In reviewing my historic gas bills, the most therms we ever used in a month was 278. Of that, 6 went to the dryer and hot water. For academic purposes (I was a math major, you see), I calculated that it turns out to be
(278-6 Therms )/ 30 days/ 24 hours x 99,976 BTU/therm = 37,768 BTU/hr in the coldest month of the past four years.
So that month happens to be January 2018, where the average temperature was 33 degrees Fahrenheit.
If I plug in a design temperature of 33 degrees, with a heating temp of 70 degrees into my heat loss calculations, I get a heat loss of 28,279 BTU/hr. This is assuming .75 air changes per hour.
So, would it be safe to say that 28,279/ 37,768 = 75% is a good approximation of my efficiency?
Now, if my exact model of boiler was properly sized, it could have an efficiency of 83%. (but in theory less because again, not every day is a design day)
But by that calculation, this massively oversized 145,000 BTU/hr steam boiler is making my gas bill only 10% higher than it should be. (83%/75% - 1)
Now, if I fudge with the infiltration numbers a bit and say my house is tighter than I thought (.5 air changes per hour), the efficiency drops to 66%, which makes the gas bill 25% higher than it could be. But definitely no more than that.
Is that calculation correct? The boiler, whom I have nicknamed Andre the Giant, was purchased by the previous owners. I can just imagine the plumber saying, "Well, I could sell you the right-size boiler, but the bigger one will keep you three times warmer..."
It seems that if we are still comfortable with this gigantic boiler (which we are!), that I have very concrete numbers saying wait until it dies to change it. Sorry polar bears and penguins, your ice caps are melting a little more because of me!
I'm curious, what kinds of numbers do you guys get when you plug in your gas bill/boiler/heat loss numbers?