Heat loss and btu heat needed

[PWC Repair]

I'm close to getting my heat up and running in my shop but wondering about ACTUAL btu's needed. After all, I won't be keeping it at 72*........more like 60*. So, I've done a few heat loss calculators I've found online and they all seem to agree at 34000-38000 btu. Is this actually what's needed in heat? Here's what I can say for sure....my shop will hold 10* more than outdoor temp with no heat on at all when it's cold. For instance, if the low for the night was 20* and I go to the shop at 9am and it's 28* out, it will be about 38* in the shop. I'm sure I gain heat from the ground. I dug into the hill on my build and the backside 48ft wall is 4ft below grade. Our frostline here is only about 14". I figure I have about 25-35% of my concrete that wants to stay 50*. Today it was 43* outside and 53* in my shop. At 2:45pm I decided to light my kerosene radiant heater (16000btu) and watch time vs temp rise. At 4:00 it was up to 58* in the shop. At 5:00 it was 60* in the shop and had dropped to 41* outside. At 6:30 it was still 60* in the shop and 40* outside. I then shut it off and locked up. Just a bit ago at 8:00 I went back down to check temp drop it was 41* outside and temp inside the shop had dropped to 55*. Shop is 29x47 inside dimension, 12ft sidewall, open span, 17ft peak height (all open), insulated with 3" foam sheets. Two 9x8 overheads, steel wrapped styrofoam core. One man door, steel wrapped foam with 9 pane glass.

Now, armed with this info can any mathematical guru tell me how many actual btu's of heat I will need to maintain 60* in the shop? The little radiant heater will bring heat up pretty quick like that if it's 38* in there too. I'm thinking that's pretty good considering the inside volume up to the roof peak.

 

[PWC Repair]

Just went out and checked shop temp this morning. Still 51* in there. Local airport weather shows it dropped to 39 at about 1am and then 37* at 5am. Still 37* out right now.

 

[theoldwizard1]

BTU calculations are more "black magic" than mathematical !

Remember that "thermal mass" from the floor and the hillside works both ways ! It will want to pull the inside temperature down to 50*. If you could put temperature probes spaced every one inch from the outside wall to say 2' away you would see this.

 

[PWC Repair]

3" foam sheets, r18 everywhere.

 

[Jim greengo]

What kind of heater are you planning on using?

 

[PWC Repair]

Hydronic floor heat, also hung and plumbed in a fan coil unit. I'm confident everything will work, just trying to figure out how much it will run. I'm using a 50 gallon electric water heater. Pics on last page of my shop build in link below.