carbon rich fuels such as gas and oil. Zero carbon and carbon neutral fuels have revolutionised domestic heating whilst, advances in technology have lowered the cost of the energy saving appliances that were only available to a select few.
UNIT 304
Zero carbon and carbon neutral fuels Terms used to describe fuels that neither contribute to nor reduce the amount of carbon (measured in the release of carbon dioxide) into the atmosphere.
In Unit 304, we will build on the knowledge of central heating systems you learned at Level 2. We will investigate new and exciting technology that has the potential to dramatically cut the cost of heating our homes whilst, at the same time, reducing our carbon emissions. We will also look at new controls and components that can transform an existing wasteful installation into an energy efficient system.
The factors, which affect the selection of central heating systems for dwellings The main purpose of central heating is to provide thermal comfort conditions within a building or dwelling. The factors to consider when discussing comfort are: ■■
Humidity – The amount of moisture there is within the environment. Ideal conditions for humans require 40% to 60% humidity. Anything below 40% can make the eyes and throat very dry. Above 60% makes the atmosphere very damp and uncomfortable.
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Air Changes – Refers to the amount of air movement (not velocity) within the building. Air movement is important because it replaces used air with fresh air, which is needed for breathing. Air changes, however, lead to heat loss. Every time an air change occurs, the fresh, cold air requires heating up. Air changes account for the biggest heat loss when calculating the fabric heat loss from a building.
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Air Temperature – Air temperatures should be between 16-22°C, dependent upon the type of activity being carried out, age of occupants and the level and quality of clothing. Air temperature at feet level, not greater than 3°C below that at head level. Room surface temperatures not above the air temperatures.
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Air Velocity – This is the speed at which the air travels within the building. If it travels too fast then a draught will be felt by the occupants, but if there is no movement, then the air changes will not be satisfied. Airflow past the body is horizontal, at a velocity of between 0.2m and 0.25m per second. A variable air velocity is preferable to a constant one.
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Activity within the Building – Applies to the type of work that is being carried out within the building. The more activity that is
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carried out by the occupants, the less heat will be required, so temperatures may have to be adjusted to suit. ■■
Clothing – Relates to the type of clothing worn by the occupants of the building.
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Age and Health – A major factor in heating design. The age of the occupants will have a direct effect on the type of the systems we install. Older people feel the cold more than young people, which may mean that designs will have to modify, especially when it comes to the temperatures of key rooms such as lounge and bedroom.
The space heating zoning requirements of larger single occupancy dwellings Approved Document L Conservation of fuel and power.
Central heating systems in dwellings are subject to the strict requirements of the Building Regulations, which stipulate that every home must be divided into at least two heating zones. Approved Document L is driven by the need for energy efficiency. It lays down specific requirements as to how this must be carried out. The current Building Regulations came into force on 1 October 2010 for England and Wales. The document is divided into four specific sections: ■■
Part L1A – Installations in new domestic dwellings.
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Part L1B – Installations in existing domestic dwellings.
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Part L2A – Installations in new industrial/commercial buildings.
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Part L2B – Installations in existing industrial/commercial buildings.
The requirements of both of these very and different aspects of space heating are explained in two accompanying documents: ■■
The Domestic Building Services Compliance Guide for Parts L1A and L1B
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The Non-domestic Building Services Compliance Guide for parts L2A and L2B.
Both of these documents were produced by the government as guidance to help installers comply with Approved Document L. To support these documents, the Energy Savings Trust published a set of standards known as the CHeSS standards (Central Heating Systems Specifications), which lay down both good practice and best practice with regard to controls of central heating systems.
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Approved Document L1A and L1B
Boiler interlock
The main requirement of Approved Document L is for a boiler interlock.
Is a series of controls (cylinder thermostats, programmable room thermostats, programmers and time switches) that prevents the boiler from cycling when there is no demand for heat.
In addition, it also sets out the following boiler requirements: ■■
Every home must be divided into at least two heating zones, using a thermostat controlling a motorised valve.
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If the house is less than 150m2, then these two heating zones can be controlled by the same time clock or programmer.
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If the house is larger than 150m2, then each zone must be controlled by its own time clock orprogrammer.
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Living and sleeping areas (zones) must be controlled at different temperatures by means of a thermostat.
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Every radiator should be fitted with a thermostatic radiator valve, unless the radiator is being used as the reference radiator for a thermostat situated elsewhere in the room.
These requirements apply every time a home is built. Where existing installations are concerned, the requirements of Document L are made retrospectively. In other words, if an existing system does not comply with the regulations, then the system must be updated: ■■
Every time a home has an extension or change of use.
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Every time more than one individual component, such as a boiler is replaced in a heating system.
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Simple boiler servicing is exempt from this, but the recommendation is made that radiator thermostats should be fitted when the system is drained down.
The criteria used when selecting heating system and component types All too often, installers will install what they know and not what is best for the property or the customer. Heating design requires a careful consideration of specific criteria if the heating system is to fulfill its potential to the customer as an efficient and economical system. These criteria are: ■■
Customer’s needs – The overriding concern when designing any central heating system is that the customer requirements are satisfied. However, legislation will also need to be taken into account, as the system must comply with the requirements of the regulations in force. Where a conflict exists between the customer requests and the legislation, it must be explained to
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the customer that their wishes cannot be fulfilled, as the system would not meet the requirements of the regulations. Consultation with the customer is most important and the design needs to be approved with them before work can begin. ■■
Building layout and features – The position of key components and appliances are often dependent on the layout and the features of the property. Many designers will look at the plans of the property and assess the best methods of heating the space. Consultation with the building owner is important as they may have their own ideas of what they want within a particular space or room.
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Suitability of system – Whilst small domestic properties do not present many design problems with regard to pipe sizing, pipework layout and routing, larger systems may require careful planning utilising a multi-zoning approach, complete with separate timing and temperature controls for each zone and may well present a necessity for a low loss header to cope with the load of each of the separate zones. The greater the number of heat emitters, the greater the possibility that a low loss header would need to be installed. Multiple boiler installations may also feature in larger properties and these would require careful and considered design practice if the system is to be economical and energy efficient when completed.
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Energy efficiency – In many cases, the controls that are installed on the system will determine whether a system is energy efficient or not. New controls such as night setback, delayed start and weather compensation will assist in making the most of good system design and this will help in creating an energy efficient and economical system. Again, zoning will help in this regard by limiting the heat in areas which, are not occupied whilst, maintaining a comfortable environment within the dwelling.
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Environmental impact – New technology developed in recent years means that no longer do systems have to rely on carbon rich fuels to maintain a warm living environment. Both carbon neutral fuels, such as biomass systems and zero carbon options such as air and ground source heat pumps, means that the opportunity to lower the carbon fuel usage is available. Biomass is particularly good when a communal heating system serving a number of dwellings is being considered.
Utilising ‘green’ technology to its full advantage reduces carbon emissions and, as a consequence, the carbon footprint of the building. Other technological advances in solar heating and Micro CHP systems have also widened the scope for significant savings, not only to the customer, but also to the environment as a whole.
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UNIT 304
Heat pumps and micro-combined heat and power Vapour
Vapour Compressor
Condenser
Evaporator
Heat pump A device, which extracts useful heat from the air, the ground or from water and uses it to heat the water in a hot water or central heating system. Heat pumps absorb low temperature heat energy, raising it to more useful temperatures using the vapour compression refrigeration cycle.
SmartScreen Unit 304 Handout 2
Fan
Liquid & vapour
Liquid Expansion valve
The principle of a heat pump 304.1
A heat exchanger, called an evaporator, transfers the absorbed heat to a liquid, called a refrigerant that boils at an extremely low temperature. During the transfer process, the refrigerant boils to form a refrigerant gas, which circulates in a closed system to a compressor. Here, the gas is compressed to a very high pressure turning the gas back to a liquid again. This process generates large amounts of useful heat, which is then collected in a second heat exchanger called a condenser where it is released into the water delivery system via radiators, underfloor heating systems or a hot water cylinder heat exchanger coil. From the condenser, the refrigerant liquid passes through the closed system to an expansion valve. Here, the pressure is suddenly released turning the liquid refrigerant to a liquid/gas mixture. During this process, the temperature of the refrigerant drops very significantly, creating the cooling effect found in refrigerators and freezers. At this point the cycle is complete and the refrigerant is ready to absorb more low temperature heat. Applications of heat pumps ■■ Space heating – The main purpose of a heat pump is for space heating. The lower the distribution temperature to the heat emitters, the greater the efficiency of the system. For this reason, heat pumps are best suited to those systems where the distribution temperatures are as follows: ■ ■
underfloor heating (delivery temperature 30°C–45°C) fan coils (delivery temperature 35°C–55°C)
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low temperature radiators (delivery temperature 45°C–55°C). Domestic hot water – Heat pumps can supply hot water at 60°C to 65°C but where the output is less than this, a supplementary heat source, usually in the form of an immersion heater, will be required. ■
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Space cooling – Heat pumps that incorporate a reversing valve to reverse the refrigeration cycle can be used in conjunction with fan coils to provide space cooling during the summer. Partial cooling of the dwelling can be achieved by using the existing underfloor heating pipework to distribute cooling. However, because of the greater risk of condensation, the temperature should be limited to 18°C to prevent problems with condensation. Reversing valve Cold gas
Cool air to room
Hot gas Compressor
Condenser (hot)
Evaporator (cold)
Liquid refrigerant
Expansion valve
Warm air intake 304.2
The reversing heat pump
Air source heat pumps An air source heat pump extracts heat from the outside air and will continue to extract useful heat when the temperature is as low as –15°C! Heat from the air is absorbed into the refrigerant that then passes through to a compressor. The compressor compresses the refrigerant increasing the refrigerant temperature significantly. Here, it transfers the higher heat into the condenser heat exchanger to heat the central heating and hot water circuits. There are two basic types of air source heat pump:
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Air to water – This system distributes heat via a wet central heating system. Because heat pumps distribute water at a lower temperature than a conventional boiler, heat pumps of this type are better suited to either underfloor heating or systems with larger radiators.
Hot water cylinder
Air in Cold air out
Pump
Air source heat pump
Air source heat pump heating system 304.3
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Air to air – These produce warm air, which is circulated through the dwelling by a system of ductwork. They do not have hot water provision. Most air source heat pumps are available as: ■ ■
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Single packaged system for outdoor installation. A single package for indoor installation. This system requires a ducted air intake and outlet to the outside air. A split system with the evaporator positioned outside the dwelling and the condenser positioned within the dwelling, both linked via pipework containing the refrigerant.
The main disadvantage to heat pumps is their size. In most cases, an air source heat pump will be larger than a boiler of the same output; for example, a 12kW single package will have dimensions of 1,500mm x 1,200mm x 750mm.
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Air source heat pump cut away
Ground source heat pumps A ground source heat pump extracts heat from the ground using a solution of water and antifreeze, which is more commonly known as brine. The brine is circulated through a system of pipes buried under the ground at around 1m depth. At this depth, the earth has a pretty constant temperature of between 8°C and 10°C. The pipes can be laid in several ways: ■■
In a horizontal closed loop, in coils, known as slinky’s, laid in a trench about 1m to 1.5m deep.
A ground source heat pump slinky
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In a vertical closed loop installed in boreholes of around 50m to 100m deep.
The heat from the earth is absorbed into the brine, which is then pumped through one side of the evaporator. The refrigerant passes through the other side of the evaporator, and the heat from the brine causes the refrigerant to boil and evaporate. The refrigerant then passes through the compressor where its pressure and temperature are increased. The high pressure, high temperature gas then flows through the system to the condenser. Here, the heat is passed to the heating system via the condenser. As the refrigerant loses its heat, it condenses into a liquid before circulating to the expansion valve where the pressure is released, creating a very cold refrigerant gas. At this point, the cycle begins again. There are three main types of ground source heat pumps: ■■
Closed loop indirect circuit – A brine mixture is circulated through plastic pipework buried horizontally in a shallow trench or vertically in a bore hole. This type of system can also be submerged in a lake.
Hot water cylinder Ground source heat pump Pump
Two way manifold Pump
Buried captor or 'slinky' Pump
Top view
Underfloor heating circuits The flow and return connections to the thermostatic mixing valves enter side by side
A ground source heat pump heating system with underfloor heating 304.5 ■■
Closed loop direct circulation, direct expansion (DX) – Here a copper coil is buried in the ground through which refrigerant is pumped. The heat transference is much better because no heat exchanger or pump is required. These are not common in the UK because they are subject to much more stringent regulations with regard to the use of the refrigerant and environmental protection.
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Open loop – Used where there is a good source of ground water. The water is extracted and circulated directly through the heat
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pump heat exchanger and either discharge back into the water source or re-injected into a suitable aquifer. A water extraction licence and discharge consent is required for this system for which there is likely to be a charge.
A typical ground source heat pump
Co-efficient of performance The co-efficient of performance (CoP) is a measure of the efficiency of heat pumps at a specific operating condition recommended in EN 14511. It states: ■■
The standard rating condition for air source heat pumps is A7W35. This simply means: input air @ 7°C, Water output @ 35°C. The standard rating condition for ground source heat pumps is B0W35. This simply means: ■
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brine @ 0°C, Water output @ 35°C.
CoP is measured in Kilowatts (kW) and can be calculated by using the following equation: Efficiency rating =
Q W
Where: Q = Heating input W = Total power consumed by the system including the fans, pumps and controls
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Example A heat pump output is maintained at a steady 10kW/h. The power used by the fans and other equipment totals 2.5kW/h. What is the co-efficient of performance and subsequent system efficiency? 10 =4 2.5 Therefore, for every 1kW of power used, the heat pump gives 4kW of heat. An efficiency of 400%. The CoP of a heat pump will dictate the running costs of the system. For example, a condensing gas boiler will give the same performance of around 93% efficiency, irrespective of whether it is fitted in to a new build property with underfloor heating or an existing installation with radiators. A heat pump, however, will give good efficiencies of up to 400% providing it is running with a low flow temperature of 35°C. If the system design means that the flow temperature is increased to 50°C, then the efficiency can drop to 300%, significantly increasing the running costs. It is, therefore, critical that the system is designed to comfortably heat the building with as low a flow temperature as possible, and that serious consideration should be made to insulate the building as much as possible before a heat pump installation is considered.
SUGGESTED ACTIVITY A heat pump output is maintained at a steady 9kW/h. The power used by the fans and other equipment totals 2.3kw/h. What is the co-efficient of performance and subsequent system efficiency?
Micro-combined heat and power Often referred to as co-generation, micro-combined heat and power (Micro CHP) provides both heat and electricity from a single energy source. Unlike larger CHP systems, Micro CHP is typically limited to a single dwelling. Micro CHP is not a renewable technology, unless it is fuelled by a renewable biofuel. It can, however, help to reduce the carbon emissions and total energy costs for a dwelling. It will not replace the need to be connected to the national electricity grid but it will provide at least some of the dwellings electricity needs and allow any surplus electricity to be exported into the national grid system. Micro CHP systems usually have an electrical output of around 5kW with a heat output that is sufficient to replace a domestic central heating boiler. Typically, domestic Micro CHP systems require the same installation and maintenance techniques as a modern condensing gas boiler.
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How Micro CHP works 1 Baxi Ecogen looks like a conventional wall-hung boiler and uses a conventional gas boiler burner, connected to the mains gas, to heat helium.
2 The helium in the hermetically sealed sterling engine expands and presses down a piston. Cold water flowing around the boiler absorbs the heat, the gas contracts and the piston rises again. The heated water flows out of the boiler and into the hot water cylinder. Cold water flows into the engine and the process starts again - the piston is driven up and down 50 times a second.
3 The piston has a magnet attached to it and as the magnetic field passes through a coil at the bottom of the engine it generates up to 1kW of electricity.
4 Much of the heat expelled in the exhaust gasses from the system is captured by a heat exchanger and reused for generating domestic hot water.
A typical Micro CHP
Types of micro-combined heat and power systems Micro CHP units use a number of different technologies to generate the heat and power requirements: Micro-generation Certification Scheme Under the Micro-generation Certification Scheme (MCS) the Micro-generation Installation Standard: MIS 3007 provides approved guidance on the installation requirements for Micro CHP installations – this is particularly important if government funding is being sought for a project but does, in any case, provide a useful source of information.
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Stirling engines – This is the only Micro CHP engine that is recognised by the Micro-generation Certification Scheme. The Stirling engine does not use the internal combustion process. Instead, a sealed cylinder containing a gas, such as helium, is heated at one end of the cylinder. It is then cooled by the return heating water. This makes the gas expand and contract regularly and uniformly, which drives a piston connected to an electrical generator. The heat for expanding the gas can be provided from a variety of sources, including solar and natural gas. A Stirling engine Micro CHP will normally convert 10% of its total power into electricity. The remaining 90% goes to the heating/hot water system.
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Heat source
Heat source
Regenerator
Stirling engine
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Rankine engines – There are only a handful of manufacturers of 304.007 alternative below this type of system on the market. They are based on the same system that drives the larger power stations, whereby a fluid is heated until it turns to a gas which then drives the turbine. The fluid, however, is not water. It is usually an organic fluid such as a refrigerant or silicone oil, that requires a much less temperature and pressure than a water based system.
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Internal combustion engines – This is a well-established method of producing electricity for small generators and can be powered by petrol, diesel, LPG, natural gas or biofuels. With a Heat source Micro CHP system, heat is taken from the engines cooling water and exhaust system. They are noisier and larger than the Stirling engine types and are not usually supplied for the UK market.
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Fuel cells – A chemical reaction drives a fuel cell which contains natural gas. Natural gas contains hydrogen and this reacts with oxygen taken from the air. The reaction generates electricity and Heat source heat. Known as solid oxygen fuel cells (SOFCs), they consist of a solid electrolyte sandwiched between an anode and a cathode layer. When the natural gas passes over the anode, hydrogen is released. This then combines with the oxygen from the cathode to produce electricity and heat. Sale of these state-of-the-art units is Regenerator due to begin in the UK in 2014.
Typical heat outputs for central heating and hot water delivery, total between 4kW to 8kW for every kilowatt of electrical power. For a 5kW of electrical output, between 20kW and 40kW of heat output is achievable making them a viable alternative to normal central heating boilers. If extra heat power is required, then an additional heat exchanger fitted within the unit, is used to supply the heat directly to the heating system.
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Flywheel/alternator Heat engine Engine heat recovery 1
Electricity supply
Inverter
Hot water supply
Exhaust heat recovery 2 Heat exchanger Engine exhaust
Heat reserve
Mains cold water
A typical Micro CHP system 304.08
The applications of Micro CHP Micro CHP units are being developed as a direct replacement for domestic and small commercial boilers. The requirements of a Micro CHP unit are very similar to a boiler in so much that they require connections to the gas supply and the heating system. However, there is an extra requirement for connection to the electricity system. The average UK home has a requirement of 1kWh of electricity to 6kWh of heat when taken over a 12 month period and this is easily matched by a Micro CHP unit. Micro CHP works at its optimum performance when operated for extended periods. For short running periods of an hour or less, the electricity consumed by the unit can significantly reduce the benefits. A way to combat this would be to install a thermal store. This would reduce cycling and store heat for use later in the day. Low temperature heating systems, such as underfloor heating can use heat from the thermal store and provide lower return temperatures, thus improving the efficiency of the Stirling engine type Micro CHP. Regulation and planning The Domestic Building Services Compliance Guide explains the requirements for Micro CHP under the Building Regulations for England and Wales. Unlike gas fired and oil fired boilers, the performance of Micro CHP is not given in terms of season efficiency. Instead Micro CHPs are rated by the Heating Plant Emission Rate (HPER) for particular applications in a particular property. The HPER indicates the carbon emissions related to the needs of the property in kg/CO2. HPER is given for each kWh of generated heat for heating and hot water. The carbon value, however, includes that for both heat and power generation. Under the Micro-generation Certification Scheme (MCS) guidance on the installation requirements of Micro CHP are given in Microgeneration Installation Standard MIS3007. 50
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Multiple boiler installations and the use of low loss headers – The layout features of multiple boiler installations and the positioning of its components For a boiler to work at its maximum efficiency, the water velocity passing through the heat exchanger needs to be maintained within certain parameters. This is especially important for condensing boilers as they rely on a defined temperature drop across the flow and return before the condensing mode begins to work effectively. Installation of a low loss header allows the creation of two separate circuits. These are shown in the diagram below:
Boiler management control
Zone controller
Zone controller Outdoor sensor Zone valves Primary circuit
Boiler shunt pumps
Boiler
Boiler
Boiler
Shunt pumps
Secondary circuit
Hot water cylinder
Low loss header
Expansion vessel Pressurising unit
A multiple boiler installation with a low loss header ■304.9 ■
The primary circuit – The flow rate within the primary circuit can be maintained at the correct flow rate for the boilers so that the maximum efficiency of the boilers is maintained regardless of the demand placed on the secondary circuit. Each boiler has its own shunt pump so that equal velocity through the boilers is maintained.
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The secondary circuits – The secondary circuits allow for varying flow rates demanded by the individual balanced zones or circuits. Each zone would be controlled by a shunt pump set to
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the flow rate for that particular zone. A two-port motorised zone valve, time clock and room thermostat controls each zone independently, and these are often fitted in conjunction with other controls such as outdoor temperature sensors. In some cases the flow rates through each secondary circuit will exceed what is required by the boilers. In other cases, the opposite is true and the boiler flow rate will be greater than the maximum flow rate demanded by the secondary circuits, especially where multiple boiler installations are concerned. Water velocity is just part of the problem. Water temperature is also important. There are two potential problems here: ■■
If the difference in temperature between the flow and return is too great, it puts a huge strain on the boiler heat exchangers because of the expansion and contraction. This is known as thermal shock.
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For a condensing boiler to go into condensing mode, the return water temperature must be in the region of 55°C. In some instances, temperature sensors are fitted to the low loss header to allow temperature control over the primary circuit.
A typical low loss header
Installation of a low loss header
The low loss header is ideal for use with systems that have a variety of different heat emitters. It is the perfect place for installing an automatic air valve for removing unwanted air from the system. Drain points can also be fitted for removing any sediment that may collect in the header. Both of these features are usually fitted as standard on most low loss headers.
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Sealed heating systems – positioning, fixing and connection of components Sealed heating systems are those that do not contain a feed and expansion cistern but are filled with water directly from the mains cold water supply via a temporary filling loop. Large systems would be filled via an automatic pressurization unit. The expansion of water is taken up by the use of an expansion vessel and the open vent is replaced by a pressure relief valve which is designed to relieve the excess pressure by releasing the system water and discharging it safely to a drain point outside of the dwelling. This is vital as the water may be in excess of 80°C. A pressure gauge is also included so that the pressure can be set when the system is filled and periodically checked for rises and falls in the pressure as these could indicate a potential component malfunction. The system is usually pressurized to around 1 bar. There are several types: ■■
sealed systems with an external pressure vessel
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system boilers that contain all necessary safety controls
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combination boilers.
All fully pumped systems, such as those with two or three, two-port zone valves (known as the S Plan and the S Plan Plus) or a three-port mid-position valve (known as the Y Plan) or a three-port diverter valve (known as the W Plan), can be installed as sealed systems or they can be purpose designed heating only systems using a combination boiler with instantaneous hot water supply.
Fully pumped systems with two or three, two-port zone valves (known as the S Plan and the S Plan Plus) The S Plan has two, two-port motorised zone valves to control the primary and heating circuits separately by the cylinder and room thermostats respectively. This system is recommended for dwellings with a floor area greater than 150m2 because it allows the installation of an additional two-port zone valve to zone the upstairs heating circuit from the down stairs circuit (the S Plan Plus). A separate room thermostat and possibly a second time clock/programmer would also be required for upstairs zoning. A system by-pass is required for overheat protection of the boiler.
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Upstairs timer switch Upstairs room thermostat
System by-pass
Cylinder thermostat
2 port zone valve to hot water 22mm flow and return pipework 2 port zone valve for upstairs circuit
Expansion vessel
Downstairs room thermostat
2 port zone valve for downstairs circuit
Wiring centre Programmer Pressure gauge Filling loop Pressure relief valve and discharge pipework
The S Plan Plus system 304.10
The two-port zone valve 54
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Fully pumped systems with three-port mid-position valve (known as the Y Plan) or a three-port diverter valve (known as the W Plan) The three-port valve mid-position (Y Plan) or diverter valve (W Plan) controls the flow of water to the primary (cylinder) circuit and the heating circuit. The valve reacts to the demands of the cylinder thermostat or the room thermostat. This system uses an expansion vessel in the place of the feed and expansion cistern. It is filled directly from the main cold water supply via a filling loop. A pressure relief valve safeguards the system from over pressurisation. The system below shows a typical Y Plan system complete with all the same controls found on an open-vented Y Plan.
Pressure gauge
Temporary filling loop with double check valve arrangement Pressure relief valve and discharge pipework
Expansion vessel
The Y Plan system
This system contains a system by-pass fitted with an automatic by-pass valve, which simply connects the flow pipe to the return pipe. The by-pass is required when all circuits are closed either by the motorised valve or the thermostatic radiator valves, as the rooms reach their desired temperature. The by-pass valve opens automatically as the circuits close, to protect the boiler from overheating, to allow water to circulate through the boiler, keeping the boiler below its maximum high temperature. This prevents the boiler from ‘locking out’ on the overheat energy cut-out.
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Sealed system components As we have already seen, sealed systems do not contain a feed and expansion cistern, nor open vent pipe. Instead, these systems incorporate the following components: ■■
an external expansion vessel fitted to the system return
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a pressure relief valve
■■
the system is filled via a temporary filling loop or a CA disconnection device
■■
a pressure gauge.
The expansion vessel The expansion vessel is a key component of the system. It replaces the feed and expansion cistern on the vented system and allows the expansion of water to take place safely. It comprises of a steel cylinder, which is divided in two by a neoprene rubber diaphragm.
The expansion vessel with filling loop, pressure relief valve and pressure gauge
The vessel is installed on to the return because the return water is generally 20°C cooler than the flow water and this does not place as much temperature stress on the expansion vessels internal diaphragm as the hotter flow water. If installing the vessel on the flow is unavoidable, it should be placed on the suction side of the circulating pump in the same way as the cold feed and open vent pipe, on the open vented system.
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On one end of the expansion vessel, is a Schrader air pressure valve where air is pumped into the vessel to 1 bar pressure and this forces the neoprene diaphragm to virtually fill the whole of the vessel. On the other end, is a half inch male BSP thread and this is the connection point to the system. When mains pressure cold water enters the heating system via the filling loop and the system is filled to a pressure of around 1 bar, the water forces the diaphragm backwards away from the vessel walls compressing the air slightly as the water enters the vessel. At this point, the pressure on both sides of the diaphragm is 1 bar pressure. As the water is heated, expansion takes place. The expanded water forces the diaphragm backwards compressing the air behind it still further and, since water cannot be compressed, the system pressure increases. When cooling, the water contracts, the air in the expansion vessel forces the water back into the system and the pressure reduces to its original pressure of 1 bar. Periodically, the pressure in the vessel may require topping up. This can be done by removing the cap on the Schrader valve and pumping the vessel up to its original pressure with a foot pump. The operation of expansion vessels was discussed in more detail in Unit 303: Understand and apply domestic hot water system installation, commissioning, service and maintenance techniques (in the companion book for 6189). The pressure relief valve The pressure relief valve (also known as the expansion valve) is installed on to the system to protect against over pressurization of the water. Pressure relief valves are usually set to 3 bar pressure. If the water pressure rises above the maximum pressure that the valve is set to, the valve opens and discharges the excess water pressure safely to the outside of the property through the discharge pipework. Pressure relief valves are most likely to open because of lack of room in the system for expansion due to a malfunction with the expansion vessel. This can be caused by: ■■
■■
The diaphragm in the expansion vessel has ruptured allowing water both sides of the diaphragm, or; The vessel has lost its charge of air.
A pressure relief valve
The filling loop The filling loop is an essential part of any sealed system and should contain an isolation valve at either end of the filling loop and a double check valve on the mains cold water supply side of the loop. The filling loop is the means by which sealed central heating systems are filled with water. Unlike open vented systems, sealed LEVEL 3 NVQ DIPLOMA IN PLUMBING AND HEATING
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systems are filled directly from the mains cold water via a filling loop. The connection of a heating system to the mains cold water supply constitutes a cross connection between the cold main (fluid category 1) and the heating system (fluid category 3), which is not allowed under the Water Supply (water fittings) Regulations. The filling loop must protect the cold water main form back flow and this is done in two ways:
The filling loop
■■
A filling loop has a type EC verifiable double check valve included in the filling loop arrangement, or
■■
The filling loop must be disconnected after filling creating a type AUK3 air gap for protection against back flow.
The filling loop is generally fitted to the return pipe close to the expansion vessel and may even be supplied as part of the expansion vessel assembly. Permanent filling connections to sealed heating systems It is possible to permanently connect sealed heating systems to the mains cold water supply by using a Type CA back flow prevention device. The Type CA backflow prevention device, when used with a pressure reducing valve, can be used instead of a removable filling loop to connect a domestic heating system direct to the water undertaker’s cold water supply. This is possible because the water in a domestic heating system is classified as fluid category 3 risk. A CA device can also be installed on a commercial heating system but only when the boiler is rated up to 45kW. Over 45kW, the water in the system is classified as fluid category 4 risk, and so any permanent
Pressure gauge
Type CA devise
Pressure relief valve and discharge pipework
Expansion vessel
Cold water supply
A sealed system with CA backflow prevention device 304.12 58
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connection would require a Type BA RPZ valve. An example of a CA backflow prevention device can be seen in Unit 302: Understand and apply domestic cold water system installation, commissioning, service and maintenance techniques (in the companion book for 6189). The device contains an integral tundish to remove any discharge should a backflow situation occur. Under normal operation, the valve should not discharge water. However, the valve may discharge a small amount of water if the supply pressure falls below 0.5 bar or 11% of the downstream pressure. The Pressure gauge This is to allow the correct water pressure to be set within the system. It also acts as a warning of component failure, or an undetected leak should the pressure begin to inexplicably rise or fall.
System boilers A system boiler is an appliance where all necessary safety and operational controls are included and fitted directly to the boiler. There is no need for a separate expansion vessel, pressure relief valve or filling loop and this makes the installation much simpler. The system boiler has all the components for a sealed system contained within the boiler unit. It is filled directly from the mains cold water via a filling loop which is often fitted by the boiler The system boiler has all the components for a sealed system contained within the boiler unit. It is filled directly from the mains cold water via a filling loop manufacturer. which is often fitted by the boiler manufacturer.
Pressure gauge
Expansion vessel
Temporary filling loop with double check valve arrangement
Pressure relief valve and discharge pipe
A sealed system with a system boiler 304.13 LEVEL 3 NVQ DIPLOMA IN PLUMBING AND HEATING
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Room thermostat Time clock Combination boiler Expansion vessel
Pressure relief valve and discharge pipe
Filling loop
Heating flow
Heating return
Hot water supply
Mains cold water
A sealed system with a combination boiler 304.14
Combination boilers In recent years, combination boilers have become one of the most popular forms of central heating in the UK. A combination boiler provides central heating and an instantaneous hot water supply from a single appliance. Modern combination boilers are very efficient and they contain all the safety controls; ie expansion vessel or the pressure relief valve, used on a sealed system. Most ‘combis’ also have an integral filling loop.
Underfloor heating (LO1, LO2, LO4) There are four assessment criteria within these Learning Outcomes: SmartScreen Unit 304 Handout 5
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1 Clarify the design principles for underfloor central heating systems: a. combined with radiators b. stand alone. 2 Identify the layout features of underfloor central heating systems. 3 Analyse the working principles of underfloor central heating system pipework and component’s: a. use of manifolds THE CITY & GUILDS TEXTBOOK
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b. controls system application – time and temperature to space heating zones c. underfloor pipework arrangements from manifold to room. 4 Specify the positioning and fixing requirements of components in underfloor central heating system’s: a. manifolds b. pipework arrangements (cabling) c. pipework installation techniques: solid floor suspended timber floor. Underfloor heating has been around for many years, the Romans used a warm air system 1,500 years ago to great effect. It is only fairly recently that its benefits have been rediscovered. With the arrival of new technologies, such as air and ground source heat pumps and solar heating, underfloor heating becomes not only a viable option for the domestic dwelling but one that will save money, energy, reduce CO2 emissions and, as a consequence, help save the fragile planet on which we live.
The design principles of underfloor central heating systems An underfloor heating system provides invisible warmth and creates a uniform heat; eliminating cold spots and hot areas. The temperature of the floor needs to be high enough to warm the room without being uncomfortable underfoot. There is no need for unsightly radiator/convectors because the heat literally comes from the ground up. Underfloor heating creates a low temperature heat source that is spread over the entire floor surface area. The key word here is low temperature. Where as most wet central heating systems containing radiators and convectors operate at around 70°C to 80°C, underfloor heating operates at much lower temperatures making it an ideal system for air and ground source heat pump fuel sources. Typical temperatures are: ■■
40–45°C for concrete (screed) floors
■■
50–60°C for timber floor constructions
Traditional wet central heating systems generate convection currents and radiated heat. Around 20% of the heat is radiated from the hot surface of the radiators and if furniture is placed in front of the radiator, the radiation emission is reduced. 80% of the heat is convection currents, which makes the hot air rise. This adds up to a very warm ceiling! Underfloor heating systems, however, rely on both conduction and radiation. The heat from the underfloor heating system conducts through the floor, warming the floor structure, LEVEL 3 NVQ DIPLOMA IN PLUMBING AND HEATING
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heat
Radiator at 70º 18º C
UNDERFLOOR HEATING
20º C 80º convected heat
24º C Radiator heating
Underfloor heating
Hot air rises 20º C
21° C
20% radiated heat
Cooler air decends
Underfloor pipework at 35º - 40º 18°C - 29ºC
Radiator at 70º 18º C
304.15
The principle of underfloor heating 20º C
Underfloor heating
20º C
making the floor surface a large storage heater, and the heat is then released into the room as radiated heat. Around 50% to 60% of the heat emission is in the form of radiation, providing a much more comfortable temperature at low room levels when compared to a traditional wet system with radiators and, with the whole floor being heated, furniture positioning no longer becomes a problem because as the furniture gains heat, they too emit warmth. 2.7m
Underfloor pipework at 35º - 40º 18°C - 29ºC
Theoretical ideal heating
Underfloor heating
Radiator heating on inside wall
Warm air heating
160 200 240
160 200 240
160 200 240
160 200 240
Eye level 1.7m
304.15
Heating theory 304.16
During the design stage, the pipe coils are fixed at specific centres depending on the heat requirement of the room and the heat emission (in Watts) per metre on pipe. The whole floor is then covered with a screed to a specific depth, creating a large thermal storage heat emitter. The water in the pipework circulates from and to a central manifold and heats the floor. The heat is then released into the room at a steady rate. Once the room has reached the 62
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desired temperature, a room thermostat actuates a motorised head on the return manifold and closes the circuit to the room. Such is the nature of underfloor heating that many fuel types can be used, some utilising environmentally friendly technology. Gas and oil-fired boilers are common but also biomass fuels, solar panels and heat pumps. Floor coverings are an important aspect for underfloor heating. Some floor coverings create a high thermal resistivity making it difficult for the heat to permeate through. Carpet underlays and some carpets have particularly poor thermal transmittance, which means the heat is kept in and not released. Thermal resistivity of carpets and floor coverings is known as TOG rating. The higher the TOG rating, the less heat will get through. Floor coverings used with underfloor heating should have a TOG rating of less than 1 and must never exceed 2.5. Quite often underfloor heating is used in conjunction with traditional wet radiators, especially on properties such as barn conversions. The higher temperatures required for radiators do not present a problem because the flow water for the underfloor system is blended with the return water via a thermostatic blending valve to maintain a steady temperature required for the underfloor system. Zoning the upstairs and downstairs circuits with two-port motorised zone valves and independent time control for the heat emitters also helps in this regard.
Hot water cylinder
Pump
Two way manifold Pump
Top view
Boiler
Underfloor heating circuits
Typical underfloor heating system combined with wet radiators 304.17 LEVEL 3 NVQ DIPLOMA IN PLUMBING AND HEATING
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Table 1: The advantages and disadvantages with underfloor heating Advantages
Disadvantages
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The pipework is hidden underfloor. This allows better positioning of furniture and interior design.
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Not very suitable for existing properties unless a full renovation means the removal of floor surfaces.
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The heat is uniform giving a much better heat distribution than traditional systems.
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Can be expensive to install when compared to more traditional systems.
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These systems are very energy efficient with low running costs.
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Heat up time is longer as the floor will need to get to full temperature before releasing heat.
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Environmentally friendly fuels can be used.
■■
■■
Underfloor heating is almost silent with low noise levels when compared to other systems.
Slower cool down temperatures means the floors may still be warm when heat is not required.
■■
Cleaner operating with little dust carried on convector currents. This can help those people who suffer allergies, asthma and other breathing problems.
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Greater installation time.
■■
More electrical installation of controls is required, as each room will need its own room thermostat and associated wiring.
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System maintenance is low and decorating becomes easier as there are no radiators to drain and remove.
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Individual and accurate room temperatures as every room has its own room thermostat that senses air temperature.
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Less possibility of leaks.
■■
Greater safety, as there are no hot surfaces that can burn the elderly, infirm or the very young.
■■
Better zone control as each room is in effect a separate zone.
The layout features of underfloor heating Underfloor heating uses a system of continuous pipework, laid under a concrete or timber floor in a particular pattern and at a set centreto-centre pipe distances. Each room served by an underfloor heating system is connected at a central location to a flow and return manifold, which regulates the flow through each circuit. The manifold is connected to flow and return pipework from a central heat source, such as a boiler or a heat pump.
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Hot water cylinder Ground source heat pump Pump
Two way manifold Pump
Buried captor or 'slinky' Pump
Top view
Underfloor heating circuits The flow and return connections to the thermostatic mixing valves enter side by side
Typical underfloor heating system using a ground source heat pump 304.18
The manifold arrangement also contains a thermostatic mixing valve to control the water to the low temperatures required by the system and an independent pump to circulate the water through every circuit. Each underfloor heating circuit is individually controlled by a room thermostat, which activates a motorised head on the return manifold to precisely control the heat to the room to suit the needs of the individual.
The working principles of underfloor central heating system pipework and components As we have already seen, underfloor heating works by distributing heat in a series of pipes laid under the floor of a room. To do this, certain components are required to distribute the flow of heat to ensure that the system warms the room. However, the components must be controlled in such a way so as to maintain a steady flow of heat whilst, ensuring that the floor does not become too hot to walk on. This is achieved by the use of: ■■
manifolds
■■
a thermostatic blending valve
■■
a circulating pump
■■
various pipework arrangements to suit the floor and its coverings
■■
the application of system controls – time and temperature to space heating zones.
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The use of manifolds In technical terms, the manifold is designed to minimize the amount of uncontrolled heat energy from the underfloor pipework. The manifold is at the centre of an underfloor heating system. It is the distribution point where water from the heat source is distributed to all of the individual room circuits and as such, should be positioned as centrally in the property as possible. Room temperature is maintained via thermostatically motorised actuators on the return manifold whilst, the correct flow rate through each coil is balanced via the flow meters on the flow manifold. Both the flow and return manifolds contain isolation valves for maintenance activities, an automatic air valve to prevent air locks and a temperature gauge so that the return temperature can be monitored. Most manifolds contain a circulating pump and a thermostatic mixing valve, often called a blending valve. These will be discussed a little later. Automatic Air Vent Flow Meter Isolating Valve
Pump Actuator Head
Pressure Guage
Isolating Valve
Blending Valve
Typical underfloor heating manifold
The thermostatic mixing (blending) valve The thermostatic mixing or blending valve is designed to mix the flow and return water from the heat source to the required temperature for the underfloor heating circuits. They are available in many different formats, the most common being as part of the circulating pump module as shown in the image below. The temperature of the water is variable by the use of an adjustable thermostatic cartridge inside the valve.
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