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Reply by clin

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Posted on Heating/Conditioning the Shop year round?

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clin

1121 posts in 1880 days


#1 posted 08-29-2017 03:30 PM

I’m a little late to this party. But to the OP, I can’t recommend mini-splits strongly enough. Most are extremely efficient. While Mike is happy with his setup, relative to what he had, his experience is still unusual because of the small amount of insulation he has.

Your shop is 384 sq-ft. Assuming it has nominal insulation of R-30 ceiling, R13 walls, you probably can’t buy a mini-split that is too small.

There is an AC rule of thumb, of 400-600 sq-ft per ton of AC. This will almost always be too much. This is the number that guarantees a customer won’t call back saying it doesn’t get it cold enough. It’s not uncommon that this can result in 2-4X too much AC. Too much means more up front cost, much lower efficiency, and it won’t reduce humidity as well.

However, every situation is different and that is why rule-of-thumb calculations are a poor replacement for actual design. Many factors affect how much AC you need. Your local climate of course, all construction details, type of roof, windows, floor type, common walls with other structures, north-south orientation. Heck the color of the roofing makes a difference. No one of these makes a huge difference (other than climate), but 2-dozen little things add up.

But, your shop is small enough that even the smallest AC mini-split I have seen (7,000 BTU/hr = 0.59 tons) would still be 658 sq-ft/ton and therefore only just slightly above the rule-of-thumb that is itself probably 2-3X more than you need.

Point is, if you have normal insulation don’t go throwing a 2-ton AC mini-split in your shop. It will cost a lot more up front, and be much less efficient. Mike did it because his shop is not well insulated, yet.

Ideally, a properly sized AC unit will run nearly all the time during the hottest days of the year.

As to running it 24/7 or not. If you are not in the shop during the work week, then it would make sense to set the thermostat back. I’m sure turning it off completely would be the least amount of electricity. This is true if you won’t be using it for days at a time.

Even with regular heating and cooling, there is little advantage to setting thermostats back on a daily cycle. The common thing is to let the house warm up all day while you are at work. The thing is, then the AC runs all evening to pull all that heat out of the house. It saves little energy and you have an uncomfortable house when you come home.

With mini-splits it’s not even a question for daily use, just leave them running 24/7. I’ve barely been in my shop this summer, and I still just leave it temperature controlled. Though set to 82 or so. my shop is about 300 sq-ft and being part of a converted garage, it shares 2 walls with the house and has a relay well insulated and sealed doors. So it doesn’t take much to keep it cooled.

All that said, I’d go with a mini-split, and then keep the shop controlled at a higher temperature during the week, like 82 deg (or whatever). Then have it set to 74 deg (or whatever you want) Saturday morning through Sunday evening.

I think there is also an advantage to not exposing your shop to extreme temperatures. Glues and finishing products will store better. And really, just about everything will hold up better over the years if it isn’t being cooked every summer day and frozen every winter night.

If your shop is well sealed and insulated, I wouldn’t be surprised if you had a hard time seeing the increase in your electric bill. Especially given if you probably have a house you are already running AC in and like most of us, that AC is probably old-school, inefficient, and over-sized.

Good luck, and if you do the DIY mini-split, please report back.

-- Clin


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