• Should I choose a multi-fuel or wood stove?

In short, if you want to burn solid fuel (coal) you definitely need a multi-fuel stove. If you want to burn wood you can choose either, but dedicated wood burning stoves will usually give you more space to burn larger cuts of wood.

Wood burns best from the top downwards on a thick bed of ash. Therefore, wood burning stoves will have no grate or ash pan. The wood is burned flat on the base of the stove and the ash is allowed to build up. When too much ash has accumulated you scoop some out, but it’s best to always leave a good bed of ash. If you are burning dry wood in an efficient stove, and burning it well, then very little ash should be produced. The only time you may need to empty out all of the ash is if your chimney sweep asks you to.

To burn solid fuel, a multi-fuel stove with a grate and ash pan is needed. Multi-fuel means the stove is capable of burning both solid fuel and wood. Solid fuel burns more fiercely than wood and produces more ash. To avoid the ash clogging the fire and restricting the supply of oxygen, a grate is needed for the ash to drop through and into an ash pan. It then needs to be cleaned out regularly. Some multi-fuel stoves, have a grate with moveable bars, known as a riddling grate, to help you push the ash through. Charnwood multi-fuel stoves have a riddling grate which can be lowered to provide a flat bed, so in effect they can be either dedicated wood or solid fuel burners.

Wood will burn perfectly well in a multi-fuel stove, you would just ignore the grate and let the ash build up over it. However the grate can take up space that could be used by bigger logs. So if you only need a small stove and you’d prefer to burn wood, it can be best to choose a wood stove rather than a multi-fuel. Most Charnwood stoves, for example, let you choose a design with or without a grate. If you ever change your mind and want to burn solid fuel, Charnwood allows you to buy a conversion kit to turn your wood stove into a multi-fuel.

  • Which size stove should I choose?

There is a calculation you can use based on the dimensions of the room, which will give you a good idea of the heat in kWs needed for the space. See below. This won’t take in to account several other important factors though. How well insulated is the room? Is there a staircase? Do you always leave doors open to other rooms? All these are taken into consideration by our surveyors when they come to quote for installation. They can then give a recommendation based on many years of experience!

Measure your room in metres. Multiply the length, width and height. Divide this by 14. You will then have the number of kWs needed.

(Length x Width x Height in metres) ÷ 14 = kWs

It is important not to choose a stove with an output far above what the room requires. If you try to burn a large stove under capacity, the gases aren’t burnt off quickly enough and the chimney tars up. The air wash system, to keep the glass clear, also won’t work well. If you choose a stove which is too small and regularly fire it over capacity you run the risk of damaging the stove.

  • Do I need a lined chimney?

Stoves should ideally be fitted to a 6″ lined chimney. (Occasionally a large stove will have a 7″ flue spigot and require a 7” chimney). This is not a legal requirement but is highly recommended.

The efficiency levels which make wood stoves so much more practical, kinder to the environment and cheaper to run are determined during testing on 6″ wide chimneys. You will not be getting the best out of your stove without one.

When a stove is fitted to a larger chimney there may be problems controlling the fire. On still days the chimney might not draw the smoke up properly, making it harder to maintain a good fire and producing more tar. On particularly windy days the chimney may draw too much, so that it’s difficult to control how fast the fire burns.

The warmer the chimney, the better the draw. The flexible stainless steel liners we use are insulated so that heat is retained. This is particularly important when installing highly efficient stoves that allow less heat to be lost up the chimney.

In older chimneys there may be concerns over the condition of the brickwork. It would be a considerable safety concern if fumes were to leak out into upstairs rooms. Stoves produce far more Carbon Monoxide than open fires.

It may be that your chimney is already suitably lined. If your chimney was built after 1966 it should have been built with some form of liner to meet building regulations. For the purposes of a stove this should ideally be 6″ wide. If the chimney is quite a bit wider it may be advisable to re-line.

There are many different methods and materials and all have their uses, advantages and  disadvantages.

Ceramic liners are the most common method for newly built houses. Terracotta low fired square and round liners with socket ends are used. These should be installed (as all liners should) with the male downwards, and back filled with an insulating lightweight concrete. The disadvantage of these is that they have a very low insulation value and so tar up easily. A chimney fire can cause them to crack and shatter, when the only solution is to remove them and reline, quite a major job and one that COSI is having to do more and more of. Top quality ceramic liners are available for the relining market that can withstand thermal shock and high temperatures but, of course, these cost considerably more. In short the terracotta lining is the bargain basement of the chimney linings and is only suitable for low cost new build.

We usually install flexible twin wall stainless steel liners. They have a very limited insulation value if not back filled with a dry insulation, (vermiculite, perlite or leca), and can tar up reducing its useful life. COSI install many flexible liners each year but we always prefer back filling/insulating to ensure a long life. One of the advantages is cost as, usually, no holes need to be opened in the chimney breast. The liners can also sometimes withstand a chimney fire, and are easily and quickly removed if they need to be replaced after a fire. They are most commonly used for multi-fuel and wood stoves.

When installing boiler stoves, the higher, more expensive 904 grade stainless should be specified.

If you are replacing a gas fire it may be that you already have a steel liner. However, gas liners are designed to carry away fumes, not to deal with high temperatures, and so aren’t suitable for wood and multifuel stoves.

  • Do I need an air vent in the room?
Contura wood burning stoves

5kW Contura wood stove. Available with double doors and as an inset stove. Black or grey.

Multi-fuel and wood burning stoves pull in air from the room to allow them to work. If the stove is over 5kW you do need an air vent in the room. In modern, airtight, houses an air vent should be installed whatever the stove’s output.

Some stoves can have an external air vent fitted. The air is then drawn in from outside directly into the stove, bypassing the need to have an air vent in the room which could create draughts, and keeping a modern house airtight. Burley, Charnwood and Contura stoves all come with this option.


  • I don’t have a chimney, can I still install a wood stove?

Yes, a chimney can be created for you.

five kilowatt wood stove

5kW Contura wood stove. Grey or black stove body. Cast iron, steel or soapstone top. Also available with soapstone sides.

A solid insulated twin wall stainless steel flue can be erected inside or outside your building and can be used for any appliance or fuel. With an inch of mineral insulation, tar will not be a problem if dry wood is burned.

The flue can either be stainless steel or have a black coating. Most often, we install stainless steel twin wall flues. Customers often expect these to be intrusive, but as they reflect the colours around them, they tend to sit better in their surroundings than the black coated option.

It is particularly important to have a site survey if you’re considering a twin wall flue. Internally, you need to make sure that there is enough space to install a stove, hearth and twin wall, and maintain a suitable distance from combustible materials. Externally, the twin wall flue needs to be a certain height, and a suitable distance away from taller buildings. This needs to be considered particularly when installing a twin wall system in a single storey extension to a two storey building.

Contemporary wood stoves, or stoves with store stands to give them more height, can look fantastic on their own against a wall. However, if you prefer a more traditional look and have the space to install a fireplace, COSI has many years experience of creating beautiful fireplaces from nothing!

  • My fireplace is too small for the size of stove I need, what can be done?

More and more of our installations involve building work to open up a fireplace to fit the right size of stove. This can transform the room, and provide a welcoming, cosy, focal point to a sittingroom. Our surveyor has many years experience in working with customers to achieve the right ‘look’ for them. There are examples of our work in the gallery, and on our facebook page, which is regularly updated with ‘before’ and ‘after’ photos.

If there is no scope to enlarge the fireplace we can fit an inset stove which takes up less room.

  • How big does the hearth need to be?

All multi-fuel and wood burning stoves must sit on a raised, non-combustible hearth of at least 12mm thick.

There needs to be a minimum of 225mm, or 9″, of hearth in front of the stove door. This increases to 300mm, or 12″, if the stove can properly be used with its door open. Since the vast majority of stove manufacturers state that their stoves should not be used with the door open, 9″ can be taken to be the norm. However, we tend to find that extending the hearth to 10 or 12″ gives a more substantial look.

There also needs to be at least 150mm, or 6″, from the side of the stove to the edge of the hearth where the hearth abuts a combustible material. So when a stove sits inside a fireplace the distance to the non-combustible brick chamber can be less than 6″. If the stove is free-standing, there needs to be at least 6″ of hearth surrounding the sides and rear of the stove.

The wall behind a stove needs to be suitably heat resistant, otherwise more than 6″ is required.

These requirements are all explained in more detail, and with diagrams, in Building Regulations Part J. However, it’s important to read the manufacturer’s installation instructions before deciding on a size of hearth. Their requirements will sometimes be more than those stipulated by building regulations, and they must be met.

  • Why should I use a HETAS registered installer?

Stove installations must be registered with the local authority, whoever they are installed by. HETAS are the official body recognised by the Government to approve and register competent installers of wood and solid fuel appliances. Just as ‘Gas Safe’ (and previously CORGI) are for gas installers.

It really is vital to be able to prove that your stove installation is safe. If you should ever have a chimney fire, your insurance company will need to see that the stove had been installed correctly and certified as a safe installation. If you sell the property in the future it’s also likely that you’ll be asked for a HETAS certificate.

Using a HETAS registered installer gives you the peace of mind of knowing that your stove has been installed safely by an experienced fitter. Our senior fitter has worked for us for 20 years and I’m sure he long ago lost count of the number of stoves he’s installed! We supply you with a HETAS certificate, send a copy to HETAS themselves, and they then inform the local authority. If you use someone who is not registered to install the stove, you need to inform the local authority yourselves. They will then charge to come out and certify the installation. Of course, at that point, the liner has already been installed and hidden away inside your chimney, and it isn’t possible for them to know that it has been installed correctly. We’ve been asked to rectify enough poorly fitted liners to know that builders and DIYers often get it wrong!

  • Should I choose a stove with a back boiler?

With the rising costs of oil and gas, we’re finding customers increasingly ask us if it’s worth choosing a boiler stove to help run the central heating and / or hot water. This can be a great way to reduce bills, but running a boiler stove can be trickier!

The addition of a water jacket to a stove cools the combustion process, which if you’re not careful, can tar up the stove and the chimney. This means it’s all the more important to ensure that the wood you’re burning is very well seasoned, and that you’re burning it well. A stove pipe thermometer can be a great help as it tells you whether the temperature you’re burning your stove at is within the right range.

You will also go through a lot more wood to adequately heat the room and run radiators in other parts of the house, or the hot water. Whatever heat goes to the boiler is taken away from the room. You need to plan for this. The cheapest way to buy wood is to buy it in large loads, green or semi-seasoned, and dry it out yourself. This requires quite a bit of space and takes 1-2 years.

Charnwood now make a ‘flue saver boiler’, which takes the waste heat from the chimney to heat water so it does not affect the combustion. A further 2-3 Kw of heat can be extracted this way. See www.charnwood.com  When connected to a thermal store type cylinder these are a superb method of reducing you carbon foot print by about 40%.

If you would like to speak to our heating engineer for more detailed guidance or a site survey, please let us know. He isn’t often in the showroom, so it’s best to call first.

  • Do I need a Carbon Monoxide detector?

Yes. For COSI to be able to provide you with a HETAS certificate to confirm that the stove has been safely and correctly fitted, there must be a working carbon monoxide detector installed. This needs to be built to British Standards EN 50291:2001 (battery powered). To work properly, the detector needs to be fitted high up on the wall or ceiling between 1-3m horizontally from the stove. We can provide the detector for you.


See our FAQs for more information on how to operate your stove.