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View Full Version : AUF non-forced air duct size



berkshires
11-22-2016, 10:22 AM
Hi all,

I have kind of a jerry rigged evaporator setup, but I'm doing what I can to make it as efficient as possible. I converted an old wood-burning stove into an arch last year. One of the things that was not optimal was that there was no grate - the coals and fire were right on the floor of the stove. I know this means the fire didn't get as hot as it could have. So I have built a grate for the fire to sit on. Now I want to insure that as much air as possible is getting under the grate to go up through the fire to the pans, so I get a nice hot fire, instead of letting too much cold air in to cool down the pans.

Last year I just left the door open a crack, otherwise I'd smother the fire. But I know this was letting too much cold air in, so my syrup pan was not as hot as it should be. I don't really have the option to do forced air (no electricity) but I have a lot of stack and a really good draft. So what I want to do is close the door, but feed a duct through to direct air under the middle of my new grate. I think the draft will suck a lot of air through the duct and give the fire a good AUF boost from below.

My question is how big to make the duct? The working area of the firebox is about one foot wide, 18 inches deep, and 18 inches high (between the grate and the bottom of the pans). So a pretty small area for a fire. Does this mean I want a pretty small aperture for the duct? Like maybe two inches? Less?

I know it's a funky setup, so maybe y'all don't have a good idea how much air I'll need to let in. I'll be fabricating the duct myself out of old stovepipe, so I guess I can experiment a little, but I'd appreciate any thoughts, so I can hopefully get it right the first time.

Thanks!

GO

Super Sapper
11-22-2016, 12:13 PM
At a minimum I would have an equal opening to the area of your stack. If you get too much draft you can close it up some or shorten your stack. As long as the air is coming from below the gates you should not have to worry about cooling the pans.

Zucker Lager
11-22-2016, 12:22 PM
Hey Berkshires:
When I built my arch I over did the draft door size because I didn't know how big to make it. I'm running a 2 by 3 flat pan and made the fire box about 20 by 20" with the draft door opening 6 by 14" When I run I usually have the door just cracked about 1/2 " at the bottom to keep stack temps over 700. You can see the door is just cracked in the pic. It doesn't seem to take much opening to get proper draft. Jay

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