Heatpump vs Propane Heat for Garage

[Traderfjp]

I am moving to the mountains of nc where it gets below 30 on a routine basis at night. I have 35 x 40 sq ft. garage with 10 foot ceilings and it's insulated. I had a pipe freeze right before closing that was in the garage. I want to heat and cool it. I currently have a 400amp service running two heat pumps. It's a 4,400 sq ft house. While I looked at some split heat pump units the AC was about 36k btu as was the heating. I'm wondering if I'm better off getting a 80k propane heater (2.50 a gallon) and a portable AC unit vs the split heatpump. Worried the heatpump will not have enough heating btus for the garage. These units run about 1800.00. A portable AC is around 900 and the propane heater runs around 600.00. My electric is around .11kw. Any advice appreciated. Probably will keep the heat at 60 for the winter and 74 for the AC.

 

[Jadnashua]

WIthout doing a manual J calculation, it would be hard to say, but for just a wag, unless you want the garage like a sauna or refrigerator, 80K BTU is probably at least 3x bigger than needed.

Is there any insulation in the garage? Are the doors insulated? WHat about the ceiling? Lots of windows? If there isn't insulation, adding some and sealing air leaks, you might find a one-ton unit might be more than enough. If the doors are opened often and for longer periods, a bigger unit would let it recover faster, but especially in the summer, humidity control is the big thing, and a smaller unit running longer will dehumidify much better than a larger unit that cycles on/off frequently.

To just keep the pipes from freezing in the winter, all you need is air leak sealing and keep the interior above probably 40. IF you work in the garage and don't want to wear a jacket, you'll want it to be able to get warmer.

 

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[Traderfjp]

I'm planning on keeping the temps at 50 during the winter and turning it up to 70 when I need to complete a project. The garage is insulated and there are no windows. The garage doors are insulated but when they close there is a gap at top of about 3" so maybe I can get some type of rubber insulator. In the summer I want to keep the garage at 75 and will turn it down when I'm working in there. Like you said getting rid of the moisture in there is paramount.

WIthout doing a manual J calculation, it would be hard to say, but for just a wag, unless you want the garage like a sauna or refrigerator, 80K BTU is probably at least 3x bigger than needed.

Is there any insulation in the garage? Are the doors insulated? WHat about the ceiling? Lots of windows? If there isn't insulation, adding some and sealing air leaks, you might find a one-ton unit might be more than enough. If the doors are opened often and for longer periods, a bigger unit would let it recover faster, but especially in the summer, humidity control is the big thing, and a smaller unit running longer will dehumidify much better than a larger unit that cycles on/off frequently.

To just keep the pipes from freezing in the winter, all you need is air leak sealing and keep the interior above probably 40. IF you work in the garage and don't want to wear a jacket, you'll want it to be able to get warmer.

 

[Jadnashua]

There are numerous heat load calculators, some are better than others, but if I had to guess, and believe me, that's not a great idea!, in your relatively mild climate one of the mini-split heat pump systems would be by far your most economical solution. Now, pinning down the correct size is critical. Most of those calculators severely over estimate the size needed. The nice thing about many of those mini-splits is that they can modulate, and retain efficiency if they're oversized, but that's all within certain limits. 80K BTU is way oversized for your needs unless you kept the doors open.

If humidity control is your major goal, oversizing the a/c is the worst thing you can do for comfort, efficiency, and longevity. You want the thing to run most of the time at a low level which will draw the most moisture out of the air. A big one will not and you'll end up with a cold, damp environment. If you get the humidity down, you'd also be comfortable at a higher temp on a super hot day even if it couldn't keep it at 70.

 

[Dana]

Got a ZIP code for that mountain location in NC? (For design temperature/weather purposes.)

 

[Traderfjp]

Got a ZIP code for that mountain location in NC? (For design temperature/weather purposes.)

The zip code is 28792
Thanks

 

[Traderfjp]

This is the unit I was thinking of. It runs around 2k. Does 36k btu for heating and cooling. No soldering and the unit is already charged with refrigerant. https://www.alpinehomeair.com/viewproduct.cfm?productID=453076828

 

[Dana]

Your 99% outside design temp is about +15F give or take a degree or two, which is close enough to the AHRI capacity testing temperature of +17F for mini-splits.

The AHRI file on this unit indicates a capacity of 21,200 BTU/hr @ +17F outside, 70F inside. When it's 0F outside during a Polar Vortex disturbance event it's going to be less than that at an indoor temperature of +50F, but probably still enough to keep a 1400' garage from freezing, assuming it has insulated doors and at least some weatherstripping.

Is that mini-split programmable to a 50F indoor temperature? Many won't go below 60F, which may require a workaround.