Is It Safe And Will It Work? - HVAC - DIY Chatroom Home Improvement Forum


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Old 10-29-2008, 01:35 AM   #1
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Is it safe and will it work?


I have a 400 sqft workshop in which I plan to heat with wood. Since I work with glass and therefore need powerful ventilation I have an 1800 cfm fan essentially changing the air every two minutes. Currently I keep a window open for return air. With winter coming I have come up with an idea to preheat that return air. I would love opinions on if it is safe and if it will work.

My stovepipe will have about a 4 ft horizontal run in between the interior stack and the exterior stack. I want to fashion a 12'' x 16'' x 5' sheet metal box that I can have a 16'' intake on the outside and a 16'' return on the inside, thus heating the air on the way in.


I plan to seal the stovepipe that runs through the box with a high temp sealer, so not to let any smoke into my return air. I will also run a 16'' exterior duct far enough from the stovepipe to not take in any wood smoke.

My biggest concern is; it this a big enough "thimble" for fire protection. The studs are 16'' oc. I will be putting the 12'' span in between the studs leaving almost 2'' clearance, per side, to anything combustible except the exterior siding, 3/4'' yellow pine, which will have direct contactwith the "thimble" but, in (my) theory, will be air cooled. The stovepipe will be 6'' leaving another 2'', per side, clearance to the sheet metal collar. When I am done working for the night I will shut off the ventilation and the fire will still burn for some time leaving that "thimble" to heat without the cold intake air.

any thought are appreciated.

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Old 10-29-2008, 11:04 AM   #2
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Is it safe and will it work?


The principle is as old as wood stoves and very common. They are called stack robbers.

They way you are going about it is dangerous and could cost you your life.

First off with the 1800 cfm fan you are more than likely pulling air down the chimney, through the firebox and into the room. You should have an intake vent for the wood stove that takes all it's air from the outside.

Second, the sheet metal will just go away if you ever have a flue fire. You would be able to watch the sheet metal melt as the creosote gets up to 3000 degrees. Shortly after that you will be standing outside of your shop watching it burn to the ground and hearing sirens off in the distance.

Third, high temperature sealer is not used for a wood stove where temperatures are just too high for it to handle.

If you are going to do this you have to have a fabricated steel box with a hole in it for your pipe. Then you can put in a thinner pipe like HVAC pipe and put that through the wall.

When you do this have some baffles to turn the air in different directions so that it doesn't just come in the shop and go out the other end. Air will create tunnels and go from the inlet to the outlet and have very little effect on the surrounding air. You have to mix it up.

With an 1800 cfm fan you will have to have one hell of an opening to heat that air and will have to have a really large fire to heat that air that quickly.

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Last edited by Marvin Gardens; 10-29-2008 at 11:07 AM.
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