complex domestic heating systems layouts and controls

Level 3 6035 Diploma in Plumbing Studies

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Complex domestic heating systems layouts and controls

Unit 306: Central heating



Legislative documents relating central heating design and installationBuilding Regulations Part L – Conservation of fuel and power:

L1a - …in new dwellingsL1b - …in existing dwellingsL2a - …in new other that dwellingsL2b - …in existing other that dwellings

•BSEN 12828 – Design of water based heating systems•BSEN 14336 – Installation and commissioning of water based heating systems•Domestic Heating Compliance Guide•CHeSS – Central Heating Efficiency System Specification (basic and best practice)•Manufacturer’s instructions•Domestic heating compliance guide•Gas safe installation and use regulations•Part P



Typical pipework layouts – Types of systemsModern domestic central heating systems fall into two categories, which are based on the way the system is filled with water and the pressure that the system operates at.

Low pressure, open-vented systems:These are fed from a feed and expansion cistern located at high level. These can be fully pumped or gravity systems.

Sealed, pressurised systems:These are fed directly from the mains cold water and incorporate an expansion vessel to take up the expansion of the heated water. This type of system is a more modern fully pumped or combination boiler system.



One-pipe gravity system

• These systems are not compliant to Building Regulation Part L.• Boilers connected to these systems are low efficiency.



• This system has gravity circulation to the hot water circuit, but pumped circulation to the heating circuit. The control on the heating was sometimes by a room stat, which only switched off the pump, but the hot water had no such control and was often very hot.

• These systems are not compliant to Building Regulation part L. Boilers connected to these systems are low efficiency.

Two-pipe semi-gravity system



C plan two-pipe semi-gravity system

• An updated version of a two-pipe semi-gravity system that includes a cylinder thermostat and zone control on the hot water.

• The C plan plus is the minimum acceptable compliant system with Building Regulations Part L1b.



Fully pumped mid-position valve – Y plan system

• The three-port mid-position valve controls the flow of water to the hot water cylinder and heating circuit. The valve reacts to the room thermostat and cylinder thermostat.

• The system has an automatic bypass valve which connects the flow and return pipe. The bypass opens if the system pressure increases when circuits close down due to them getting to temperature. This allows water to flow through the boiler – stopping ‘lock-out’ and also prolongs the circulator life.



Fully pumped two two-port valves – S plan system

• This system uses one zone valve to control the hot water, which is activated by the cylinder thermostat; and a second zone valve which is activated by the room thermostat. The zone vale acts as an isolator, closing the flow of water off. If a property is over 150m2 an additional heating zone valve should be fitted allowing independent control of upstairs and downstairs.

• As with the Y plan, an automatic bypass is fitted to the system.



Y Plan S PlanFull thermostatic control

Building Regulation compliant

Recommended for larger properties

Can be used with sealed systems

Can be used with system boilers

Can be zoned

Boiler interlock

Types of systems



Key components within heating systemFeed and expansion cistern:

Used in an open-vented system to fill the system up with water. Commonly located in the loft to locate it at the highest point of the system.

The base of the F&E should be at the level as the cold water storage cistern and be fully supported over its base.

The main purpose of the F&E is to allow the heated water in the system to expand and to operate at atmospheric pressure. The system water will expand by 4% when heated and this expansion needs to be taken up in the F&E which is commonly an 18-litre, black plastic cistern.



The 22mm openvent from the primaries is located over the top of the F&E and must rise a minimum of 450mm above the water level and should not have any valves.

The F&E should be located a minimum on 1 metre above the circulator. The vent needs to be a minimum of 450mm above the water level, to prevent the surge effect of the pump.

Minimum vent pipe size 22mm.



CirculatorsThe circulator is usually located in position by full-bore valves on either side, they are used to allow the pump to the removed without draining the system.



As the system requires positive pressure, the position of the circulator is important in relation to the cold feed and vent pipe.



Air separatorThe use of an air separator allows close proximity or close-coupled pipework, when positioning the cold feed and vent pipe. The air separator is basically an oversized pipe with a cold feed and vent pipe connections. The water swirls around separating the air.



Instead of an air separator, the configuration shown below is commonly used.

The vent and the cold feed need to be installed on the flow from the boiler, on the suction side of the circulator with a maximum of 150mm between them. Ensuring that the circulator always draws on the cold feed. The correct layout of pipework, ensures the‘neutral point’of the system is at the base of the cold feed.



Two-port motorised valve: Commonly known as a zone valve, this can be activated by a room or cylinder thermostat. This allows both hot water and the heating circuits to be controlled separately. When there is a call for heat, the zone valve opens and allows water from the boiler to circulate around the pipework. Likewise when the thermostat is up to temperature the zone valve is closed. (Isolates)

Three-port mid-position valve:This valve controls the flow of water to both the heating and hot water circuits. It reacts to both the cylinder and room thermostat.

The three ports are labelled:AB = the flow for the boilerA = central heatingB = hot water



Cylinder thermostat:Situated third of the way up a hot water cylinder governing the temperature of the hot water. This is linked to the zone valve or three-port valve. It can be a bi-metallic strip, volatile liquid or gas type. Commonly set to a maximum of 60ºC.

Room thermostat:Situated 1.5m up a wall and approximately 3m away from a heat source and not on an outside wall. It can be a basic bi-metallic type or, nowadays, a programmable, Wi-Fi enabled, app-activated or digital type, according to the customers requirements.More commonly a compensating room thermostat can be fitted.



Boiler thermostat:This is located on the boiler and governs the temperature of the primary water. This water heats up the heat exchanger in the hot water cylinder, as well as heating the heat emitters in each room.

These commonly have a phial attached to the return pipe of the boiler indicating when the system is up to temperature.



Frost thermostat and pipe thermostat:These are incorporated into a system to stop vulnerable parts of the system from freezing.

They override the programmer and other thermostats and should be set to between 30–50ºC.

The frost thermostat is positioned in vulnerable areas such as garage installations and the pipe thermostat is fitted to exposed pipes. They will activate the circulator and boiler briefly to warm up the system water.



• These allow independent timed control of both the hot water and the central heating. Commonly situated in the kitchen or airing cupboard of a property.

• They often have a‘boost’or‘override’ button to allow the system to operate outside the programmed timings.

Programmers



Water compensation This control uses an externally fitted temperature sensor fitted on a north or north–east facing wall, so as not to be in the direct path of solar radiation. As the external temperature rises, the weather compensator reduces the circulation temperature of the flow from the boiler.



A good system of controls must:•Ensure that the boiler does not operate unless there is demand. This is known as‘boilerinterlock’. •Only provide heat when it is required to achieve the minimum temperatures. There are two levels of controls for domestic properties and these are set out in Central Heating System Specification (CHeSS) CE51 2008:

HR7 – Good practice for systems with a regular and a separate hot water storage:i. full programmerii. room thermostatiii. cylinder thermostativ. boiler interlock v. TRVs on all radiators, except in rooms with a room thermostatvi. automatic bypass

HC7 – Good practice for systems using a combination boiler or Combined Primary Storage Unit:i. time switchii. room thermostatiii. boiler interlock iv. TRVs on all radiators, except in rooms with a room thermostatv. automatic bypass valve



Boiler interlock: is not a physical device but an arrangement of system controls (room thermostats, programmable room thermostats, cylinder thermostats, programmers and time switches), which ensurethe boiler does not fire when there is no demand for heat.

In a system with a combi boiler this can be achieved by fitting a room thermostat.

In a system with a regular boiler this can be achieved by correcting the wiring interconnection of the room thermostat, cylinder thermostat and motorised valve(s).

An automatic bypass valve: controls water flow in accordance with the water pressure across it, and is used to maintain a minimum flow rate through the boiler and to limit circulation pressure when alternative water paths are closed.

A bypass circuit must be installed if the boiler manufacturer requires one, or specifies that a minimum flow rate must be maintained while the boiler is firing.

The installed bypass circuit must then include an automatic bypass valve (not a fixed position valve).



Sealed systemsSealed 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.

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 pressurised to around 1 bar.



Multiple boiler installations that incorporate low loss headersFor a boiler to work at its maximum efficiency, the water velocity passing through the heat exchanger needs to be maintained within certain parameters.

Boiler manufacturers should tell you what the limits are for each make and model of boiler. 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. A low loss header acts as a neutral point in the system – all circulators must pump away from the header, thus, ensuring pipework and boilers are all under positive pressure.



Flow ratesThe primary circuits: 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.



Flow ratesThe 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 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.

A low loss header acts as a neutral point in the system – all circulators must pump away from the header, thus ensuring pipework and boilers are all under positive pressure.



Thermal shockIf the temperature difference between the flow and return is too great, it puts a strain on the boiler heat exchanger – through thermal expansion and contraction.

The temperature of the water passing through the heat exchanger is important, particularly with a condensing boiler. For a boiler to enter into ‘condensing mode’ the return temperature should be no greater than approximately 55ºC. In some instances, temperature sensors are fitted to the low loss header to allow temperature control over the primary circuit. 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.



Wiring diagrams for central heating systems

Typical Y plan



Typical S plan



Safe isolation of electrical supplies



Multi meter, test leads and probes Non-contact voltage indicator

Approved voltage tester with proving unit

Locked off and labelledGS38 The HSE testing

equipment standard

(Test probes max 4mm preferably 2mm)

Safe isolation



Safe isolation1.Test the voltage indicator on a ‘known source’ (proving unit or adaptor plug).

2. Prove appliance is ‘live’. Place probes on to the live and neutral on the appliance mains connection – voltage indicator lights up.

3. Identify the source for the supply ie MCB or fuse.

4. Switch off MCB, lock and label, or remove fuse, lock and label.

5. Go back to appliance and prove appliance is ‘dead’. Place probes on to the live and neutral on the appliance mains connection – voltage indicator does not light up.

6. Re-test the voltage indicator on a ‘known source’ for a second time (proving unit or adaptor plug).



Safe isolationThe correct locations to carry out safe isolation are:

•consumer unit

•fused spur

•switch fused spur.

Always lock and label to avoid the supply being switch back on by mistake (retain fuse).



Electrical testsEarth continuity test

This is where the installer tests to make sure all the metal work (pipes, boilers, appliances, metal sinks, etc) is safely connected to the main earth bar at the consumer unit. This keeps everybody safe.

• Find a known earth source (screw on cover or earth terminal).

• Multi-meter on ohms (or buzzer).

• Place one probe on the known earth.

• Place the second probe on the appliance casing or pipework.

• It should show a low reading (or buzz).



Polarity testPolarity test must first be carried out ‘dead’ before allowing any electricity:

1.Is live connected to live?

2.Is neutral connected to neutral?

3.Is earth connected to earth?

4.On a circuit, you can connect the live and earth together, then go to the other end of the circuit and test. The multi-meter needs to be set on ohms (or buzzer).



Polarity testIf the dead test proves okay, then you can make the system live and re-test. This time the multi-meter needs to be set on voltage (240V).

Probes: Live and Neutral = 240VLive and Earth = 240VNeutral and Earth = 0V

If these reading are different polarity is incorrect.



Short circuit

• If the live accidently touches the neutral, a short circuit will occur. This means the fuse will blow or the MCB will trip.

• If the live accidently touches the earth, a short circuit will occur. This means the fuse will blow or the RCD will trip.



Socket testerA simple way to test polarity in a domestic socket is by using a socket tester. Plug it in and switch the socket on – if all three lights come on, the socket is okay.



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complex domestic heating systems layouts and controls

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