Central heating... How does it work?

Just about every customer I visit seems to feel they don't fully understand exactly how their heating system works. This, I think, is due to the lack of an overall user instruction leaflet. There will be a boiler leaflet, a programmer leaflet, thermostat leaflets, but no overview of how they all work together as a system. Here's my attempt to explain it...

An average, ordinary modern domestic heating system has the following major components. (The first five have controls that the user can set or adjust):

Boiler

Programmer

Room thermostat

Hot water cylinder thermostat

Radiators

Hot water cylinder

Diverter valve

pump

And here's how they work together as a whole system...

1) The boiler has two separate functions. To heat your hot water tank and to heat your house.

3) Your programmer contains a clock and two time switches (channels) which turn the hot water and central heating services on and off at pre-selected times of day (or night).

4) Once the system is turned 'on' by the programmer, the boiler will be turned on and off by either the "room thermostat" and the "hot water cylinder thermostat". Each thermostat will turn the boiler on when heat is needed, then turn it off again when it isn't.

5) The "diverter valve" is an electrically operated valve which switches the hot water flowing from the boiler to the radiators or hot water cylinder depending on which thermostat is calling for heat. If both are calling for heat at the same time the valve will assume a 'mid-position', sharing the flow between cylinder and radiators. The diverter valve is sometimes called a 'mid-position valve' for this reason.

And that's it. Easy, eh?

And here are a few other things it is useful for you to know...

1) The water in the boiler is separate from the water that comes from your taps. Heating system water goes around and around the boiler and radiators indefinitely, until it is drained out by a heating engineer for one reason or another... This slows down internal corrosion, and also allows the circulating water to be dosed with inhibitor to completely prevent internal corrosion.

2) The hot water cylinder has a long coil of pipe inside, through which the boiler water flows. This is how the boiler water is kept separate from the domestic water. Heat is transferred through the wall of the pipe into the water in the cylinder.

3) The control knob on the front of your boiler sets the temperature of the water it produces, and this is the temperature your radiators will be when your heating is 'on'. This control also needs to be set so the boiler delivers water at a higher temperature than set on the cylinder thermostat, or the cylinder will never reach the selected temperature and the cylinder thermostat will never shut the boiler down.

4) Modern boilers often have a "pump over-run" facility. This can be puzzling to users getting to know their system. The pump continues to run for a few minutes after the boiler has been turned off. It removes the risk of static water in the heat exchanger being boiled by residual heat immediately after the gas flame is extinguished, and extends heat exchanger life.

This has been a description of a "fully pumped system".

The other common type is the older, and less fuel-efficient "convection hot water, pumped heating" system. The boiler is energised all the time - the boiler thermostat keeping the boiler hot constantly. The hot water cylinder is heated by boiler water circulating by natural convection, and domestic hot water assumes the boiler water temperature. There is a pump fitted to the radiator circuit and the room thermostat simply switches the pump on and off, circulating hot water around the radiators until the room thermostat is satisfied. The hot water cylinder needs to be physically located above the boiler, and heating the hot water takes much longer than a fully pumped system.

Hope that all helps!