how should you think about heating load for a garage?
I’m building a 3-bay garage (L-shaped with one 40′ tandem portion and a 20′ single car portion) which is approx. 600 sq ft in size and has 13′ ceilings.
I’d like to have this space semi-conditioned. 90% of the time, I will not heat or cool the space. However, because I’m planning on having solar solar batteries and electric vehicles in the garage, I’d like to heat the garage to just above freezing on the coldest days of the year when the temps fall below freezing (I’m in Southern Ontario for reference) so that the batteries continue to operate effectively.
My question is how should one think about heating load in a space like this where if the garage door opens up (to bring a car in or to take one out) all of the conditioned air in the space can be replaced with cold air within a matter of seconds.
My original plan was to use a single 1T ductless hyperheat minisplit for this space. But my concern is that if I open the garage to bring my car in on a cold winter’s night, and replace all of the 40 degree air in the garage with 10 degree air, will the single minisplit be powerful enough to raise the temps back above freezing in a reasonable timeframe?
A gas-fired heater is not an option in this space since I’m building an all-electric house. But other forms of electric heat are on the table if a mini-split isn’t the best answer.
Thanks!
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Replies
A minisplit will be fine for MAINTAINING the temperature when everything is closed, but the minisplit isn't a good option to deal with "catch up" after the door has been opened.
I think the easiest option for you here is to install a minisplit and an electric resistance unit heater. Set the unit heater's thermostat 2 degrees or so below the minisplit's setpoint. This way, after the door has opened and the temperature rapidly drops, the unit heater will kick on and do most of the work of bringing the space back up to the target temperature. Once the temperature get's close to what you want, the unit heater will shut off and the minisplit will take care of the last little bit of temperature increase. The minisplit will do ALL of the work of MAINTAINING the setpoint temperature, which is what you need most of the time, and the minisplit is much more efficient for long periods of operation.
Bill
Most of the heat capacity of a building is not in the air, it's in the building itself.
A one ton heat pump can raise the temperature of about 400 CFM of air by 30F. Your building is 7800 cubic feet, so that's 19 minutes of runtime to reheat all the air.
As a wild-eyed guess I would say your building has a heat capacity on the order of 20,000 BTU/deg F. Changing the temperature of the interior air by 40F takes about 4000 BTU. When that cold air comes in, heat will flow out of the building and warm up the air, because the temperature difference is pretty big. The building temperature won't have to drop by much -- a fraction of a degree. So the air temperature will rebound quickly to almost where it was, and then the heat pump will run for a while to get all the way back.
As a reference, I have a large 3 car garage that is well insulated and heated/cooled with a mini split. The wifey goes in and out of there a few times a day. 99% of the time the garage door is closed.
Concentrate on your air infiltration and your insulation. I think your mini split will do what you are asking.
BTW, why not build a well insulated box around the batteries inside the garage? I had 8 deep cycle solar batteries in a box like that in my cottage crawl space in Parry Sound during the winters after we had closed the cottage down. The solar panels continued to charge them and the batteries give off heat. They never froze.
Note that you should vent any boxes housing batteries like that.
Bill
Thanks everyone. This gives me a lot of comfort that I can plan for the single mini-split for now, and then worst case, can either add a small electric resistance heater near the solar batteries if needed.
If you're only worried about the batteries, you probably won't need the electric unit heater. I was thinking you wanted the garage comfy for people. The batteries won't cool down enough fast enough from a "door opening event" to really matter much -- the minisplit alone should be fine for that. For people, the cold air wouldn't be as comfy, and the unit heater would help to get the indoor AIR temperature back up quickly.
Bill