A programmer is simply a timer which controls one or more switches. The timer can be mechanical or electronic.

Programmers for central heating systems typically have two sets of switches - one for hot water and the other for heating. It is normally possible to have two or more different switching times each day, and on many types the hot water and heating times can be adjusted separately.

Inside a typical 2 channel programmer

Two channels means one set of switches for hot water and another separate set for heating.

Internal diagram of a typical 2 channel programmer Internally, the clock or timer is powered from the mains supply and activates the switches at the times selected by the user. The clock can either be a mechanical type, or more likely, an electronic device.

The diagram shows the most usual arrangement. Terminals are 1=HW Off, 2=CH Off, 3=HW On, 4=CH On.
Most programmers have a standard plastic backplate which is fitted to the wall. The actual programmer clips onto this plate, making it very easy to replace.

Switching is done at mains voltage. In the 'off' position, the 'off' terminal is connected to line, or 230 volts. In the 'on' position, line is connected to the 'on' terminal. Both sets of terminals work in the same way.
Depending on the rest of the system, one or both 'off' terminals may not be connected.

Traditional programmer settings

Most traditional 2 channel programmers have two sets of times, and additional buttons or sliders to select between 4 different modes.

These four modes combined with the two channels (hot water and heating) result in 16 different combinations (4x4).
For systems with gravity hot water where the hot water cannot be controlled separately, 6 of the combinations are not permitted, resulting in 10 available combinations.
Some programmers have a switch on the back to select either 10 or 16 options. However as gravity systems are not common, and certainly wouldn't be installed today, the 10 or 16 choice is becoming less relevant.