Choosing the right output for your multi fuel stove or wood burner

Choosing the correct heat output will be fundamental to your future enjoyment of your wood burning or multi fuel stove and its potential running costs. Just because you have a big space or a big hearth to take a big stove, don’t be lured into thinking that you actually need a big stove. If you have to pay for your wood then you should ensure that your stove doesn’t cost any more to run than it needs to.

The reality is that, the ‘ideal’ room temperature you want to achieve is always going to be a matter of personal preference. Here are some considerations you should bear in mind…

As all wood burning and multi fuel stoves are designed to work best with the right fuel load it is better not to choose a stove which has a much bigger output than you really need. Choosing a big stove and reducing the fuel load to compensate means that the fire-chamber may not reach the correct operating temperature and thus the body – the bit that radiates the heat in a traditional radiant stove and helps convect the heat in a contemporary convection stove – may not be able to do its job properly. What you’ll get by doing this is a lacklustre fire and disproportionally poor heat output.

Reducing the temperature in the fire-chamber will also have a negative impact on the effectiveness of the flue’s up-draught making the flue gases rise slower consequently slowing them down further, all of which could create problems with creosote and condensates. You’ll more than likely also have problems keeping the glass clean as a low fire-chamber temperature reduces the effectiveness of the stove’s Airwash system which relies on the exceptional heat to burn off any dry particulates which may settle on the glass.

From an environmental viewpoint choosing a big stove and then reducing the fuel load and thus the fire-chamber temperature also means that you will simply end up producing more smoke (and therefore more soot) and more pollution. If you live in a UK Smoke Control Area, even with a DEFRA Smoke Exempt stove (also known as a Defra Approved stove) you could leave yourself open to prosecution through the creation of nuisance smoke.

Conversely, choosing a stove which is too small and then having to stoke it up like the Flying Scotsman to get warm on very cold days could potentially permanently damage your stove and flue system through over-heating of the bodywork and stove and flue components.

Apart from the size and volume of the room, to estimate the right heat output you need you will also have to consider whether your room is well or poorly insulated. Large window areas, other ambient heat (eg central heating in other parts of the house), single story buildings, strong prevailing winds etc, are all important considerations. An understanding of how well your house is (or isn’t) insulated will enable you to choose one of the three insulation categories (A, B or C) in the table below which you’ll need to choose to help you make the best calculation.

You will also need to consider how you are going to ‘live’ with your stove. In a typical square sized living room, with a typical sized stove, the stove will be placed in the fireplace and this doesn’t usually present a problem. If though, you have a long living room (typically say, two rooms knocked together) in theory the calculations to heat that volume of room will determine that you need a stove with twice the heat output of the individual stoves needed to heat the original rooms before they were knocked together. For example this could mean one 10kW stove as opposed to two 5kW stoves. But beware… in theory we know that the 10kW stove will heat that particular space – but how do you begin to live with it? See the diagram below to see what we mean. A 10kW stove is an extremely powerful stove and one that’s probably too hot to sit in front of for any length of time.

For typically sized living rooms, one view is, that it is better to have a slightly larger stove than shown by the calculation result, perhaps rounding the calculation up to no more than an additional 1kW. Remember, the standard industry calculation shown below is only a rule of thumb and is not exactly scientific. This little bit of extra heat output will afford you some flexibility when it is particularly cold without ever over-firing or slow burning. Some of Alfred Poppins customers want the little bit of extra output so that they can avoid running the central heating at particular times of the year, like late Spring and early Autumn, letting the excess heat permeate into the hall and other adjoining rooms. However, do please check out our diagram above on ‘Stove output versus practicality and comfort.

We must not forget that there’s also the aesthetics to consider too. For example, if you have a large imposing fireplace, then you could be forgiven for thinking that a smaller stove of the correct kW rating will not be visually appropriate. In this case you may like to consider a ‘slimline wide-bodied’ stove which where the firebox has been squashed and stretched to create a bigger impression without actually increasing the heat output (they take longer logs too).

So in the end choosing the correct output of stove for you will be a combination of theory (you have to start somewhere), practicality, personal preference and perhaps some compromise along the way. Experienced installers will also be able to offer their opinion and advice onsite which will have been arrived at over many years and through involvement in many, many installations.

How to work out the stove output you need

This is based on the standard industry calculation using a multi fuel stove or wood burner to heat a room to a comfortable temperature (22Cº) when it’s 0Cº outside and it generally works pretty well.

Firstly, multiply the room Length x Width x Height (in metres) and divide the total by one of the three figures listed below, depending on your interpretation of the insulation standard of the room where the stove will be operating. The three figures (10, 15 and 25) are based on the approximate volume of air that 1kW of heat produced by a stove in various insulated room situations would warm, from virtually no insulation (eg Victorian house) to state-of-the-art insulation (eg modern dwelling). This will provide you with a good estimate of the heat output that you’d need from your stove and taking into account the considerations outlined in the section above this should help you arrive at the ideal output for your particular requirements.