Boiler stove central heating basics


Thermosyphoning relies on the fact that the hot water (from the stove boiler for example) will tend to rise. To encourage thermosyphoning the part of the loop that carries hot water from the wood boiler stove (the flow) is run continuously uphill to the heat loss radiator. To help the thermosyphoning along this section of pipe should be wide bore 28mm. The larger diameter pipe is much easier for the water to flow through.

So in this first section of rising, wide bore, pipe the water is encouraged to flow up to the heat loss radiator and then back to the stove.
Running a whole system by thermosyphon does not meet regs now, but it is useful to know about because the heat loss radiator still needs to work through thermosyphoning if the power or pump fails.

Heat loss radiator / heat leak radiator

If your wood boiler stove is running and there is a power cut, or the pump fails, then your stove will still be making hot water. You need to get rid of the heat somehow or the water will start boiling in the stove boiler. You cope with this simply by making sure that the heat from the stove boiler can thermosyphon to a radiator. This radiator is known as the "heat loss radiator" or "heat leak radiator" and gives the heat somewhere to escape to. Often the heat loss radiator is fitted in the bathroom as this radiator may be a little warmer than the rest.

Balancing a central heating system

What does balancing mean? You may have several radiators connected to your heating loop and balancing them is a way of making sure that each one gets enough hot water. For example you might find that the first radiator closest to the stove gets very hot, but that the others do not. By closing down this radiator a little you encourage the hot water to flow further down the loop to the other radiators. By tinkering with how open or closed each raditor is you should be able to 'balance' the system so that each radiator gets hot.

Balancing is especially important for heating systems using thermosyphoning, but is also necessary in other central heating systems.

The float for the ball valve should be metal

Because steam may be vented into the header tank the float for the ball valve should be made of metal. If the ball is made of plastic then the steam can cause the ball to deform.

Automated valves should be normally open and unrestricted

If there is a power failure then automated valves used to control the heating system should be of the type that remain open when there is no power (fail-safe), and the valves should not restrict the flow of water. This is to allow the heat loss device to work. Valves in parts of the system not effecting the heat loss device may not need to be normally open or unrestricted - consult your heating engineer.


At the highest point in the circuit (usually a radiator) will be a vent to remove airlocks.

A pump will usually push air out but you have to be careful on a thermosyphon system to ensure that all the flow pipework is slightly on the rise so that any air automatically travels up to the highest point where it can be vented.