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Old 03-01-16, 09:07 AM   #1
gtojohn
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Default Preheating furnace combustion air

I'm considering building a preheater for my furnace's combustion air. Winter is almost over so unless weather gets weird it might not be a factor until the fall. I currently have 2x 50kbtu natural gas furnaces with 80% afue existing in the same attic space. This attic space has a super insulated floor and 3-4 layers of radiant barrier and a ridge vent. Something I always notice is the roof above the adjacent attic space will frost while this one doesn't. Seems like there is a lot of heat energy left in the 20% going out the flue. Because of our short and warm winters 80% furnace are the norm and I have 5-10yrs left on my equipment's life span.

I have found a few papers on using combustion preheaters to increase industrial equipment efficiency and possibly reduce some emissions. On a small scale it might not do much, but as we know here gaining 1-2% overtime adds up. My current plan is to wrap my 4" flue double wall type b vent with 6" single wall snap, nearest to furnace connect a 4-4-6 wye and feed 4" aluminum flex into my furnace. My Ruud unit has a stub for a combustion air duct. As the furnace burns it will draw fresh air from the 6" pipe wrapped around the flue. If I really want to go all the way I could replace the double wall 4" with single wall, this pipe gets way hotter. Double wall flue clearance is 1" from combustibles vs 4-5" for single wall.
http://energy.gov/sites/prod/files/2..._preheated.pdf
http://energy.gov/sites/prod/files/2..._preheated.pdf

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