by ericmark »
Fri Aug 08, 2014 11:33 pm
I had a friend who in spite of having three wood burners himself thought they were destroying the woodland and wanted me to write a web page saying how bad they were.
Before writing the page I wanted to find out if what he said was true so started doing research.
He did have some valid points but in the main only for poorly installed units the main point is you need to stop tar build up and particular emissions and to do that the fire needs to run at a critical temperature too high and the energy is waisted and too low and you get particular emissions and tar build up.
So the stove has to run at a set heat and extra air has to be added for after burn which is clearly no good on its own so it needs a heat store so it can run at this critical heat as a batch burn with any excess energy stored for latter use normally in a large water tank.
When this system is installed then the flue temperature is constant at 150 degrees C and as a result you know exactly what insulation is required to ensure there is no condensation and resultant corrosion.
So with a correctly installed system everything is finally tuned and to alter any insulation may upset the balance and either pump out energy to atmosphere or cause tar build up.
Nearly all wood produces the same amount of heat by weight but the wood stove has a volume limit not weight so that volume has to be adjustable to suit the wood being burnt.
The are some simple ways to control the flue temperature including using sterling engines driving fans but most DIY set ups are very inefficient and wasteful and one has to accept this and clearly with a very hit and miss system no one can really answer what insulation is required.
To be fair not all DIY systems are bad. The "Rocket" system is a condensating system and very very efficient. The flue is slightly below horizontal to allow the condensate to run out and the stone work retains the heat for days. In fact the only commercial condensing system is still under test and relies on electric power which to me is rather dangerous as with a power failure one would need to rake out the fire.
What my research did point out was there is a huge vibration in wood burners with efficiency between 20 and 99% and without knowing exactly what control method is used no one can really answer your question.
The research caused us to fall out as I would not write a web page saying how bad wood burners were without him at least installing flue thermometers to his fires. I also pointed out each fire is matched to a density of wood so one may need hard wood another soft wood and some a mixture but he wanted to claim because it worked with his stove that was the way to do it and would not consider each stove will be different and unless you can alter the fire size then there is no such thing as a multi-fuel stove. With some you can remove or fit fire bricks to adjust for different fuels and this works but unless you can then the fire will only burn efficient with one fuel others may burn but not efficient.
In the UK it is near impossible to convert from gas to wood burners within the law. I think form memory you are allowed a drop in efficiency not more than 5% and even old gas boilers and fires are in the main too efficient to allow converting to wood burners within the law.
The efficiency of electric is far better than gas so also converting from electric to gas is questionable but one can argue the power stations are not that efficient so it would required a test court case.
Since you clearly can't regulate an illegal system there are clearly many who are doing rather bad things to our environment with wood burners but as far as you are concerned all it means is there is as a result a lot of miss-information on wood burners. What I think is a crime is DIY sheds can sell wood burners which they know can't be used within the law.
But of course it is the same when the sell consumer units which they know can only be fitted by the DIY guy by paying the LABC loads of money.
Sorry no real answer to your question but at least you should realise why you can't get the answer.
Personally I would love to fit a "Rocket" wood burner but don't think I would get insurance on my house if I did.
