Need help hooking new duct to existing duct.

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Feb 8, 2010
106
North Arkansas
I have put in a new wood furnace in my basement and now need to hook my new ductwork from it to the existing ductwork. My question is about pushing hot air up through the coil for the existing AC unit. Can I just fabricate a piece of metal that will fit right on top of the coil? There is a perfect spot for this if it will work. Or will the heat damage the coil? I know they make backdraft dampers but even they would let heat to the coil on the A/C right? Can I get away with just a backdraft damber on the woodfurnace side?

Thanks.
 
You cannot have the heat of the wood furnace running through the coil. It must be placed above the coil, with a backdraft damper between. With a proper damper, the heated air will not backfeed into the coil. Could you post some pics of what your working with? Every setup will vary.
 
The air unit is located on the floor of the main living area, the basement is directly underneath. The vents are in the floor of the main living area. So the A/C runs down through the coil and into the trunkline. My furnace will be ducted into the bottom of the trunkline flowing upwards. Basically, I don't see much, if any room for installing a damper between the trunkline and the bottom of the A/C coil. The reason is because two individual duct lines running off of the trunkline are installed at the very top with just an inch of clearance before the trunkline connects to the A/C unit. Perhaps I will need to build a new trunkline and locate the ducts lowerer so I will have room to install a damper?

Laynes, check your PM box for about a week ago.
 
I cannot get a straight answer out of my HVAC shop, which is building the ductwork for me and I'm installing. My existing LP furnace sits directly on top of the A/C unit and blows hot air through the coil also, but I do not use the LP furnace. It has been retired.

This is what I dont understand, if the LP furnace can blow hot air through the coil, why can't the wood furnace blow hot air to the coil? Thanks so much for any help.
 
It's a simple outage that can cause many problems. Alot of heat will buildup in the ducting. In the past on our old wood furnace we had an outage and I could watch water droplets dance and sizzle on the ductwork. There's alot to look at. You want the proper static pressures in the ductwork, clearances, size, etc. I find it odd that a hvac company cannot help out or give ideas, maybe liability who knows. I got your pm, my response never sent. Pics would make things much easier, I cannot picture things well in my head. Has any hvac techs looked at it?
 
Its not that they dont want to help, they just do not know, and have run some like that just not sure if its right.

They did figure my size of plenum and ductwork and have built it for my install. I just provided the measurements, bends, etc.
 
More like ruin the refrigerant. I am sure it has some upper temperature rating, at that is probably less than your hot air furnace. It may have work for good old freon, but with the R whatever used these days you would have to check the specs. Pics or a drawing would help.
 
Hunderliggur said:
More like ruin the refrigerant. I am sure it has some upper temperature rating, at that is probably less than your hot air furnace. It may have work for good old freon, but with the R whatever used these days you would have to check the specs. Pics or a drawing would help.

Typical compressor discharge temperature limit is in the 135 degC to 145 degC (295 degF) range, which is to prevent breakdown and loss of lubricity of the entrained oil, but not a problem for R134A or what have you. Either way you don't want to get your AC coil really hot.
 
FURNACESETUP-1.png
 
I can't read anything in the pic, it's too blurry.
 
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