What causes build-up and blackened glass on a wood-burning or multifuel stove?

Excessive build-up of creosote in your woodburner or multifuel stove is invariably a result of incomplete combustion.   These days most high-efficiency stoves are designed with intelligent airflow systems which aim to reduce the natural possibility of incomplete combustion but there are still many factors within the user's control which can (and do) result in unnecessarily poor levels of combustion, resulting in excessive build-up regardless of how well the stove is designed.  These are mainly incorrect use of air controls, and incorrect use of fuels.   

What can I do to minimise build-up and prevent the glass going black so quickly?

Luckily, there are some very simple things you can do to put yourself in a better (and less maintenance-intensive) position with your stove. 

1. Burn only well-seasoned or kiln dried wood, with less than 20% moisture content.

This is an absolute must for many reasons, but namely for better efficiency, better safety, longer lifespan of the stove, and with lower levels of maintenance required.   Excessive moisture in the fuel being burnt results in lower heat output due to the water content consuming so much of the heat energy in order to be burnt off / evaporated so that combustion can continue.  This process causes excessive smoke as there's not enough energy left to achieve complete combustion, while also releasing all that water vapour into the fire chamber and up the flue / chimney - and of course, this is going to cause a higher rate of build up, not just in the stove and on the door glass, but also in the flue / chimney system - which in turn quickly becomes a fire safety issue - creosote build-up is one of the main causes of chimney fires, and of course a narrowed flue passage caused by this build-up also increases the chance of failed evacuation of carbon monoxide, which we all know can very easily be deadly.  

Unfortunately there is a lot of misunderstanding about how wood should be seasoned, and through this misunderstanding many users believe they are burning seasoned wood when in fact it if it has been dry-stored in an enclosed space with no airflow and no exposure to wind, the wood is likely to still even be green after a long seasoning period.  To dry out properly, the process requires exposure to fresh airflow whilst being kept dry. 

If you purchase your fire wood from a licensed reputable supplier, you should find that the wood is certified 'Ready To Burn' and will be tested to ensure it is below 20% threshold - this is now a requirement in the UK for firewood merchants.  But, in the name of being vigilant and protecting yourself from unwanted wet wood, you can purchase wood moisture meters at very affordable prices which allow you to take readings of the wood moisture content before using it in your stove.  Ideally, we like to see 15% or less for our wood burners. 

2. Ensure your fuel is stored correctly

Whether you burn wood or other solid fuels, they must be kept dry.  Damp fuels of any kind are going to cause the same issues described above; and invariably will cause you unnecessary, avoidable problems with build up;  and in addition, can cause a nuisance to the user as wet fuels are seldom easy to burn.  Always ensure your fuel is dry stored, even if this means you have to purchase smaller quantities, you will achieve better efficiency from your fuel and have less problems with build up and other maintenance issues. 

3. Avoid burning waste, treated/painted/varnished wood, and any fuels not approved by the stove manufacturer.

For us chimney sweeps it's not uncommon to find the mess left behind by burnt plastics and other waste in fireplaces and stoves; and it goes without saying really, that these really should not be burnt in your stove.  It may seem harmless, but realistically creates build-up in all sorts of unwanted ways, and unfortunately we see a lot of damage caused to stoves used in this way.   From melted plastics jamming what should be operable mechanisms, to nails out of pallets and fence panels fused into the griddle grate preventing correct function; it might be convenient to throw waste on the fire, and to burn all sorts of free wood, but inevitably burning waste is going to cause you more problems. 

Play it safe, and only burn what the stove was designed for!  The stove will perform better and last longer if it is used as-intended with the correct fuels. 

4. Use your stove's air controls in accordance with the manufacturer's guidelines

In general, nobody likes to feel they are wasting fuel, and therefore wasting money; and as such many keen fire-stokers tend to have a habit of shutting the air controls down too much, allowing the fire to stay lit, but ultimately under minimal airflow to reduce the burn rate; so the fire smoulders.   This unfortunately also results in incomplete combustion which means not only are you going to get more build-up and blackened glass, but you'll also be wasting much of the calorific value (the energy) that the fuel has to offer, which works against the perceived economical benefit of smouldering the fire.   

If you look at the way air intakes are designed on most DEFRA approved stoves for example, they are mostly restricted to prevent the user from being able to shut the vents down completely because this mitigates the problem described above, and thus creates a cleaner smoke; with much less of the unwanted particulates caused by incomplete combustion.   Of course, this design element of DEFRA approved stoves is more to do with the particulates expelled out of the chimney; but effectively this is the same principle.  By allowing a reasonable amount of air-intake at all times, the issue incomplete combustion can be massively reduced. 

So on this basis it is important to observe the recommendations of the manufacturer in the user instructions for your stove.  

5. Get your chimney swept regularly

Hopefully this is something you keep on top of anyway, but this can easily be a cause of build-up and an ongoing struggle to keep your stove glass clean.   If your fire is struggling to draw enough air to achieve complete combustion, this will cause a smouldering fire similar to closing the air vents too readily as mentioned above.   This can very easily happen when the chimney has not been regularly swept, as the build-up of soot reduces the cross-sectional diameter of the chimney, restricting the up-draught, resulting in limited draw of air into the fire.   If this is happening there is also a very real risk that toxic gases such as Carbon Monoxide are going to fail to evacuate and could end up spilling into your home instead.   Because of this, we would advise against using the stove at all if it is struggling to draw, until the root cause has been identified and resolved - we offer an advanced chimney sweeping service for situations such as this. 

6. Check door rope seals are forming a seal

Over time the rope-seals around your door degrade and need replacing; this can be purely due to age and compression, or they may have become loose or frayed over time - if you can see visual damage, or can see that the rope seals are not firmly clamped between the door and the stove body when fully closed (you can test this with a sheet of paper - if the door is shut on the piece of paper the rope seal should create resistance preventing you from simply pulling the paper out - there should be resistance), then they are not likely to be forming a seal and will be compromising the burn efficiency and throwing off any correct use of the air controls mentioned in point #4. 

7. Avoid over-fuelling / over-building the fire bed

Ticking all the above boxes but still having issues?   The height of the firebed matters!   

This is something we have found almost nobody mentions when advising on keeping stove glass clean, but with a lot of in-house testing here at Calon yr Adref, we have found that allowing the firebed to build up higher than necessary allows build-up to form on the lower parts of the glass a lot more - it is logical that as the fire bed height increases, the airwash system is unable to then effectively clear the lower portions of the glass; allowing build-up to occur much quicker. 

If your stove needs some TLC and you're based in Pembrokeshire please visit our stove servicing & maintenance page where you can book in with us for a whole range of stove servicing.  We also offer professional certified chimney sweeping across Pembrokeshire.