Matching furnace blower CFM to AC unit tonnage

2131 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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user_12345a

· #2 · Jun 27, 2020

furnace likely grossly oversized for that space if insulated. if it's a 80%er not as bad.

The cooling fan speed can be reduced to minimum but it may still move too much air for a 2 ton a/c. You may in fact only need 1.5 ton - what calculations did you do? Load calc or tons per sq ft?

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user_12345a

· #4 · Jun 27, 2020

probably meant 1600cfm.

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user_12345a

· #8 · Jun 28, 2020

Based on your description even 40k btu/1.5 ton is probably too much - spray foamed places are incredibly tight.

You may need an hrv or fresh air intake in the return.

You could very well have a heat loss of 20 to 25k, in which case you should get a 30k furnace. (lennox and goodman make. carrier makes a 26k)

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