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Dirty glass is either caused by poor fuel choice or incorrect operation of the stove's combustion air control(s). It's rarely due to the stove. Never let the residue on the glass build up as this will make it harder to remove. Use a proprietary glass cleaner when the glass is cold and ensure that it is completely dry before you light your stove.
Harder to remove baked brown residue is unburned condensate from the wood which includes creosote and you can be sure that what you see on your glass is going to be much worse inside your flue system. It must be avoided at all costs as it can be a cause of dangerous chimney fires and blockages. Its appearance usually means that the logs you're burning are unseasoned (aka wet or green) or that you are 'slumber' burning to the extent that there's incomplete combustion of the wood which produces the creosote – or perhaps it's both. Either will have an impact on the effectiveness of the airwash system and its ability to remove the dirty deposits.
It's worth noting that some dry sooty deposits on the inside of the glass can occur at the end of the burn cycle with some stoves and is usually noticeable when you relight the stove the next day. This is perfectly normal and happens because the effectiveness of the airwash is reduced as the fire chamber naturally cools down which allows the soot to settle. This type of deposit is usually very easy to wipe away. A bit of dry kitchen roll or newspaper dipped in the ash bed can make an effective dry 'scrub' which won't damage your ceramic glass.
In order to stop the glass getting dirty again review your choice of fuels and ensure that any wood that you burn is fully seasoned with a moisture content of less than 20%. You can easily check this by investing in a low cost moisture meter. If you haven't got a moisture meter then there are some obvious tell-tale signs that can be used to confirm if the wood is seasoned. Firstly, it should feel comparatively light and make a clacking sound when knocked together (as opposed to a dull thud). In addition the bark should be peeling, or be easy to peel, with some evidence of cracks around the outer surface heading to the middle of the log. When you buy your wood you should look for the 'Ready to Burn' logo which is your guarantee that the logs will have a moisture content of less than 20% and will burn cleanly – they'll also help you do your bit for local air quality by reducing the emissions that your stove produces.
Try to avoid turning down your stove to the point where you can see wisps of smoke rising off the logs as this indicates inefficient dirty burning. A flue pipe temperature gauge is well worth investing in (if you have exposed flue pipe) as this will allow you to see when the flue gas temperature drops below 120ºC, the point at which creosote is produced and likely to settle on the glass. If your stove has separate primary and secondary air controls, when you burn wood the primary air should be shut off (except perhaps for the initial combustion stage) and the flame then regulated using the secondary air control which allows the airwash to function properly. Only reduce the secondary air when you're sure that the all of the logs are burning brightly and the surface of the logs has turned ash grey. This will maintain the heat in the fire chamber, deliver greater fuel efficiency, but most importantly, allow the airwash to do its job of keeping the glass clean.
Finally, avoid placing the ends of logs pointing towards the glass. Use a heat resistant gauntlet to place the logs exactly where you want them. The end of the log is where much of the gas and condensates escapes during combustion and in the early stages, when the fire chamber is not yet at its full operating temperature, this condensate which includes creosote, will condense and cool on the glass and then stick around to be baked on later during the burn cycle. Ensure that all traces of this staining is removed when you next light the stove. If it's brown looking then this will be creosote and you will need to use specialist stove glass cleaner and ensure the glass is dry before you re-light the stove.
Download the Hetas Ready to burn technical bulletin here
