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darrel...
darrel...

Posted: Mon Oct 31, 2011 7:43pm

Post Subject: Tar on roof

We have recently installed multi stove burner, but the roof has got alot of tar on it now and it also runs down the side of the boat. Is the chimmley too short or not lined, How can we stop this?

Tony-B...
Tony-B...

Posted: Tue Nov 01, 2011 8:35am

Post Subject: Tar on roof

I reproduce the text from an answer I gave in the magazine in 1997. I would add that if you have a Coolie hat fitted please remove it because that will encourage condensation and drip it onto the roof as well. "When the fuel is burnt efficiently the flue gases will be mainly CO2 and a bit of water vapour (from the hydrogen bound up in the fuel). As you shut the stove damper to control the rate of burning you start to produce CO and increasing amounts of carbon. If the temperature is not high enough or if you have insufficient air to burn it you will also produce vaporised hydrocarbons (tar). The amount you produce will depend upon the fuel you are using. House coal and wood are especially bad for this. The internal flue pipe usually gets hot because there is no wind around it to keep it cool and its in a warm cabin, however the chimney is another matter. The chimney is cooled so the water vapour and tar condense on its inner surface and trickle down - the inner skin on the chimney is supposed to direct this straight back down the flue pipe where it will eventually get re-vaporised, burnt, or cleaned out manually. Clearly something is going wrong with this theory. Remove the chimney and look at the underside. The double skin should have a "disk" of metal supporting its lower end that completely blocks the space between the inner and outer skin. This disk should be located just above the point that the upstand on the roof finishes inside the chimney when the chimney is fitted. The inner skin should hang down so that when you fit the chimney it actually enters the flue, or at the very least is directly above it with only a very small gap. I suspect that your flue pipe may be smaller than "normal" so the inner skin has a larger diameter. This will allow the tar to drop onto the top of the upstand and trickle out between the upstand and the outer skin. The way to cure this is to make yet another internal skin that is small enough to drop the tar back down the chimney. I would seal any joints with silicon sealer and that may well be enough to secure it. If it is loose use pop-rivets, small self-tapping screws, or small nuts and bolts so fix it. You may have a chimney where the inner skin is not baffled at the base, so fumes can find their way into the cavity between the two skins. This will allow them to condense and trickle down the outside skin. I have heard that filling the void with spray foam from a DIY outlet (the sort sold for filling gaps in buildings) often cures this problem - but make sure it is not flammable when cured." If you want the accompanying diagrams then I need your email address - send to Tony@TB-Training.co.uk.

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