Unit heater size help

[2fst4u_7]

Just framed up a 40x80x16' pole barn and trying to figure out how big of a propane unit heater I should put in. Walls will be r21 and ceiling will be blow in with as much as I can throw up there. I found a few BTU calculators online that range from 60-110k BTU as a recommendation. That just seems very low to me. Any advice would be much appreciated! Thanks

 

[TurnipTruck]

I have an 80kbtu furnace easily heating my 40x60x14 in the banana belt of Alaska. R21 walls and R60 attic.
What is the rule of thumb? 35btu/ft sq?

 

[2fst4u_7]

I have an 80kbtu furnace easily heating my 40x60x14 in the banana belt of Alaska. R21 walls and R60 attic.
What is the rule of thumb? 35btu/ft sq?

Thank you sir!

 

[Bert_]

60-80 seems about right. At work I've got a 100k tube heater in a 40x100x16 and the heater is a little too big. Located in Nw Iowa

 

[beltfeed]

I would go with 2-unit heaters. If you have your heat source at just one end your temps are going to drop a lot at the other end. Try to induce an oval air movement path to help keep the temps even. You also need to look the efficiency of your heaters. Most ceiling hung unit heaters are only 80% efficiency. For the cubic foot of your building and your location I would use (2) 60-75K ceiling heaters if that is the heat source you are looking to use. One thing that comes into play will be if you are going to maintain a constant temp or keep a base line lower temp and increase when you are going to be in the building. A layout like this with a couple of fans would work decently. How low are your winter temps?

 

[ToolsRCool]

Are you heating it continuously, idle till needed, or on-demand then off? I would initially also suggest a 2-heater system, in case you do need a burst of heat the first one can't recover from quickly, such as opening a large overhead door or on-demand usage. Did you get insulated doors?

Propane is costly, any chance you would be able to use a high efficiency home furnace but jetted for propane instead of natural gas? I think they all have that option. I'm attempting some testing on this right now on a natural gas version, and it is going surprisingly well. It would gain you another 14% or so in heating efficiency.

 

[PoorUB]

I haven't seen a building I couldn't heat with 30 BTU per square foot, and you might get by with less. Using 30 BTU per sqft the OP's comes out to 96,000 BTU. Figuring 80% I would safely assume a 125,000 BTU, 80% heater would do it, perhaps a 100,000 BTU, 80% might do it.

 

[Airforce_NavyDAD]

I haven't seen a building I couldn't heat with 30 BTU per square foot, and you might get by with less. Using 30 BTU per sqft the OP's comes out to 96,000 BTU. Figuring 80% I would safely assume a 125,000 BTU, 80% heater would do it, perhaps a 100,000 BTU, 80% might do it.

30 is a good number unless there are ridiculously tall ceilings. We figure 25 here and works great.