Pairing separate furnace with AC unit question

[JamesBondo]

I need some advice about choosing the correct ac/furnace system for my new pole barn. It is a 40x80 building. I am turning 32ft x 40ft into a conditioned space (see attached pictures). The side walls are 14ft and have 2.5 inches of closed-cell spray foam. The perimeter has a concrete rat wall with R7 foam board insulation 3ft down in the ground between poles (that was a pain to dig). I am doing a pier and beam raised wood floor, so the end result will be a ~4ft insulated crawl space, with 10ft ceilings. I plan on using blown-in insulation ~r45 for the ceiling and the dividing wall will be r19.

When the shell was being built, I kinda jumped the gun and bought a lightly used Goodman high efficiency 80k btu LP furnace for $200. Now that I am getting closer to the install, I am wondering if that may be way overkill for what I need. From what I am reading, I most likely need a 1.5 - 2-ton ac unite. An ac unit that size usually requires a blower fan speed of around 600-800 cfm. The lowest my furnace can go is 1200 cfm. My concern is that I will be moving to much air over the evaporator coil for proper dehumidification. Some of the calculators I have tried are saying a 30k system would work for my space. I am located in central Iowa, so we get hot summers and cold winters I was wondering if anyone had any recommendations or advice? Thanks!

 

[TRWham]

It is possible to bypass air around the coil. We did this routinely in some commercial applications where we needed more air for air changes but the total cfm exceeded the cooling requirement. In those cases, however, we were building custom air handlers and had 100% control of every aspect of the system. You might find a contractor who can handle this, but that might also be the tail wagging the dog. Start with a clean sheet of paper and determine what you would have done if you did not have this unit on hand. Depending on how far off the ideal specs are from the unit you have, you can decide what to do.

 

[Jim greengo]

A 3t drive is normally the smallest you're going to find on forced air furnace,I wouldnt sweat it too much.
As far as the ac goes with the sq footage your talking about I'd say a 2t would be on the edge of being undersized,but I guess it depends on how good that spray foam works.

 

[yeldogt]

What's the reason for that type of construction .... why not just build it on a slab ? Or is the building two levels ?

What are you doing over the dirt?

I can keep my 1600sf studio warm with 15k in PA.

How are you going to use the space ? Some guys want an evening/ weekend working space where they are making lots of noise ... just want it warm vs cold. I'm doing more quiet work for longer times so I want quiet and a bit more control of the temps ...

 

[nadogail]

IMHO, I have had good luck with Goodman furnaces. I hope it serves you well.

 

[JamesBondo]

Thanks for all the info. I recently contacted several online distributors such as HVACDirect.com and they pretty much all recommended a 40k btu furnace and a 2.5 ton ac system. I think I will purchase a new system and use the furnace I currently have for the other 48x40 part of my building when I get around to insulating it in the future. Yeldogt, I plan on using the finished space as an entertainment area/office. In my opinion, wood floors are nicer to stand around on plus less overhead space to heat. It also makes plumbing and installing air ducks simple. I also plan on making storage racks under there that can be accessed from the other half of the shop for wood and metal storage. I will also put a 6 mil plastic vapor barrier down as well.