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Hello, I have heard of heating systems that recover some of the heat from the chimney of your woodburner and duct it around the house. We will be installing a wood cooker and stove to heat our house - initially to heat water to power radiators via a heatstore. However, I'm now curious as to whether ducting heat directly from the flue wouldn't be a more efficient use of the energy. Any comments/experiences?
We have a wood burning stove in our living room, the flue is single walled and exposed up to the ceiling, painted black it gives off considerable extra heat to the room.
I Canada I have seen the flue from a wood stove go across the kitchen ceiling, into the lounge then up through the floor and into the bedroom, across the room at shoulder height across the landing to the other bedroom and up out through the roof. All in cast iron single skin pipe and in a timber frame house too boot! King early version of central heating? 1800 ish. they didn't have fire regs back then and the pipes went through oversize 50mm square holes and the roof flashing was thi through cedar shingles. Apparently the biggest danger was from sparks going up the chimney igniting next doors roof.
I would talk to your cookers manufacturers about this one. The problem is that if the flue is too cold, a, it won't draw properly and b, tars and acids will condense in the flue and run down to rot out your cooker. Frank
There shouldnt be much heat from the cooker flue, I can get my cooker above roast on the gauge but still hold the flue pipe. Cookers usually have chimney dampers and flue diverters so you can really hold the heat in the appliance by reducing the flue draft. An insulated flue is essential for a cooker making it hard to retrieve any lost heat. A stove flue will generally be hotter (depending how good the cooker is ). In principal (I have done it before) a single skin flue pipe running through a bedroom boxed in brickwork, with vents at floor and ceiling level will give convected heat, you would have to insulate the top section of chimney so as not to cool gasses too much. The problem is you are not allowed to do it under building regs. Even if you use an insulated chimney system the manufacturers advise to box it in. The worry would be in years to come the flue may rot and leak fumes into the room.
Building regs only alow 2 x 45 degree bends running for no more than a meter on the angle in one flue. Any bend in chimney acts as resistance to the flue draft.
I asked something very similar a while back, but am still unsure -
If you pull all the heat out of the waste gas, then you need some other method of getting moving the air through the system. Thats easy enough - an electric fan on the chimley cowel would consume, im sure, much less energy than the hot gas holds. You could control the burn rate with the speed of this fan, thermostatically, if you liked.
Cooling the air would precipitate out the unburnt tars, so some method of cleaning the heat exchanger would be needed, or, even better, design / controll your system to burn sufficiently hot to burn these compounds anyhow (ie burn cleanly)
Ive spent some ammount of time looking for some kind of 'retro-fit' exchanger to fit on a flue, but havent found anything yet. Nor found any real flaw in the concept either.
No answers from me, im afraid, but id be very interested in what you conclude?
If you use a dedicated cooker then you will probably limit the combustion by damping to acheive the correct cooking temperature, which will probably not result in the most efficient combustion, so expect tar and noxious gases, which are best (!) taken away at as high a temperature as possible. Contrast with the oven as part of an efficient woodburning setup, and the heat extracted from the flue gases into a large thermal store (fire brick, soapstone, etc).
I don't have specific advice with regard to woodburner flues, but I know we install whole house heat recovery ventilation systems that can recover some of the heat from woodburning stoves. Some customers request an internal extract register to be situated close to the woodburner, or other heat source, the warm air is taken into the ducting of the system, passes through the heat exchanger in the MVHR unit, and thus some heat is thansferred to the filtered, fresh, supply air. Obviosly there are fire considerations to take into account, but there are specially designed fire-registers and dampers that can be a solution as a safety precaution.
I know we also don't generally suggest that cooker hoods are direcly connected to our systems. This is partly because of tar, grease and other pollutants (as mentioned in previous forum sequence comments) effecting the ducting and the filters in the MVHR system, and partly because the fans in cooker hoods disrupt the balance of the MVHR system. But we recommend re-circulatory hoods, and always put 2 extract registers in kitchen, when possible of course. Again, capturing much of the heat generated in your kitchen.
Heat from bathrooms is also recovered, and humidity levels controlled through ventilation... but I won't drag on, any more !
Seems like a good possible solution to me though. Steve W.
May also be an extra precaution against carbon monoxide and other gases etc.. Don't want to get into a very serious subject that I don't know too much about ! ; but as long as the system does not have any cross contamination of supply and extract air, it might be an 'extra measure', in theory at least..
When I installed mine, (A small 4KW stove), I ran the twin walled flue pipe ran through an upstairs bedroom. Part J calls for sepcific clearances and for ventilation round the flue. I boxed in the upper part in the bedroom (controlling access in is a necessary part of part J), with a vent into the room at ceiling level. (I sealed off the flue ventilation with fireboard at rafter level, (rafters part is ventilated to the sky via draughts from round the flashing). I'd guess I reclaim 300 watts from the top vent, which warms the bedroom nicely.
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