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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.

I agree with @SuperTech & @tim smith above. The number don't favor mod cons unless you can keep them condensing.

If the Homeowner can do his own service that helps.

I am very Leary due parts availability and parts prices. A blower motor assembly can cost 1/4-1/3 the cost of a whole new boiler. After 10-15 years (maybe less) the mfg has moved on to a different design and parts are no longer available


I have seen mod cons work great on the right installation.


We used then to heat a 100,000 gallon tank located outside that stored fire sprinkler water at 50 degrees. They condensed all the time and two 200,000 btu mod cons replaced an oversized 1,500,000 btu gas fired atmospheric boiler.

They save enough in gas costs to pay for the entire job in less than a year. But that is a commercial job.

As @tim smith mentioned residential is different.

Here's another thought. Have you considered an Energy Kinetics EK1 Frontier? Again, I'm a homeowner not a pro, but many pros here have high praise for the Energy Kinetics. The benefits for you are that it's higher efficiency than a standard cast iron boiler, but it's non-condensing, so you don't get the problems associated with a condensing boiler. If you get the stackable EK1 Frontier, you get a compact system with a 40-gallon water tank for your hot water.

https://energykinetics.com/system2000-quietest-most-efficient-boiler/

So that might be the best of both worlds for you: higher efficiency with the good reliability track record of the Energy Kinetics, without going to a condensing boiler that brings more problems. And the stackable design is more compact than a traditional cast iron boiler with an indirect tank.