My project is installing a duel fuel propane/wood furnace for shop heat. It uses 6 foot logs that will light off with propane. My idea is to get the wood started with the propane and then just turn the gas valve off to save on fuel. I intend to put the furnace in the lean-to that is attached to the shop. I have a question about the return air. How much return air does a furnace need? The return air plenum opening is 20” by 18”. Do I need to pull the return air form the shop or is it ok to pull its air from the lean-to? If it can use air from the lean-to other than screen to keep the pack rat out what would I need? Would some type of damper door be a good idea? The return air plenum does use filters on ether side of the blower motor and I guess it would be a good idea to retain them. Shop is 24X30.Thank you for any advice.
- Thread starter ks willie
- Start date
State of Jefferson
Member
You never want to restrict return air if the furnace has a 18x20 plenum, you want the duct to be at least that size. Depending on the kindof work you do in the shop, I THINK you are required to use outside air for the return, to avoid running excess dust and or fumes through the furnace, that could cause fire or explosion.
If it were me, I would set it up so I could pull return air from either the shop, or the lean to. That way, if it's 20 degrees outside, and you are just puttering in the shop, you can use the already warmed shop air. But if you are painting, welding, sanding, etc you could switch it and draw clean outside air through the furnace.
That's just how I'd do it. Building codes may tell you differently. I try not to pay to much attention to codes;-}
Ben
If it were me, I would set it up so I could pull return air from either the shop, or the lean to. That way, if it's 20 degrees outside, and you are just puttering in the shop, you can use the already warmed shop air. But if you are painting, welding, sanding, etc you could switch it and draw clean outside air through the furnace.
That's just how I'd do it. Building codes may tell you differently. I try not to pay to much attention to codes;-}
Ben
Jaden
Member
- Location
- Eastern Nebraska
You need to pull return air from the heated area, or you are essentially trying to inflate the room like a balloon. Return air from the heated area pulls the cold air out, routes it over the heat plenum, and returns it heated to the shop. Is the furnace well insulated so as not to give off heat in the lean-to?
If you cut a hole thru the wall between lean-to and shop, into the furnace cold air return,
that will be your (only) warmest spot in the shop.
If you run a cold pipe to the opposite wall in the shop, THAT spot will be the warmest spot but will help to heat what's in between it and where the heat comes in.
Utopia (which never happens), would be a cold air return pipe to each of the far corners of the shop floor, back to the cold plenum, if you got BTU enough.
You are moving the cold air with the blower. Because nature abhors a vacuum, heat will flow up (and over) to fill any space in which the cold air has been removed.
that will be your (only) warmest spot in the shop.
If you run a cold pipe to the opposite wall in the shop, THAT spot will be the warmest spot but will help to heat what's in between it and where the heat comes in.
Utopia (which never happens), would be a cold air return pipe to each of the far corners of the shop floor, back to the cold plenum, if you got BTU enough.
You are moving the cold air with the blower. Because nature abhors a vacuum, heat will flow up (and over) to fill any space in which the cold air has been removed.
Geo-TH,In
Well-known Member
I have a 26x44x16 shop. I put the cold air return as high as I could get it and pushed the warm as close to the floor as possible. Warm air rises and by removing it from the ceiling and sending it back to the furnace, I could use a 0.6 gallon/hour nozzle and heat the shop with oil. BYT, it would be impossible to heat without good insulation.
George.
George.
In the heated portion of my shop I mounted 3-4 inch PVC sewer drainfield pipe, about 6 feet apart. Plug the ends to keep summer birds out. Common manifold on the ends is connected to one side of an extra furnace fan in the plenum. Other fan side is blocked off, so it sucks off the ceiling only. Blows hot furnace air and recycles warm ceiling air past the stove pipe, into the room.
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