Furnace size needed for new build skid shack

I’m building an 8 x 18 foot skid shack with decent insulation- I’m looking at the Empire LP direct vent 25,000 btu unit and wondering if I can go smaller, like a 10k or 15k heating furnace?

I have a 10×20 skid house and use a 20,000BTU ventless. Works great. We had a 10,000BTU heater but that took a bit too long to get everything warmed up.. Traded straight up for the 20,000 from a buddy who had a much smaller shack & haven’t looked back.

A 15k would work. 20k would be better. Especially if you keep it warm all season. If you warm it up from frozen I would go with the 25k unit.
If you have gas lights they are good for 2k btu constant with a infrared factor that helps warm up the adjacent surfaces.

I put the 25000 btu empire direct furnace in my wheel house and haven’t regretted it for a second. My house is 6.5 x 14 and is spray foamed so it’s quite easy to heat and the furnace rarely works hard. One time on Red Lake on a weekend trip with the wife and kids it got down to about -25 and the house stayed nice and warm.

One thing I did you modify mine a little was installing a 12v brushless computer fan behind the front cover to circulate the warm air and that helps a lot. Empire offers a fan option that mounts in a similar fashion but my $5 computer fan is quiet, still moves a lot of air and has very minimal battery drain.

Also, I would definitely recommend direct vent just for the lower moisture levels inside the house. Likely one reason you are considering it but I also sleep better knowing it’s vented outside.

It’s an added expense to upgrade now but cheaper than going with the smaller unit and then having to change it out later

I went with a RV heater in my 10×24 shack. Warms up quickly and with the forced air, it moves the heat. I have done this in the last 2 houses I’ve had and have been very happy with them.

https://www.airxcel.com/rv/suburban/furnaces/sfq-series

I built an 8 x 18 wheelhouse with spray foam insulation. I have a 20,000 BTU Suburban forced air. One duct up front and I ran another to the middle of the house. I’ve spent many nights on the ice with temps below -20 and never had an issue.

If I were in your spot, I’d stick with the 25,000 and not have to worry about it. If you have a family the best way to keep them out on the ice is to keep them warm and comfortable.

Nice to see some ice talk in August!