[houtza]

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!

 

[user_12345a]

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?

 

[roughneck]

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?

 

[user_12345a]

probably meant 1600cfm.

 

[houtza]

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.

 

[roughneck]

That’s not any kind of load calculator. I tried it and it suggested my house needed 7 tons. Which I know for a fact is 3 tons too many.
Bigger equipment is not better.
Figure 350-400 CFM per ton.

 

[roughneck]

www.loadcalc.net

 

[user_12345a]

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)