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What size wood burning stove?

what size wood burning stove

What size wood burning stove for your room? 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.

Mont 5 logsSo 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.

OPT_Find-me-in-the-WAREHOUSEWhy 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 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 (before popping back here to buy of course 😉 and looking at stoves, 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).

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).

In order to calculate what size wood burning stove for a 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.

what size log burner

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 figure. 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.

Now forget about kW and look how big the firebox is 🙂

Personally I would go a larger than I think I need rather than risk it when it comes to evaluating what size wood burning stove. But bigger than 5kW and you need an air vent. So if you do not want an air vent 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. Go figure.

You CAN burn 3-8kW 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


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