Smoking furnace

[Handy Andy]

  I had a bright idea, wanted to burn wood in my shop but have the heater outside.  So I built a 6x6 steel building 4' from the shop.  Purchased a DAKA wood furnace at Menards and installed it, using metal bestos pipe for the flue.  The little building is wrapped with tyvec and r 11 insulation, and has a trailerhouse door, which fits tight.  the whole building is very tight, and I installed a supply on the furnace using 8 x 14" duct and the same size duct for a return.  Well, I tried it yesterday, and the small building filled up with smoke and was pushing smoke into my shop.  So I found if I hold the door open, it works fine.  I realized that I did not allow for combustion air, should have used a much larger return, or installed a vent for the furnace combustion air.  So think I'll go get a vent and put it in.  Just not sure how big.

[thecfarm]

You built a good tight building,too tight.  ;D  They do need air. Before the days of my OWB I was burning wood in the basement too. The wife wanted to keep the heat down there and wanted the cellar door kept close. We have a oil hot water heater about 4 feet from the stove. I smelled something odd and went down. The stove was getting it's air from the water heater. I knew better,but did not think it would be a problem. I just cracked the basement window a little bit and left the cellar door open.

[Al_Smith]

Hmm well you either don't have a large enough flue,are getting a down draft condition or just don't have enough intake air for the burner or a combination of all three .

That little building with a furnace inside is not going to freeze .I wonder what would happen if you cut a hole in the wall on the leeward side and installed  a 24 by 24 hooded gravity louver ? Maybe just an adjustable louver or a vent with a door so you could close it off when not in use .

You about have to have it screened off to keep the possums out or the neighbors cat though .Go out to fire it up only to be greeted by a litter of kittens some morning .

[doctorb]

It sounds like you were so worried about losing heat from the outside furnace that you are hindering its ability to create it.  Taking a broad look at your problem, you have two solutions.  1)  Bring the stove inside and use it like an indoor wood stove.  It could still provide heat through the current system you constructed.  Or, 2),  Burn a little extra wood, keep the mess outside, and lose a little heat to the great outdoors by giving that furnace some air. Seems like an easy choice to me. ;)

[Handy Andy]

  I put a 12 x 18" vent in today, and it was better, although I could still smell a little smoke in the shop.  So I went out and opened the stove and found the gasket hanging off the bottom of the door.  Don't know if it was just the gasket or if I will have to hook the return directly to the fan.  The furnace came with a little box to cover the fan, with a slot for a furnace filter, but didn't put it on. Checked to see if it would fit, and it will, but will have to change things around as the duct would have to come from the side and the filter is located wrong for that.  Anyway, will try a new door seal first, and if that doesn't do it will get some 8 x 14" duct and close off the return.

[r.man]

I have seen two wood furnaces mounted through walls so the door was outside and everything else was inside. These were the type of furnace that used convection and had a large tin box around everything but the door and smoke outlet. Kept the mess and raw fire outside with the heat inside. The one fellow liked it, didn't have to bring his wood in through the shop and kept the ashes and mess outside. I don't think he had a problem with combustion air.

[Holmes]

The supply and return air are separated from the combustion side of the furnace. Your furnace needs combustion air, by direct communication either to the outside of the building or from your shop [ a pipe or opening }.  A 4" dryer vent outlet installed to allow air into the shed should solve your combustion problems.

[Al_Smith]

I'm not certain a 4" intake would be enough .Reason being for example a 4" would only have 1/4 the area of an 8" flue pipe which most actual furnace types use .

In my simplistic method of thinking I wouldn't limit the amount of intake air either a furnace or stove would need .Even on the average stove if you throttle the discharge that fact itself limits the amount of air the thing can draw .It's kind of like throttling the intake of a blower which you never do ,you throttle the discharge .

Well now sure on a stove you have the intake shutters
dampers  of a sort but that's limiting the actual air not the source of same .

Does that thing  have a forced draft blower per chance ?

[Holmes]

 When you create a vacuum [ burning wood uses up the oxygen] in the furnace room the air will enter. It will come in thru a 4" opening.  As it stands now the air enters down thru the chimney.

[Al_Smith]

Well yeah I realize you kind of create a venturi effect .Now what happens if it decides it wants to go out the 4 inch intake instead of up the flue ?Put a spark arrester on the intake pipe ?

Geeze it would seem simpler rather than all this hullabaloo to just block the door open with a piece of firewood .When the fire burns out get a broom stick and chase out the cat and shut it for the night .I suppose you could turn loose a Jack Russel terrier and get about the same results plus it would be a little entertainmant as well . ;D

[Handy Andy]

  Well, I got it fixed.  New gasket for the door and hooked up 8 x 14 duct to the return.  The duct was a pain, had some salvage ductwork that had to be cut down, and the duct from the shed was laying flat ways, so stood my duct in the building on edge and just cut a hole to fit.  Had to go around the corner and connect to the rebuilt filter box, was a job but completed it.  Got cold enough for a fire this morning, so I fired it up, and went into the shop.  The fan came on and it smelled clean.  Checked the building and it works fine with the door shut. Installed the vent in the wall a week or so ago.  Anyway, SUCCESS!