How to use Thermostatic Radiator Valves
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. When fitting TRVs, it should be noted that some are bi-directional (can be fitted on either end of a radiator), but many are best fitted at one end only (either flow or return) to reduce the chance of noise in the system.
Turning up a TRV will not usually warm up a room unless the radiator is cold, as most are basically designed to switch the heat on or off as the room temperature falls below or rises above a set temperature and are not fully modulating. Some TRVs are prone to sticking at one setting, if they're never adjusted. It's not a bad idea to turn them fully up or off a few times a year to reduce the chance of failure.
Different types of Thermostatic Radiator Valves
In the past few years a number of electronic TRVs (eTRVs) have appeared on the market. These typically allow temperatures to be set with greater precision than traditional TRVs, as well as being able to vary the temperature by time of day so that, for example, living rooms may be set to a higher temperature in the evenings, or bedrooms to a lower one by day. They have a motorised head that automatically turns the valve on or off when required; however this does mean that all such vales require a battery and are not completely silent in operation. Some eTRVs can be controlled wirelessly from an infra-red remote control or wirelessly from a personal computer and may have an override button that extends the higher temperature period by an hour, which can be useful in living rooms when people occasionally stay up late to watch a movie! Some models also allow remote thermostats within the room, so they are more indicative of the general room temperature than one in immediate proximity to the radiator. (Temperatures close to the bottom of a radiator sited under a window can often be significantly lower than the true room temperature.) eTRVs can also be sold in a tamper-proof version, typically for use in hotels or similar locations, where temperatures are wanted to be controlled centrally. It's not clear what savings will typically be achieved by using eTRVs but, as always, manufacturers' claims should be taken with a degree of caution when used in a real home or commercial building.
If poor temperature recognition is one of the main problems (for example if the radiator valve is situated behind a long curtain, under a draughty window or in an area prone to direct sunlight), then an alternative to an eTRV may be to install a remote sensor on a standard TRV head. This works by monitoring the room temperature via a 2 or 5 metre capillary connected to the sensing thermostat which can be mounted in an area more representative of the room.
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. First, it's 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 and 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.