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Solid Fuel Systems

A schematic drawing of a solid fuel heating system is shown below. This is quite difficult to control in accordance with current Building regulations (U.K.)

In this example the radiators are controlled with thermostatic rad. valves and the hot water cylinder has a 2-port valve operated by a cylinder thermostat.

Some form of gravity circulation is useful for solid fuel heating systems since excess heat can build up in an unattended fire especially at night. This excess heat can be dissipated at the primary circuit to the hot water cylinder provided the 2-port motorised valve is open. Sometimes a radiator is used to remove more heat, this is called a heat leak radiator and is usually in the bathroom.

A heat leak radiator is a conventional radiator installed in such a way as to receive water by gravity circulation so that if the pump is switched off, the radiator will still dissipate heat. One way to make sure that a heat leak radiator is not inadvertently turned off is to install two lock shield valves instead of one hand wheel and one lockshield valve. A lock shield valve is one that cannot be turned off.

An injector tee aids circulation in gravity circuits when the pump is on.

It is a good idea to leave some radiators without thermostatic radiator valves. If all the radiator valves close then the pump will be pushing against a closed head. Another way to relieve pressure in a system is to incorporate a pressure relief valve situated between the flow and return pipes.

In the diagram below a pipe thermostat is used to switch the pump on and off. This ensures that the hot water cylinder heats first, before the radiators receive heat. Another way to use this pipe thermostat would be to set it at 85oC and use it as a safety feature in the case where the 2-port motorised valve is closed heat could be dissipated through the radiators.


The photographs below show some control items used for mechanical control.

The diagram of the solid fuel heating system shown below eliminates gravity circulation. The pump is switched on manually for normal operation, i.e. if space heating or hot water is required the user switches on the pump.

Heat dissipation is important in solid fuel systems. If a fire was to get hot at nighttime when unattended then heat may build up in the systems causing a very high temperature of hot water in the cylinder or even steam in the heating system. To dissipate excess heat the pump can be switched on automatically if the temperature in the flow close to the boiler gets above say 85oC. The pipe thermostat situated close to the boiler does this.

The hot water cylinder temperature is controlled as in the previous system by a cylinder thermostat. The radiators are controlled by thermostatic radiator valves as before. The advantage of not utilizing gravity circulation is the cylinder will heat up more quickly and heat can be dissipated quickly when the pump is switched on by the high limit pipe thermostat set at about 85oC.

One disadvantage of this system is that the hot water cylinder will not heat slowly at nighttime if the fire is ‘banked’. The temperature will build up in the boiler, the pump will come on and hot water will flow to the hot water cylinder and radiators simultaneously.

In summer time it would not be a good idea to turn off radiators, since these are in effect part of the heat dissipation system. In some cases it may be advantageous to have several heat leak radiators for this purpose.

The system with heat leak radiators is shown below.


The photographs below show some control items used in electrical control.

Heat leak radiators are shown in the diagram below. In the diagram the Bathroom heat leak radiator will maintain gravity circulation past the pipe thermostat thus giving an accurate water temperature at all times.

The system shown below is more like the first system discussed where heat is dissipated by gravity circulation, in this case through the hot water cylinder when the 2-port valve is open and through the heat leak radiators.

The high limit pipe thermostat may not be required if the two heat leak radiators have a high enough output.

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