Matching furnace blower CFM to AC unit tonnage

2126 Views 7 Replies 3 Participants Last post by user_12345a, Jun 28, 2020

houtza

Discussion Starter · Jun 27, 2020

Hello, I am wondering if someone with more knowledge of air conditioning systems can tell me if my situation is an issue or not. I am building a 32x40 finished room inside my 40x80 pole barn with 10 ft ceilings. I picked up a lightly used 80,000 BTU furnace with a 16,000 CFM blower. I now need to buy the AC unit. The problem is that based on the calculations I have done, the AC unit should be 2 ton or around 800 CFM for the evaporator coil. Will there be any drawbacks if twice that about of air is passing over the coil using the 16,000 CFM blower? Any advice would be greatly appreciated!

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houtza

Discussion Starter · #5 · Jun 27, 2020 (Edited)

roughneck said:
There is no way an 80,000 BTU furnace can move 16,000 CFM.
That’s a huge furnace for such a little room. Huge amount of air conditioning as well.
What sizing calculations have you done?

Oops, ya I meant 1600 CFM. For the room, I have 2.5 inches of closed-cell spray foam (around R 19 I think) and will have R49 blown-in insulation in the attic. I am in south-central Iowa, so we do get some pretty cold winters and hot summers. The furnace is HE, so the end output is 76,800 BTU.

I should say, my "calculations" were more just using several online calculators. In hindsight, I should have done more research but it was selling for cheap and I didn't have much time.

I used a load calculator like this one and came out with 60k BTU furnace
https://www.remodelingcalculator.org/hvac-load-calculator/
I figured that since the furnace is downdraft and the ducts will be below the floor in a closed-off but not heavily insulated area my furnace being slightly larger would be ok.

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