A/C size/options for 24x40 shop

[G-ManBart]

Hey all,

I'm getting to where I need to plan for A/C for my shop, and need a little advice. My building is 40x64' with 13.5' ceilings. I partitioned off 24x40' that will have one man door, and one insulated overhead door giving access to the 40x40' side. I put 2" foam board insulation under the 24x40 side and added three 300' loops of Pex for radiant heat. There is one insulated exterior overhead door (12x10) that should be around R13 as I recall.

The partition wall is insulated R13 and the exterior walls are going to be (just started) R19. The ceiling has some insulation I don't have an R figure for because it was free 4' wide by 36' long rolls that fit between my trusses perfectly. Going off the thickness, I'd say those rolls were around R13 or a touch less. After I hang the steel ceiling I plan to blow cellulose over the top of the existing insulation...haven't decided a target depth, but plan to put a lot in there since it's cheap. I may have enough of the rolls left over to lay another layer over the top before I add cellulose, but I'm not sure yet.

In the pics below you will see I have a full bathroom outlined on the floor, but I don't think that's a major factor in this decision....it will be a complete room with 8' ceilings and a storage loft on top.

When I use online calculators for what size A/C I need, they come back at around 19,000Btu. I'm thinking of going with a mini-split for a variety of reasons. Obviously that puts me in between the normal sizes, so I'm not sure which way to go. Surprisingly, we do get quite a few days here in the high 90s and low 100s in the summer, so I'd like to make sure I can work comfortably in the shop whenever I feel like it. Right now I'm mostly a weekend and evening warrior, but my plans are to be working in the shop much more frequently in a few years (nearly daily).

With all of that, what size would the experts here recommend? A few pictures of the shop to make it a bit more clear (getting ready to hang a vapor barrier, then hang the ceiling, then build out the bathroom):

 

[G-ManBart]

I should add that I'd love to hear recommendations on brands as well.

 

[MushCreek]

Go bigger. Inverter type minis don't suffer much from being a bit oversized. We heat and A/C our entire house with a single 12K Mitsubishi Hyper-Heat, but it's a small house with a very tight and well-insulated envelope. We have a 9K in the master suite, but rarely run it, even here in hot humid SC.

Make sure you compare the various ratings. We were originally going to go with a ducted mini, but the efficiency drops way off, as well as the low temperature range for heat. We just have the regular wall-mounted indoor units, with a SEER in the mid 20's, and heat capability down to -13 F.

We love the Mitsubishis. They are so quiet, you can't tell they're on, and that's in a quiet house. You can't hear the outdoor part, either, unless you're right next to it. I paid about $3400 for the two units, mounting brackets, and refrigeration tubing shipped to my door. I did my own installation, and then I had a licensed contractor qualify the system for the warranty (read the fine print if you buy on-line). The A/C only added $20 to our electric bill in July, the hottest month.

Having had a big shop in FL, I learned the key to A/C in a big shop is to get ahead of it, turning the A/C on well before it gets hot out. If I tried to cool my shop starting in the heat of the day, the 5 ton system never made a dent.

 

[theoldwizard1]

Properly designed, you can use your mini-split as the heat source for you radiant heat floor. If you have NG that would probably be a cheaper heat source. A properly sized min-split heat pump would be cheaper than a propane or an electric boiler for your radiant heat source.

Mitsubishi seems to be the "king of the hill". Fujitsu I think is #2.

 

[G-ManBart]

Thanks guys...this is good stuff! I was sort of leaning towards going larger since I'm in between sizes on all the charts.

I've done some quick research on the Mitsubishi units, and they seem to have great reviews. I was actually surprised they aren't more expensive, which is a nice bonus!

 

[theoldwizard1]

Thanks guys...this is good stuff! I was sort of leaning towards going larger since I'm in between sizes on all the charts.

I've done some quick research on the Mitsubishi units, and they seem to have great reviews. I was actually surprised they aren't more expensive, which is a nice bonus!

If you are planning on using any min-split heat pump as a heat source, carefully check out the cold weather performance. Some start loosing efficiency as high as 35F !

Mitsubishi has some models that will work down to below zero, but those typically cost a few more $$$.

Heat pumps for radiant heat are not common, at least in the US. I don't know why they are not sold as factory "pre-packaged" systems in the US, but they are in other parts of the world. An experience HVAC designer/installer should be able to size the water-to-refrigerant heat exchanger.

If you have NG, I would use it.

 

[G-ManBart]

Based on a couple of factors, I'm planning to use a combination electric hot water holding tank and an on-demand heater for the radiant heat setup.

Our building inspector is a plumber by trade, and after talking it over with him extensively, it seems like the way to go. He put this sort of system in his shop, which is a bit larger than mine (but lower ceilings) and with 1/3 less Pex than what I used, and he's very happy with it. I think he said the system will get the shop into the 70s even when it's really cold. If I can even get it into the 50s or 60s when it's really cold I'll be thrilled. He also said that his worst winter bills show it costs him no more than $30 a month to heat his shop.

There is NG at the house, but not at the shop, so that's not really an option. We actually purchased the 5 acre lot next to us, knocked the house down, and were able to keep the city water and septic, so the barn is sited where the old house was (at least the corner by the septic tank is)....the old house was on propane.

Still, for those times in Fall and Spring when it gets cool for a few days, but isn't quite time for the radiant heat 24/7, having the mini-split heat pump might be a nice option.