How can one 5kW stove be much smaller than another? How then do I get this right? What are the manufacturer's not telling us? Forget about kW's and check the size of the firebox! Read on...
To discover what size wood burning stove you need measure your room and divide by 14 (so length x width x height in metres, divided by 14). Divide by 10 if your room is particularly poorly insulated. Divide by 25 if your room is new-build super-insulated. I still recommend reading the rest of this article ;)
Stove heat output is measured in Kilowatts (kW) and the figures usually provided by stove manufacturers are “nominal heat output”, “maximum heat output” or both (nominal is often provided and is very dubious – see below).
Stove efficiency is different and is measured in % terms (how much heat emits from the stove after the rest goes up the chimney so as a rough guide a stove with 75% efficiency loses 25% up the chimney).
Sticking to basics the amount of heat a stove gives out is determined by how much wood is burned and for what period of time.
So the physical size of the firebox determines the maximum heat output of the stove, for a particular period of time, when comparing one stove against another stove.
The problem here is that in the UK the manufacturer gets to specify the nominal heat output they desire. The testers check that the nominal is “possible” with a sensible amount of fuel. It is obvious from this that the figures can be played with. Hence why “5kW stoves” can have wildly different firebox sizes.Why might a manufacturer want a stove to be advertised as 5kW when maybe he can add an extra log or two and call it 8kW? Because, due to building regs, stoves 5kW or less often do not have to have an air vent in the room (which is a selling point). Also, it is quite tricky deciding how much wood a stove can reasonably take: three small logs might be perfect whereas five small logs has a small risk of one falling out when you open the door. Taking this further one might be able to get seven small logs in with even more risk of making opening the door a little fraught. Who decides what is a reasonable loading here?
My advice: If you are in a wood stove retail outlet and looking at stoves (before popping back here to check our prices of course), then take the manufacturer’s stated kW as a guide only. COMPARE THE SIZE OF THE FIREBOX WITH OTHER STOVES AND THINK ABOUT HOW MANY LOGS A STOVE WILL REASONABLY HOLD.
Hence why I have added “Firebox size” to many stoves in the Stove Fitter’s Warehouse (one day they will all have this measurement). The figure above is for the DG Ivar 5, a 5kW stove with a huge firebox that can punch 7.75kW with a reasonable fuel load. The other figures are width, depth and useable height (uh, useable height, is considered by ourselves as 15cm above any bar to prevent fuel falling out).
Some manufacturers provide such figures as “3-6kW” or “5-7 kW”. In other words they provide a MAX output. I quite like this as it at least shows you what top performance you will get if you come over all “Casey Jones” one evening (you’ll not even begin to understand unless you once had a black and white TV).
If the external air temperature is zero degrees then then you will need about 1kW of heat for every 14 cubic metres of space in a house with average insulation and double glazing.
In order to calculate how many kW are required to heat your room, measure the width, length and height of your room and then multiply the three measurements together, i.e. width 4m x length 8m x height 2.8m = 89.6 cubic metres (room volume). If your room or house is new build and fitted with very good insulation then divide the room volume by 25. If the room has average to good insulation then divide the volume by 15. If the insulation is poor or non existent, then divide the rooms volume by 10. If in doubt use 15.
In our example above, the room is fitted with average insulation so we divide the rooms volume by 15 and the kW requirement is shown to be 5.97. Now forget about kW and look how big the firebox is as this makes a lot more sense :)
If you have two rooms with an arch in between then treat the situation as one room. If your room is open to upstairs then it really is “finger in the air” stuff. In the case of an open staircase to upstairs I would usually add 10-20% to your kW requirements figure due to heat that will “escape” upstairs. Now forget about kW and look how big the firebox is.
Something else worth bearing in mind when thinking about what size wood burning stove to get is whether or not you want the stove to heat the room without any assistance. If you have a 2kW radiator in the room and it will “always be on” then that 7kW stove can be reduced down to a 5kW stove. Personally I think it best that the stove can work alone to heat the room so that the central heating can be turned off to save energy costs. But some people will treat the stove as “an aesthetic addition” and in this case one can take other heat input into account.
What if your room suggests a 3kW or 4kW stove but there is not much choice on the market? Never worry about going a little larger than the figures suggest. "But will I not be too hot? I don't want Grandma stripping off to her underwear!". It is not quite that simple. If you burn just one twig in an 8kW stove then you will not get 8kW of heat. It is the amount of wood being consumed that dictates the heat. You can happily burn 2,3 or 4 kW of wood in a 5kW stove. Some will tell you that having too big a stove causes black glass and incomplete combustion: not really accurate. This only happens if you OVERFILL a stove with fuel, then get too hot, then turn it down to a slumber. Add a smaller fuel low and burn it well and all will be well. If you buy TOO big a stove the only problem you will have is that the floor area of the stove is large and the logs may have trouble "staying together". You could in that case knock up a small steel cage to keep the fuel together.
Now forget about kW and look how big the firebox is.
Personally I would go larger than I think I need rather than risk it when it comes to evaluating what size wood burning stove. You can always put 3kW of wood in an 8kW stove! But bigger than 5kW (nominal) and you always need an air vent (building regs). You need an air vent anyway (for all stoves) in a new build or any property that is fairly airtight (see “Do I need an air vet?“). BUT if you do not want an air vent and are not in an airtight property and a 5kW looks borderline – just buy the biggest 5kW you can find. As an example the DG Ivar 5kW is far larger than the 8kW Ekol Clarity. Darn silly testing procedure ;)
Remember: you CAN burn anywhere between 0-8kW worth of wood in an 8kW stove.
Now forget about kW and look how big the firebox is.
What size wood burning stove will your room require?
By Julian Patrick