Good central heating controls require a minimum of four things:
- Electronic timer or programmer.
- Room thermostat.
- Thermostatic radiator control valves (TRVs).
- Separate thermostatic control on the hot water system.
Timer or programmer
The electronic timer or programmer decides when the boiler should 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.
This is best located in a living room, rather than the hallway, as the hall temperature can be affected by the front door being used. The thermostat records the home's temperature and if it's either at or above the set level (and 20°C/68°F is usually adequate) it stops the boiler from operating the central heating.
Thermostatic radiator control valves (TRVs)
Older TRVs simply 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, often marked with a * and numbers from 1 to 5. The * setting is to protect against frost and 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! Newer TRVs will modulate the flow, depending on the temperature set.
Most recently, a programmable radiator controller has come onto the UK market. This allows the time and the temperature to be set in each room, without needing to change the overall programme. In some ways it works more like a separate heating zone for each room and one model can be fitted onto existing thermostatic valves, eliminating the need for any plumbing.
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, which 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 central heating controls
Intelligent heating controllers
Intelligent heating controllers 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 usually allow for different internal temperatures to be set between day and night, and may be combined with a frost feature that will switch the heating on in an 'off' period if the temperature falls below a set level. They give the very best control over central heating, although they cost somewhat more than normal controls.
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 may appear to need to be left on for longer periods when they are first installed.
Boiler energy managers
The term 'boiler energy manager' can be used for several different devices that can combine one or more of the functions of weather compensation, load compensation (which varies flow temperature with internal room conditions), optimum start control, night setback, frost protection, anti-cycling control and hot water override. They are certainly worth purchasing when recommended as an option by the boiler's manufacturer, but add-on units are of variable benefit, and care should be taken that they do not duplicate functions in the thermostat or other controller.
For example, anti-cycling devices simply delay a boiler firing and will generally only save energy through reducing performance (and in many cases give the same result as just turning down the thermostat). On the other hand, devices that modulate the water temperature based on external weather conditions are reported to make worthwhile savings, even if there is a thermostat-induced time delay to starting the heating circuit. Care should be taken that if both are in place, they do not lead to a double compensation in milder weather.
The general advice is that simple strap-on devices are probably not a good investment. However, if a boiler manufacturer recommends one for use with a specific model of boiler, then they are worth fitting when 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, especially in larger houses.
Paying for controls upgrades
Most modern controls, including timers and programmers for hot water, are relatively inexpensive, and can often be fitted by a competent central heating engineer at the same time that you have your annual boiler service.