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Stove testing: silly examples of nominal heat output and efficiency results

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The way in which a stove's heat output and efficiency is tested does not necessarily allow comparison of different stoves because the test conditions are not comparable. You can read more about this on the two pages listed below - this page will show a few examples of how the new tests are confusing and largely useless.

For more information about the stove heat output test see our stove heat output page.

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

Examples of stove test result anomalies

For example 1 and 2 we have chosen some Morso stove test results. This is just as an example as there are several other manufacturer's models which could also illustrate this point.

Example 1: The Morso Squirrel

The Morso Squirrel stove comes in 2 models: the older 'Standard Squirrel' and the more modern 'Cleanheat Squirrel'. As it's name suggests the Cleanheat is a cleanburning stove that uses preheated secondary air to allow for fuller combustion. Let's look at the efficiency and heat outputs of the two models:

 EfficiencyNominal heat outputPrevious heat output given
Standard Squirrel76%4.5 kW4.0 kW
Cleanheat Squirrel71%4.6 kW6 kW

So if I relied on the test figures I would think that the Standard Squirrel is more efficient, gives out nearly the same heat, and is cheaper so why would I buy the Cleanheat version?
However from experience I can say that the Cleanheat will give out more heat than the Standard (it used to have a maximum heat output rating of 6kW!) has about the same size of firebox and should use less wood to create the same heat. Someone buying the Cleanheat who actually wanted 4.6kW output stove will also end up with a stove that gives out more heat than they wanted (although this is not such a problem as Morso stoves can run well at lower outputs and are very controllable).

Example 2: The Morso Panther and Morso 3610 stove

The Morso Panther is a nice medium sized stove whilst the 3610 stove is much bigger - it is about 200mm higher, 150mm wider, 100mm deeper. Let's look at the efficiency and heat outputs of these two models:

 EfficiencyNominal heat outputPrevious heat output given
Morso Panther72%9 kW8.0 kW
Morso 361073%9.8 kW16 kW

Going by these figures there is not much to choose from between these two models other than looks. However a collegue has a Morso Panther in a medium sized room (about 5m x 5m x 2.3m - an old cottage with uninsulated walls) and this is the perfect size of stove for the room. Now I know if we installed the 3610 in the same room and tried to use it properly it would be like a sauna in there! The 3610 used to be rated at 16kW - much higher than the output of the Panther (of course the 3610 could happily run slowly so that would not necessarily be a problem but I'm sure you can see the point).

Our conclusion

Because there is not a level playing field when it comes to test conditions a stove customer cannot usefully compare the heat ouputs of stoves nor their efficiencies. You can't really be expected to factor in things like how open the air vents were during the test or how often the stove was refuelled and with what size of fuel - the point of the test results is to give an idea of how a stove will perform and to enable comparison between stove types. A test conducted at maximum output would give results that could at least be compared.
Our advice? For the moment ask us or speak to other experienced stove experts because the figures only show that a particular heat output is achievable and the efficiency at this output. That particular heat output is chosen by the manufacturer and hence this makes the test open to the influence of the many commercial pressures which manufacturers come under.

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