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CENTRAL HEATING CONTROLS

(also see the picture gallery)   

This is the most important part of any central heating system. A central heating system is only as good as its controls, the better the controls, the less wear and tear on the boiler and system. You will get more comfortable, controllable temperatures and in turn the running costs will be less.

Economy largely depends on which type of system that you have installed, but its usually cheaper to keep the heating on at low temperatures for long periods of time.

Programmer  will switch one, two or even three different circuits on or off at different times or even on different days. Most programmers have integral switches to set them up to your own personal preferences, for example, to have the hot water and heating to come on separately at a different time each day of the week. (Override buttons are also fitted to programmers) A programmer should be set to cover all the times that the property is occupied. A signal is sent out to the cylinder and room thermostats and these will keep everything at the desired temperatures.

Cylinder Thermostat if your system is warming a cylinder full of water then you must have a cylinder stat on it, this should be set to about 55-60 deg °C and should be positioned approximately ¹/₃ the height of the cylinder from the bottom. Once this is satisfied it allows the hot water circuit to rest, and boiler to rest if no other control is calling for heat, until some hot water is used or the cylinder cools down naturally.

Room Thermostat All central heating systems should have a room stat fitted (although a lot haven't), as this will it will shut the heating circuit off allowing the boiler and system to rest (no fuel being burnt), once the air has reached the desired temperature. As the air starts to cool slightly, the room stat brings the heating on again to maintain the set temperature (within 2 deg °C). This should be fitted about 1.5 metres high in an average temperature room with a radiator fitted, but not in direct sunlight or above a radiator, and have no other source of heat in that room. Usually these are best fitted in a hall or dinning room. Since April 2002 it is mandatory as part of the Building regulations that all new and updated systems have a room thermostat fitted preferably in the main living area and in a room with a radiator and no other source of heat, 

Note the radiator must not have a TRV fitted as this can stop the Room thermostat working correctly

Note if you have not got a room thermostat then it will pay to have one (as it is like parking up your car and leaving the engine running) if you don't have one  then it will pay to get one especially to the gas prices are rising drastically in 2006 Note this is usually an electricians job

Programmable Room Thermostat Same as above, except you can program different temperatures for different times of the day and night, also there is no need for any other timing device on this circuit (also acts as a frost stat). These are highly recommended. Some Models have a 7 day option, so the temperatures and times can be set for different each day

Note this Is the only control needed for combination boilers and is often cheaper that the manufactures time clocks that are part of the appliance 

I have a programmable stat set for 16°C at 6.30, 18°C at 7.00, 16°C at 9.00, 18°C at 15.00, 20°C at 18.00 and 12°C at 23.00. So a low temperature will turn the system off until the temperature falls to the new setting or the opposite if a higher temperature.

Note that I prefer the hard wired type as the RF (radio Frequency) do use more  batteries and are less reliable and can suffer from interference but on the plus side they are very easy to install

Frost Thermostat The purpose of this is to stop the boiler or exposed parts of the system freezing in winter. This is the same as a room thermostat except it works at lower temperatures and ignores all other timers and thermostats. These should be set at between 3 & 5 deg °C and should be placed above what is at risk of frost damage, e.g. above a boiler if fitted in a garage/ outhouse, or above a radiator in a conservatory (if not on its own circuit)

Cylinder thermostat 

this should be attached to the cylinder between 1/4 & 1/3 of the height of the cylinder so it is quite close to the bottom

it should be set no higher than 60C so as the cylinder heats up from the top down when the cylinder stat is satisfied only when the cylinder is full of hot water and will shut off the HW side of the system so all the pump energy will be put into the radiators until the room thermostat is satisfied then the boiler will shut off

if hot water is drawn out of the cylinder then cold water will enter from the bottom and the cylinder  stat will turn the boiler on again giving you constant hot water

Thermostatic Radiator Valves

(TRV's) fit on the radiator in place of the existing valve, inside the valve is a wax capsule which expands when heated and stops the flow of water to the radiator. As the wax cools it contracts and lets the water from the boiler flow into the radiator. You should only fit TRV's to every radiator providing there is a bypass fitted on the system, but even then this is not advisable.  As it could increase your fuel bills and increases wear and tear on your boiler. This is because once all the rooms are up to temperature all the TRV's will be closed, but your boiler will still be producing heat at about 78 deg °C. The pump will still be pushing the heated water around the bypass circuit, even though the house is warm enough. This will cause the boiler to cycle unnecessarily. So it is also advisable to fit a room thermostat. A TRV should not be fitted in the same room as the room thermostat as this can interfere with its operation. When the room stat is satisfied it will shut the pump & boiler off (providing another circuit is not calling). For best economy just fit TRV's in rooms that are too warm (bedrooms) or in south facing rooms and unused rooms. If you have a condensing boiler then do not fit too many TRV's, usually no more than a third of all radiators, as they will reduce the high efficiency of the boiler (see manufacturer’s instructions)

When choosing a TRV then check that they are full flow valves as some have a higher restriction, The picture on the right shows the difference, the valve on the left only has a 8mm diameter hole for the water to pass through, this can cause large radiators not to heat up correctly and can make the system noisy as well as making the system less efficient. Both these TRV's have the wax capsule head removed showing the pin that is pushed down by the head

But please note the manufactures do claim that these high restriction valves are made to the correct BS standard (BS 7478) and will Pass up to 4KW of heat on a Small bore (15mm) system.  But I personally prefer to keep the restriction down to a minimum so there is more water flow through the boiler and system

This also shows the actual size difference between 15mm and 8 mm which is why Small Bore (15mm) systems will always be better than Micro-Bore (8mm) as far grater volumes of water can pass through 15mm making the boiler work more efficantly making it more reliable

If you have a one pipe system then you must always use a full flow valve as each radiator is fed only by gravity so definitely needs a full flow valve 

The way to set TRV's up is to set each valve mid way. After the heating has been on for about 2 hours, go into each room and if the air is too warm then turn the valve down 1 number or 1/2 a number, or if too cool up 1 number. Keep doing this until you find the right setting for each room. Touching the radiator will only tell you if it is on or not at that moment in time.  once you find to correct temperature then each timer the heating is put on from cold, the radiator will come on at full and as the pre determined air temperature is reached the TRV will turn the water flow off  letting the radiator cool down, The TRV will then control the temperature of the air by opening and closing as necessary 

Motorised valves control the flow of water through the system and are used with fully pumped systems  

Shown on the right are 4 two port drive open spring return valves which are each controlled by a thermostat which will drive the valve open and turn the boiler and pump on this a called a "S" plan system 

Also a 3 port 3 position valve is available called a "Y" plan valve (mid Position) but these can only control 2 zones (usually the water and the heating) which is fine in most houses 

Energy Management Systems These are a complex type of programmer, except there are sensors connected directly to it. (Usually one on the heating return and another outside the property) These are very expensive to buy and because they use outside air temperatures to anticipate the sun warming up the property, they can be very inaccurate inside.

Load Compensation

This control  is designed to reduce fuel wastage and is to control the (boiler) only and should be use with other controls and not instead of other controls

I remain unconvinced of the benefits in domestic homes where the CH system is correctly designed and installed

These may benefit homes that do not have an electric  room thermostat  (only have TRV's on all the radiators) and in very big properties especially if you have a boiler without a modulating burner then this could possibly save fuel cost. If you want to know more about Load Compensation Control then please contact Fuel Economy Ltd.  www.savastat.co.uk 

For independent information on energy efficiency visit www.natenergy.org.uk 

2002 building Regulations

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