Heating systems require a range of controls to ensure that they operate safely, efficiently and according to consumer demand. The controls usually comprise the timer or programmer, the cylinder stat, the room stat, one or more motorised valves and the wiring to link these together. Often there will be a junction box where the wiring connects.
In order to limit the temperature of DHW, and to avoid the boiler firing unnecessarily, DHW cylinders should have a thermostat fitted to the side, about 1/4 to 1/3 up from the bottom. This is usually set to 60C and consists of an electrical switch which feeds back into the control system.
The general temperature in a house should be controlled by a room thermostat fixed on a wall at eye level in a location where the temperature it senses will be representative of the general house temperature. The thermostat is an electrical switch that feeds back into the control system.
Timers are used to determine when the heating systems switches on and off. They are also known as programmers, particularly when controlling 2 channels (e.g. HW and CH).
Programmers are essentially timers which control switches linked into the control system. Typically they will have 2 channels (HW and CH) although Combi boilers will normally have single channel programmers. The simpler programmers are also known as timers.
Room thermostats are available which combine the function of programmers. This has the advantage of putting the main user controls in one location, although as with simple room stats the location must be chosen with care to ensure that the air temp detected is representative of the whole house.
A feature of a heating control system which prevents the boiler from firing when there is no call for heat. It’s a requirement of Part L of the Building Regulations that all heating systems include an interlock. The simplest form is a tank thermostat on the hot water cylinder and a room thermostat included in the controls. Note that the room stat is required IN ADDITION to thermostatic valves on individual radiators. Some boilers include a flow switch which prevents the boiler from firing at low flow rates (eg. when all the TRVs are closed). However, not all boiler flow switches operate as interlocks. Weather compensators, some types of external temperature sensors and of ‘boiler optimiser’ also operate as interlocks.