Stove heat outputs - nominal and maximum

Stove Help & Advice Home

On our website we show, where possible the maximum heat output of the stoves (displayed as 'Heat Output'), with the nominal heat output shown separately (displayed as 'Nominal heat output'). The maximum heat output allows you to choose the correct size of stove for your room as well as letting you compare different makes of stove with each other. You can pretty much ignore the nominal heat output we only list it because it is the "Standard" measure of heat output in the UK and EU.

What is the heat output of a stove?

The heat output of wood burning and multifuel stoves is measured in kW. A kW is around 3400 BTU's. We have a kilowatt calculator which you can use to give you a guide to what the heat output of your stove should be - you simply enter in the dimensions of your room. You can then search for stoves by heat output.

How were heat outputs once measured

For many years the heat output of stoves was measured by filling the stove up, opening the air vents up and then measuring the maximum heat output. This made perfect sense:

1. It allows you to size the stove correctly as a stove should be run at near maximum output only on the coldest possible day.
2. The maximum heat output allows the sensible comparison of different stoves.

How heat outputs of stoves are measured now

In line with the European EN standards the heat output (and efficiency) of stoves are now measured in the following way:

1. The manufacturer gets to specify a refuelling period, with a minimum period of 45 mins and no upper limit.
2. The manufacturer gets to specify how open the air vents of the stove should be for the test.
3. The manufactuer can also specify the size of the fuel (within reason) - but apparently this rarely happens (I spoke to a technician at Gastec at CRE Ltd where they test a lot of stoves in the UK)
4. The nominal heat output is then calculated as the average heat output over the period.
5. The efficiency is then calculated as the average efficiency over the period.

This means that if the manufacturers are keen to promote their stove as efficient they can simply change the refuelling period and how open the vents are accordingly to get the results they want. If they would like their stove to appear to have a particular heat output then they can jiggle the fuelling period and how open the air vents are to get the figures they want.

So this stroke of genius means that:

1. We can now no longer compare the heat output of different stoves as they are all tested with different refuelling periods and with the air vents open to varying degrees.
2. The end user is left with no way of knowing how much heat the stove is going to give out when they actually use it.
3. Because the nominal heat output that results from the test has no relation to the maximum heat output of the stove there is no way to size a stove to suit a particular room without reference to the older, maximum heat output, figures.
4. Similarly the efficiency ratings of various stoves cannot be compared with each other.
5. The end user is left with no way of knowing how efficient the stove is going to be when they actually use it.

For more information about the stove efficiency test see our stove efficiency page.

The best way of shopwing how ridiculous this is is with a few examples. Click here for examples of silly stove test results.