Thoughts on proposed HVAC for detached garage in Portland

We’re starting to build a 1000 sq ft detached garage in Portland (CZ 4C). I’d like to keep the garage at a tolerable temperature in both winter and summer, so maybe heating to 55F and cooling to 80F. I’ll be doing occasional weekend projects in the garage, so I’d like to be able to adjust the conditioning a little more during those times (~ 65F heating, 75F cooling).
Our builder’s HVAC sub recommended Mitsubishi MSZ-GS18NA & MUZ-GS18NA units for the garage. I asked if he has a Man J calc to back it up, and he said that the sub “has done these enough times to know what works well”, unfortunately a typical response in my experience. So I’m hoping for some wisdom from the great folks here about this proposed system.
I ran a LoadCalc calculation with some assumptions (results below), which show about 18000 BTU needed for heating to 65F. If I read the Mitsubishi brochure correctly (also attached), it can provide up to 18200 BTU at 17F outdoors, which almost never happens around here. So I think this is a good match/fit?
Another thing that was mentioned to me here is that some systems are designed to heat to higher temperatures (e.g. 70F), whereas I want to heat to about 55F. Reading the operating manual for this system, it looks like I can select a temperature as low as 50F (attached), so that seems OK too.
Finally, it was suggested to me to consider a resistance/IR heater for those times when I don’t want to bring the whole garage up to a higher temp, but just want some heat directed at me when I’m working there. What would be a good unit to consider?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part


Replies