Saturday 1 September 2012

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. Some websites feature 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 and some basic information about your room. 

How is the Heat Output Measured?

I line with the BS and 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 the nominal heat ouput.
  3. The manufactuer can also specify the size of the fuel (within reason) - but apparently this rarely happens
  4. The test then confirms that the stove can run at the given output.
  5. The efficiency is then calculated as the average efficiency over the period.
This means that the method is not necessarily impartial. If the manufacturers are keen to promote their stove as efficient they can simply change the refuelling period and the heat output to test at to get the results they want. If they would like their stove to appear to have a particular heat output then they can just specify it. This does not mean that all manufacturers are out there testing at wildly incorrect heat outputs, but there are commercial pressures at play which do mean that a shift of a few kW here or there may well be a favourable option.

So this means that:
  1. We can now no longer necessarily compare the heat output of different stoves as the stated nominal output is not necessarily the 'true' nominal output.
  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 a maximum heat output figure.
  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.

Added Things To Consider

UK Building Regulations recommend a permanently open supply of ventilation air for stoves of 550 sq mm/kW, for every kW above 5. But the heat output that this is based on is the nominal heat output. A stove will need the highest continuous supply of air for combustion when running at maximum output. The nominal heat output achieved in the EN test does not relate to the maximum heat output (becuase it is specified by manufacturer). In extreme cases, when burning coal, insufficient ventilation can be very dangerous indeed.

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