What temp for a shop?

Tom_133

Morning Wallies,

I have a shop build we did over the summer. it's a 4800 sqft steel building with radiant. It's currently set at 63 degrees, keeps up great so far. He has used 390 gallons of propane in roughly 36 days. Plus bringing the slab up blah, blah, blah.

The question: Is there a formula that shows potential savings at 60 degrees in the shop, or how much worse it could be to maintain 70. Does the ole saying once its at its set point it doesnt take anymore to maintain it than it would with a setpoint thats 10 degrees less.

Hopefully this made sense. He nor I are complaining about fuel use, just curious.

Thanks Tom.

jesmed1

The heat loss of your steel building is proportional to the delta T, or difference between indoor and outdoor temperature, which varies of course.

In Montpelier VT, the outdoor design temperature is -6 degrees, so at 63 degrees indoor you'd have a delta T of 69 degrees. (The outdoor design temperature means that 99% of the time, the outdoor temperature will be higher, so this is your "worst case" design target for a heating system.)

If you knew how much propane you had to burn per hour to maintain that 69 degree delta T, you could then scale that number with different delta T's to find your burn rate at different outdoor and indoor temps, but you don't know that number to start with.

So an alternate method is to look up the number of heating degree days for the period over which you burned 390 gallons of propane. For Montpelier, you had 1009 heating degree days over the last 5 weeks (35 days, which is close enough to your 36 day period). Now you can divide 1009/390=2.6 degree days per gallon.

So now you know that for every 2.6 heating degree days, you burn 1 gallon of propane. One heating degree day is 24 hours during which the outdoor temperature averaged 1 degree below your indoor temperature. So with an outdoor temperature of -6 and a resulting delta T of 69, you're probably burning 69/2.6=26.5 gallons of propane per day, or 1.1 gallons per hour.

Now you have a ballpark burn rate of propane per hour based on a delta T of 69 (1.1 gallons/hr). And you can scale that for different delta T's. Suppose you want to raise the indoor temp to 70, which gives you a delta T of 76. 76/69 x 1.1 = 1.2 gallons/hr. So now you're burning 1.2 gal/hr instead of 1.1 gal/hr.

Of course, 99% of the time your outdoor temp is above -6. On a more average day, say it's 30 degrees. Then your delta T is 63-30=33, and your burn rate is 33/69 x 1.1 = 0.5 gal/hr. Or for an indoor temp of 70 degrees, it would be 70-30=40, and your burn rate is 40/69 x 1.1 = 0.6 gal/hr. So on that day, it costs you 0.1 gal/hr more propane to heat to 70 vs. 63.

So now you can play around with different delta T's based on different indoor and outdoor temperatures. For any given delta T, divide by 69 and multiply by 1.1 to get your gallon per hour burn rate.

This ignores a number of complicating factors, but it's accurate to a first approximation.

Tom_133

I am going to have to chew on this for about a week, but if I figure it out and can actually do it, I feel like at that point I should fly to Las Vegas and bet it all!

Very cool answer, I will try it out for a bit.

Where did you find the degree day info?

pecmsg

Local weather data lists Degree Days. 

jesmed1

Tom_133 said:

I am going to have to chew on this for about a week, but if I figure it out and can actually do it, I feel like at that point I should fly to Las Vegas and bet it all!

Very cool answer, I will try it out for a bit.

Where did you find the degree day info?

LOL, you'll get the hang of it. I use this website to download historical/recent degree-day data for any location:

https://www.degreedays.net/

You can pick your nearest airport and enter its airport code, then specify the time period you want the data for.

hot_rod

Temperature bin data is another way to predict or look at the history for heat loads