Efficiency Explained

Efficiency in heating appliances is the proportion of useful heat given out in relation to the energy put in. A typical 1kg wood log has the potential to release about 5kW, so if two such logs burned for 1 hour on a stove of 75% efficiency they would give out 2 x 5kW =10kW x 75% = 7.5kW, and the 7.5kW figure is the one the makers will quote as the rated heat output. A higher efficiency means proportionately less heat lost and more heat into the building. Unfortunately, for solid fuel stoves and fires it isn't that simple... 

NET AND GROSS EFFICIENCIES
The 'net' formula now used in EN Standards for calculating efficiency does not account for the heat which is wasted in merely boiling off the water present in the fuel. With coal and wood this is very significant. The more accurate 'gross' efficiency is usually about 0.9x the net efficiency, so a European appliance claiming to be 80% (net) efficient is probably closer to 72% (gross) and this gets even lower when burning 'green' or unseasoned wood with a high moisture content.

 As long as everyone uses the same gross or net value, then efficiency percentages are a "figure of merit" and it is easy to compare products, but every single log and every lump of coal is different and will burn with very different efficiency, while the quoted efficiency will likely have been achieved using carefully prepared fuels and settings which may never be encountered in practice - simply placing logs in slightly different position can vary efficiency by 10 percent, similarly mislaying the coals on a gas fire can dramatically reduce the efficiency.

Quoted efficiencies ignore heat gained or lost outside the heating appliance itself. There might be more heat gained into the building from a warm internal chimney, and therefore higher overall efficiency, notably with inset appliances to EN 13229. But then again there might be heat losses, and therefore lower efficiency, from heat storage and transfer systems, notably with independent boilers. Appliances do vary in efficiency, but the quoted efficiency figures for most solid fuel appliances should be treated as a very, very rough guide only. Efficiency figures often depend on the ingenuity of the test technician rather than any intrinsic property of the appliance.

Table of minimum recommended efficiency in the in the UK Domestic Heating Compliance Guide and in the EN Standards

	UK Suggested Minimum		European 'EN' Minimum
APPLIANCE EFFICIENCY	Gross %	Net %	Net %
Open fire (inset)	37	41	30
Open fire (Freestanding Convector)	47	52	30
Open fire (inset convector, mineral fuel)	45	50	30
Open fire (inset convector, wood)	43	47	30
Open fire + Boiler	50	55	30
Open fire + High Output Boiler	63	69	30
Dry room heater stove	65	72	50
Pellet Stove	65	72	70
Room heater with boiler	67	74	50
Independent boiler, anthracite	70	77	74
Independent boiler, wood	65	72	67
Masonry heater	-	-	70

EFFICIENCY IS A GOOD THING. BUT...
 Gas and solid fuel conventional appliances only work because of the The Chimney Effect - by losing a certain amount of heat into the chimney, they make the gases there rise and so safely evacuate smoke and pull fresh air onto the fuel to make it burn. An appliance with a very high efficiency is losing less heat into the chimney and, unless its chimney is perfect, it may encounter problems in generating enough draught to make it burn effectively, or safely evacuate smoke. This is why appliances with the very highest efficiencies, some above 90% (net), almost invariably require the flue to be lined.

MEASURING EFFICIENCY and OUTPUT
It is not normally practical to directly measure the heat output of a gas or solid fuel appliance, instead, samples of the waste flue gas are taken - usually using a flue gas analyser the chemical composition of the gas indicates how efficiently the fuel has been burned and its temperature indicates how much of the heat generated has been captured and how much is lost into the chimney.

Given the temperature of and percentage of carbon dioxide (CO₂) in the exiting flue gas, the, moderately accurate, Siegert's formula gives the efficiency from:


100 - ( (MeanFlueTemp ºC - MeanRoomTemp ºC) x (A1 / CO₂%) )
(Where A1 is: Anthracite=0.683, Coke=0.290, Bituminous Coal=0.672, Lignite=1, Peat=0.7, Dry Wood=0.650)

The heat output in kW is then calculated from:

(Efficiency x Potential heat in kW x Fuel burned in kg) / Burn time in hours)