Fire Efficiency
Your fireplace should look great – but it also has a real job to do.
Use your fire to work with your primary heating. Look for the highest efficiency possible. This will reduce both your heating bills and carbon emissions. It will also enhance the value of your house when you come to sell.
Take the chill off autumn or spring evenings without heating the entire house. Choose a glass fronted fire or a stove, with efficiencies around 80%. The additional advantage of these fires is that chimney draughts are minimised when not in use.
You need to understand the difference between radiant and convected heat. Radiant heat is infra-red radiation which goes into the room, and contributes to the ‘feel good’ glow from an open fire.
Convected heat is warm air circulating round a room. The warm air above a radiator is a good example of convected heat. In general there is no convected heat from an open fire as combustion fumes and hot air go up the chimney and are lost.
If you climb onto your roof in winter (don't try this without scaffold access!) try this: Put your hand over a chimney pot leading from a warm room below, without any fire. It is like a hot air dryer warming the outside world.
If you use your fire irregularly, and simply want a great flame picture, don’t worry about efficiency.
The following notes are not exhaustive, but they provide a guide to the main issues. You should always get qualified technical advice prior to any purchase.
Efficiency: What type of fire are you installing? |
 |  | Stoves - Wood burning, Multi-fuel, Gas
70-85% efficient.
Modern designs with secondary combustion are up to 85% efficient. Much depends on the quality of the fuel, but the principle that only enough air to burn the fuel is allowed into the stove
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 |  | Any Fuel - Open Chamber, with open grate or basket
0-10% Efficient
Very traditional and very inefficient. The large open flue area draws huge quantities of air from a room, and a roaring fire increases the rate of heat loss. An open fire generates no convected heat, as we take care to ensure that the products of combustion, which contain carbon monoxide and other nasties, are taken away.
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 |  | Any Fuel - Clay or cast iron fire back
10-25% Efficient
Better than an open chamber, as the throat is restricted, reducing the air flow up the chimney. This is the most common fireplace installation for solid fuel from the 1920’s onwards
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 |  | Gas - Ceramic Hotbox for gas fire
40-45% Efficient
The lightweight ceramic lining to this appliance improves the radiant heat output, and the shaped aperture at the top reduces the air flow further. A specially designed convector fire will give another 5-10% efficiency
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|  | Gas - Glass fronted fires and Gas Stoves
75-85% Efficient
By using glass to cover the front of the fire, and with clever design, the airflow to the fire is minimised. A lot of the heat gain comes off as convected heat. Waste gases are limited to that required to burn the gas, but no more. Balanced flue fires fit into this category.
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|  | Flueless Gas Fires
70-80% effective efficiency
Manufacturers claim 99% efficiency as no wasted heat goes into the atmosphere. However, the draught and heat loss from the 100cm² wall vent is significant. A CO monitor is essential as flue gases are discharged into the room.
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|  | Electric Fires
33% efficient
100% efficient inside your house, but 2/3 of the energy is lost in generation and transmission, so Building Regulations assume an efficiency of 33%. This explains why electricity prices are usually about 3 times the price of gas per kW hour.
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