Combination Boilers

These are the simplest systems, as the only controls are those for central heating.

Principle of operation

A combination boiler, or combi, is a boiler which heats hot water on demand, and additionally can function as a normal boiler to heat hot water for radiators.

There are no controls for the hot water section - hot taps are plumbed directly to the boiler. Incoming mains cold water is heated by the boiler and passes directly to the hot taps.
The boiler is activated by a water flow sensor inside the boiler, and the temperature is set using a control built in to the boiler.

The central heating section is similar to a conventional boiler, in that a timer or programmer is used to determine when the heating is on, and a thermostat is used to regulate the air temperature. Some boilers have the timer/programmer built in to the boiler, others require a separate programmer.

Power supply

All combination boilers require a power supply. This is usually provided by a 3A switched fused spur adjacent to the boiler. A 3 core flex connects to the boiler terminals, which will be the standard L N and E.

For those boilers which have a programmer or timer built in, the only other device required is a room thermostat. There are two main types - 2 wire and 3 wire. The choice will depend on the boiler.


The circulator pump for the central heating is normally an intergral part of the boiler, so no additional wiring is required.

Programmer / Timer

Many combination boilers have an integral timer or programmer. For those that do not, an external unit should be installed. An external programmer can also be fitted where the internal unit has failed or additional features are required. The internal unit must be disconnected before an external one is installed.

As the only function will be central heating, a single channel programmer is required. There are many types available, and they generally have volt-free contacts. These can be used with any type of combination boiler.

Where a programmer has mains voltage switching, it is only suitable for a boiler with mains voltage controls.


A boiler will function without a room thermostat, but this will be very inefficient, since the boiler will run continuously, and the rooms will get excessively hot.

While the boiler will have a temperature control, this will only adjust the temperature of the water inside the radiators, which will do almost nothing to control the room temperature.

Combined thermostat and timer

Combined units are available which are typically battery powered. These are connected in the same way as a normal thermostat.