Propane Unit Heater Sizing

[cr250dave]

Hello,

I've got a 30x40x12 pole barn that I had closed cell spray foamed at 3-4" on the ceiling and 2-3" on the walls. I've got one insulated 10x10 door, a man door and three small double pane windows. The slab is 5" thick and uninsulated. My location is near Colorado Springs at about 7,400'. We do have weeks in the single digits during the winter but most are in the teens and twenties.

I've tried calculating my BTU needs and have gotten anywhere between 83k to 250k BTU's.

I run CNC's out of my shop and ideally would need to keep it around 60F degrees.

My only option seems to be propane since I can't spare high load electric and want to keep the wood and pellet burners to the house only.

What size heater do you think would be appropriate?

 

[olytdi]

Can you elaborate on how you're getting such wildly divergent BTU results?

 

[The Cobbler]

my 525sqft 10' high walls well insulated garage, with large window, insulated sectional door & 2 man doors with lites , 2" foam under the floor does fine with 30K BTU ( similar temps to you )
I maintain 40° when i'm mot in there and up to 70° when I'm working in it

 

[cr250dave]

Can you elaborate on how you're getting such wildly divergent BTU results?

Here is the calculation I found that got me on the high side:

"If you have really good insulation you can simplify this number to 0.5, average insulation to 1, weak to 1.5, and 5 if you have no insulation. Take this number and multiply it by both the cubic feet and the amount of temperature rise that you want. Roughly the formula comes out to this: (Insulation * Cubic Feet of Garage * Temperature Rise) / 1.6 = # of BTU."

(1 x 14400 x 30)/ 1.6 = 270,000 BTU.

All the other online calculators and calculations put me in the ~80-100k BTU range.

 

[toyotadriver]

If you’re going to heat it all the time, I’d go with a 50k. If you are going to only heat it when you work out there, then you’ll need more BTUs and will probably want to go with a 60-80k.