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):
Hot water cylinder thermostat
Hot water cylinder
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
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
Hope that all helps!