Renfrewshire Council

Solid fuel heating systems

In solid fuel heating systems the fire heats up the water in the boiler, circulates it to a hot water storage cylinder, then a pump sends it to the radiators.

This type of heating burns a wide variety of solid fuels that need to be purchased by the user. Solid fuels vary enormously in price and quality, and this affects the performance of the heating system.

Wood is cheap but has a very low output - so it can't maintain high enough water temperatures to keep a comfortable heating environment. It also has a high tar and residue content, which transfers into the chimney, increasing the likelihood of blockages and chimney fires.

Coal is more expensive and has a higher output than wood, but can also vary in its ash and volatile gas content - which in turn will reduce its heat output.

Smokeless fuels - manufactured or natural - have a higher output than coal and release this heat over a longer period of time so are ideal for closed appliances.

With solid fuel systems, never think that cheap is best. The better quality the fuel the more expensive it is to buy, but the better the results.

Cheap fuels have a high ash and soot content and a low heat output, so generate lower water temperatures in the boiler and consequently lower water temperatures at the taps and radiators.

The lower quality the fuel, the more the chimney will need swept. The higher the ash content, the more often it will need the ashes emptied and the less air will be transferred through the firebed.

Types of solid fuel system

There are two types of solid fuel heating systems:

Open fires
Open fires have a fire basket bolted onto a high output boiler. Fuel is loaded onto the fire basket and the smoke/gases produced are vented  into the chimney.

Designed to burn a variety of fuels - wood, coal and some smokeless fuels - they are limited in terms of their efficiency and ability to run radiators.

They suffer from a slight lack of user control because although the air intake underneath the fire can be controlled to a degree, the air drawn over the top of the fire can't be controlled, which means they have to be re-fuelled regularly and can be difficult to keep lit for long periods i.e. overnight.

Because of the nature of the fuels burned and the relatively low flue gas temperatures produced, open fires produce a lot of soot so it is important that the chimney is swept regularly - at least three times a year and possibly more.

Closed fires (Room heaters)
Closed fires are more efficient and more controllable than open fires. With these the fire burns behind a door - or two doors in the case of multi-fuel appliances.

Because the fire is burned behind a door, the burning rate of the fire can be more easily controlled as all air introduced into the firebox is via the thermostat flap.

As well as the user being able to turn the thermostat manually to adjust its setting, it also has a degree of automatic control.

It's very important for safety to make sure the appliance is cleaned regularly as soot and ash will accumulate and could block the flue ways.


Solid fuel systems are a bit more basic than gas systems but the same controls can be introduced. Thermostatic radiator valves (TRVs) can be fitted to radiators so that once the room temperature coincides with that of the TRV setting then the valve will shut the radiator down. If the temperature drops below the TRV setting then the valve will open up and the radiator will begin to heat again.

Programmers can be introduced into solid fuel systems but can often be more bother than they are worth.

One control often fitted on a solid fuel system is a device known as a high limit thermostat. This is a safety device designed to stop the hot water from overheating.

Hot water

With a solid fuel system the water heats constantly when the fire is lit.

Once the hot water is up to temperature then the central heating pump can be turned on - but always make sure the fire is turned up to a high setting so that it can heat both water and radiators.

Get the best from your system

Always buy the better fuels
It's a false economy to buy cheap. The quality of the heat and hot water produced is governed by the fuel in use. The appliance can't be adjusted in any way to alter the amount of heat the fuel produces.

Fill the firebox to its maximum capacity
To heat the water in the back boiler the fuel needs to be in contact with the boiler surfaces, so filling the firebox to less than its full capacity is also a false economy and will lead to disappointing outputs.

De-ash the appliance every morning and night (at least)
Allowing ash to build up can affect the parts and in turn this will reduce the heat transfer through to the boiler.

Sweep the chimney regularly
Open fires and multi-fuel appliances burning coal or wood should be swept a minimum of three times a year.

Check your settings
In winter the appliance will need to be burned at its highest setting to provide sufficient hot water and good radiator temperatures. This will lead to you burning more fuel to keep your house warm.

During summer where there's less demand for heating, the appliance can be operated on a lower thermostat setting.

Avoid a low setting or partly-filled firebox
Never run the central pump with the fire in a low setting or with the firebox partly filled - this leads to lukewarm water circulating through the system with poor water and radiator temperatures.

It also causes condensation in the boiler which in turn causes corrosion.