El Mac;
I'll add my 2 cents worth in hopes that it helps. I am not an installer, but have one in the family. My shop is brand new, and the heater is too. I live in an area that sees a few months of 20s' for lows, and 40's for highs. We can go weeks here without seeing the sun if we get a big blast of cold & wet from the north.
The big deal on choosing appears to be cubic feet of heated space, not so much square feet.
A 20x30 area in my shop has 9 ft finished ceilings. 3 windows dual pane, and an insulated 8x16 door.
Another "wing" of the shop is 24x30, with 12 ft. walls and scissor trusses to a finished
14~ ft. ceiling. That area has 3 small dual panes and an insulated 10x9 roll-up door.
So about 1320 square ft., I can't remember the cubic ft. number.
Propane is the only real choice for me for clean heat in the shop. We heat the house with a woodstove. My guy told me 80k BTU in a 86% efficent furnace would be more than sufficent. I have about 2 lbs. of metal in my spine, I hate the cold, and wanted Tee-shirt and jeans comfort.
He said okay we can bump that up. I pushed it beyond, and now have a 125k BTU Reznor Shop heater hanging on the wall. It is overkill, I know, but I tend to go that way.
We also spent an extra $50.00 or so on a smart thermostat, which can hold a min temp and then come up to my 68ish work temp at the times I plan to play. My guy says it costs less to create a base temp and hold it than to let the shop get colder and heat it up to work temps.
We shall see.
I look forward to being warm in my playroom this winter.
