I have a steel 24x32 ft building, (768 sq ft.), with 2 inch spray foam on the walls and 3 inch on the ceiling, (10ft high ceiling), and was thinking of getting a heat pump to keep it above 50F. I have a wood stove but don't always want to deal with starting and tending a fire if I'm just going to work in it for a few hours, and if I don't use it for a few days I don't want the building to get cold soaked. I'm in central NY state. Since I'm not looking to heat up to 70 F most of the time I'm wondering if I can downsize the units I'm looking at, and what my electricity cost might run, or is it better to go bigger to deal with a few really cold days. We mostly see 20-35F but get a few days in the teens and near 0F. Also how the SEER rating would affect my costs. I've looked at some Daikin units that are on sale, I guess they are older units since the SEER rating seems to be lower. Cooling is not a concern as I have good shade in the summer and with the doors closed it's never gotten above 72F.
Just read the OP, didn't bother with any of the 2-pages of replies. I have a shop that's 2 floors on 20x30 footprint, spray-foamed walls and roof, six large Andersen 400 windows, two 10x7 foot machined M&T PVC overhead sectional doors with single-pane glass (19 sq.ft./ea?), and a solid wood man door. So, the walls and roof are super-efficient, but the doors are not the most efficient, as beautiful as they are cosmetically.
I run a Mitsubishi minisplit with MXZ3B30 outdoor, a MSZFE18 on first floor, and MSZFE12 on second floor, with the MHK1's controlling each. I keep it set with auto-changeover to stay within 55F - 85F year-round, whether I'm there or not, with my goals to avoid ever having to worry about paints and chemicals stored out there, and to not walk into a freezing or blistering shop when I have a free hour to get something do on a weekday evening.
Wonderful system, really happy with it most of the time, with the following exceptions:
- It gets pretty expensive to run when our temps drop below 20F. I assume this is because it executes more frequent defrost cycles, in addition to the efficiency drop you'd expect for a heat pump in cold weather, but whatever the reason... it pulls some serious juice.
- I cannot heat the shop very well at all in the teens°F. If it's 55F when I go out to the shop on a 16°F day, and I turn it up to 65°F, it might still be 55°F (or even 53°F!) at the end of the day.
- It's awesome having an air-conditioned shop, but opening overhead doors too frequently on an ultra-humid hot day can cause the coils to ice up.
- The drain pans can clog with wood dust (my shop is used for a lot of weekend-warrior woodworking), such that they need to be sucked/blown clean.
On that last point, insist your installer run a separate drain line from each indoor unit to outside, rather than using a tee/junction inside the wall. This way, you can simply connect your wet/dry shop vac to each outside drain for a few seconds as standard yearly maintenance, and avoid ever having a clog.