Heat pump size for detached garage (mostly heating)

[old__man]

I built a garage 3 years ago and heated it with resistance electric. It was expensive. I moved and am building a new garage. I would like to heat it (and cool during our 2 week summers) with a heat pump. I'm stuck between 12K and 18K. There will be no other source of heat. If it gets a little cooler on cold days so be it. I also realize when it snows and everything gets buried that I will have to shut the heat pump off until I can dig it out again.

-24 X 24 X 10
-2X6 framing 24" O.C.
-Wall will be insulated with fibreglass and attic will be blown in. Vapour barrier through out.
- Vinyl siding over OSB.
- 12X8 garage door, 36" man door, 2 24X36 dual pane windows.

I live in a somewhat cold, damp, and very windy place. Here is a link to the climate data.

18K unit I am thinking about. AHRI certificate

12K unit I am thinking about. AHRI certificate

 

[wssix99]

Are you going to heat the space at a constant temperature all the time? Or do you expect that you'll turn up the heat when you are working there?

 

[old__man]

I might just leave it at temp and let the heat pump do it's thing. I don't think heat pumps allow low temp settings nor do they like doing the yo-yo of temperature changes.

 

[billybek]

You might want to check some of the manufactures of these products sites. Many mini split and split systems are able to produce heat at a much lower ambient temperature than the heat pumps of years past.
Daikin is one manufacture that claims 100% heating capacity down to -5 deg F and some capacity at -13 deg F.
Supplement with some resistance heat and you should be good to go except in the coldest of cold weather.

 

[nadogail]

Is electricity your only available fuel?

 

[kelpaso1]

I am in a similar climate and I went with a 18K unit for my 22x24 attached garage. I heat it 24 hours in the winter and it is pretty much fine. Only time it has problems keeping it at 65F is when it gets to -15C or colder. Still works but garage cools down some but not freezing. I built a steel frame sitting on a concrete patio stone that keeps the outside unit about 3ft up off the ground. Never had a problem with snow but I do snow blow a couple paths in front of it. I don't recommend wall mount brackets because it will vibrate the wall it's mounted on. I did that the first year and it drove me nuts. Making a stand on the ground and now I cant even tell it's running. Also I estimate it costs me $30 a month to run it in the winter.

 

[old__man]

Is electricity your only available fuel?

My other options are;

1) Wood. Not really feeling having a constant source of ignition in work space where fumes might be present. I've had a gas leak or two in the past. Also, the stove won't load itself, I have to be there. Although I do have a wood stove in the house so the fuel is there.
2) Oil. Ludicrously cost prohibitive to install an oil tank and furnace. Running it would not be cheap either. Source of ignition as well.
3) Propane. Similar in install to oil I would imagine.

Most homes being built in the area are heat pump with baseboard electric back up.

 

[old__man]

I am in a similar climate and I went with a 18K unit for my 22x24 attached garage. I heat it 24 hours in the winter and it is pretty much fine. Only time it has problems keeping it at 65F is when it gets to -15C or colder. Still works but garage cools down some but not freezing. I built a steel frame sitting on a concrete patio stone that keeps the outside unit about 3ft up off the ground. Never had a problem with snow but I do snow blow a couple paths in front of it. I don't recommend wall mount brackets because it will vibrate the wall it's mounted on. I did that the first year and it drove me nuts. Making a stand on the ground and now I cant even tell it's running. Also I estimate it costs me $30 a month to run it in the winter.

Nice. That is the sort of info I was looking for. Good tip about the stand. I do have room back there for one. What make/model of heat pump do you have?

I just move back to this town after being away for 3 years. Blowing snow is an issue. This was the last good storm I endured here before I left. Vid gets interesting about 30 seconds in.

 

[kelpaso1]

The brand name is KERR, I got it it 5 years ago when I moved into this place. It cost me $2700 installed.

 

[wssix99]

I might just leave it at temp and let the heat pump do it's thing. I don't think heat pumps allow low temp settings nor do they like doing the yo-yo of temperature changes.

Exactly. You are in a little bit of a pickle because if you go too big, you may run a little less efficient. If you optimize the unit, then you can't go back and throttle the temperature if you want to save on electricity costs.

I understand that the mini split heat pumps are designed to handle varying loads better. You may find that the best and most versatile solution for you as suggested above.

 

[theoldwizard1]

-24 X 24 X 10
-2X6 framing 24" O.C.
-Wall will be insulated with fibreglass and attic will be blown in. Vapour barrier through out.

How much insulation in the attic ? 30cm ? 100 cm ? 500 cm ? Big difference !

FYI, "low temperature" heat pumps (i.e. those capable of supplying heat down to about -30C, actually run a "defrost" cycle. This is critical, especially in your climate (cold, damp), to prevent the external unit from icing up. I am not sure if this is enough to prevent snow packing.

If you have any water lines inside, you need to have a backup heat source in case of a power outage.

 

[old__man]

How much insulation in the attic ? 30cm ? 100 cm ? 500 cm ? Big difference !

It will most likely be around 13-14 inches giving me an R40 value. The walls will either have an R20 or R24 insulation. Not sure what I can source locally yet.

FYI, "low temperature" heat pumps (i.e. those capable of supplying heat down to about -30C, actually run a "defrost" cycle. This is critical, especially in your climate (cold, damp), to prevent the external unit from icing up. I am not sure if this is enough to prevent snow packing.

Indeed. They usually also have heated drain pans as well.

If you have any water lines inside, you need to have a backup heat source in case of a power outage.

No water lines but there will be no back up. Most houses I know have no back up heat in case the power goes out. It takes a long time for water lines to freeze in case of power failure though. With modern day PEX damage from freezing will hopefully be minimal. My last house had no back up and was on a well. Power failures really sucked there.

Come to think of it, the only back up heat source that I can think of that doesn't need power is either a portable kerosene/naptha heater or a wood stove.