Simple Guide to Central Heating Controls

Modern central heating systems can run efficiently providing warm and comfortable homes at a surprisingly low cost. The overall effectiveness of a heating system depends on two things: a high-efficiency boiler, that ensures little potential heat is wasted, and good controls, designed to ensure that the boiler is only working when heat is needed in the home.

Good heating controls require a minimum of four things:

• an electronic timer or programmer
• a room thermostat
• thermostatic radiator control valves (TRVs)
• separate thermostatic control on the hot water system

Additional controls that may be worth considering to get the very best from a central heating system include:

• intelligent heating controls
• a weather compensator
• a boiler energy manager
• full zone control

Basic Controls

The timer or programmer

The electronic timer or programmer decides when the boiler is able to run. It is not true that boilers work best when they are running continuously or that energy is saved by leaving the heating on all day, even if the home is unoccupied. Whenever the boiler is firing it is using energy, and whenever the home is being heated to a temperature above that outside, it will be losing heat to the outside world. In spring and autumn there is no need to keep the heating on all day; a reasonably well insulated home can be left to cool down slowly with the heating timed to come on perhaps an hour or so before people return home from work.

A seven day timer is also strongly recommended, so that it is possible to set a different heating pattern for weekdays and weekends. Some timers allow different patterns for each day of the week; this can be useful for those working part-time or on shifts that vary from the conventional Monday-Friday work pattern.

Some programmers incorporate built in thermostats and temperature sensors. These need to be sited in a living room rather than by the boiler, but can often represent a good investment.

The room thermostat

This is best located in a living room, rather than the hallway, as is commonly done, as the hall temperature can be affected by the front door being used. The thermostat records the home's temperature and if it is at or above the set level (and 20°C/68°F is usually adequate) stops the boiler from operating the central heating.

Thermostatic radiator control valves (TRVs)

These switch individual radiators on or off, depending on how warm the room that they are located in is. They usually have a fat valve at one end, marked with a * and numbers from 1 to 5. The * setting is to protect against frost; it will typically leave the radiator switched off unless the temperature falls below about 6°C. For a normal living room, the setting of 3 or 4 is likely to be about right; for a bedroom a cooler temperature will normally suffice. Turning the dial up when the radiator is already on will not increase the room temperature!

TRVs are not expensive and can be fitted by DIYers who are comfortable with plumbing. Generally, one radiator should be left without a TRV and left permanently switched on, unless the boiler is fitted with a flow meter to detect when all the radiator valves are closed. This special radiator may be a bathroom towel rail (where the heat is always likely to be useful), or in the same room as the room thermostat. It is not a good idea to have a TRV on the radiator in the same room as the main thermostat, as if it turns the radiator off at a lower temperature, it can mislead the main thermostat into thinking that the house is cooler than it really is.

Thermostatic controls on the hot water system

This section does not apply to combination (Combi) boilers, but only to systems with a separate hot water tank. Firstly, it is most important that the hot water can be controlled by the programmer separately from the central heating. Some older systems only allow the heating to run when the hot water is on; this can be quite wasteful of fuel. Secondly, there should be a thermostat on the hot water tank - this is usually strapped to the outside fairly near the bottom. This controls the water temperature - it should not normally need to be set higher than 60°.

The room thermostat and the hot water thermostat should be wired up to the boiler in what is known as an "interlock". This means that if both the house and hot water are at temperature, the boiler will be switched off. If this does not happen, when the water temperature inside the boiler itself falls, an internal thermostat will cause the boiler to fire to heat up this water - a process known as "dry cycling". All the energy used in this cycle is wasted as it is not used for any useful purpose.

More advanced controls

There are a number of more advanced controls now on sale.

Intelligent Heating Controllers

Intelligent heating controllers, such as the "Dataterm", combine several of the functions above and can also learn how long it takes for a house to heat up in different weather conditions. These also often allow for different temperatures to be set between day and night. They give the very best control over central heating, although they cost somewhat more than normal controls.

Weather Compensators

These measure the temperature, either internally or externally, and delay switching on the central heating on milder days. Simple ones are quite inexpensive and replace a normal room thermostat; they are well worth considering, although the programmer will appear to need to be left on for longer periods when they are first installed.

Boiler Energy Managers

There are many kinds of boiler energy managers on the UK market, ranging from simple devices that delay a boiler firing (and work rather like just turning down the thermostat!) to complicated ones optimised for a particular model of boiler. The general advice is that simple strap-on devices are probably not a good investment, but that if a boiler manufacturer recommends one for use with a specific model of boiler, then they are worth fitting at the time that the boiler is installed.

Full Zone Control

Most homes have a single heating zone - the only controls in the rooms are by TRVs. However the need for heating in the main living rooms can be quite different from that in bedrooms, with the latter requiring lower temperatures for longer hours. At the time a new central heating system is installed, it is possible to fit a full zone control that has different pipe loops and separate thermostats for two (or more) areas. This can save significant amounts of fuel in larger houses.