One of the great advantages of central heating is the ability to control the temperature inside the home, and to respond rapidly to changing conditions without wasting costly energy. The system controls make this possible.
The minimum requirement – for a system with a gravity-fed hot water circuit – is a programmer, a boiler thermostat and a room thermostat. More sophisticated systems – with pumped hot water circuits ЎЄ may also have motorised valves to direct the flow of hot water where it’s needed, a cylinder thermostat, radiator thermostats and an external thermostat to guard against frost.
The programmer is the central control, and determines which parts of the system are in operation and at which times. Simple programmers offer a choice between hot water only or hot water plus central heating, for one or two periods in each 24-hour cycle. Simple programmers are not very flexible; for example, it is not generally possible to programme them to provide hot water only for one on-period and hot water plus heating for the other in the same day. More sophisticated digital programmers are available which work on a 7-day cycle, with more on-periods in each 24 hours and a more flexible choice of hot water only or hot water plus central heating.
Thermostats are simply temperature-controlled switches. The boiler thermostat controls the temperature of the water flowing out of the boiler. If the temperature is below the setting of the thermostat, and the programmer says that the system is on, then the boiler comes on. The room thermostat senses the temperature inside the room and switches the heating on if it has the ‘permission’ of the programmer to do so. In practice, in a simple system this means starting up the pump to circulate water around the radiators. The room thermostat is usually located in the main living-room.
A cylinder thermostat is used in conjunction with a motorised valve, where the hot water system is pumped rather than relying on gravity flow. It works in the same way as the room thermostat, turning on the pump when the temperature of the water in the cylinder drops below the pre-set value. The motorised valve responds to inputs from both the cylinder thermostat and the room thermostat, directing the outflow of the pump accordingly.
An external thermostat is used to override the other controls when the outside temperature drops to a level where frost damage might occur.
Thermostatic radiator valves (trvs) control the flow through individual radiators. They’re very useful in rooms which might otherwise get too warm before the temperature in the living-room has reached the level set on the room thermostat. Most trvs incorporate the thermostat within the valve itself, but there are types with remote thermostats which can be placed on the wall some distance from the radiator.