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Gas Bill Math - Consumption versus boiler size

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Member Posts: 114
edited April 2019
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?

• Member Posts: 23,655
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Your math looks OK to me -- there is a lot of slop in all the numbers for heat loss (as we all have noticed before!). Your apparent efficiency is, perhaps, a little low -- but not so much so as to be concerning, never mind surprising. And illustrates quite nicely why I, at least, don't fret about replacing a perfectly good boiler just because it is oversized. It may help with the warm fuzzy feelings, but it doesn't pay. When you do replace it, somewhere down the road, you can get it more neatly sized.

I enjoyed Andre the Giant. Great guy, great comic, great actor. And perhaps it is perverse of me -- but his performance in The Princess Bride is really wonderful.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 15,840
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I agree with most of the above, Oversized boiler will have larger jacket loss (heating the basement) which is not all bad but you want the heat upstairs but the larger boiler is keeping your floors warmer so all is not lost. 2d thing is increased standby and chimney loss do to a boiler that cycles more has more standby loss, cooling off between cycles and then reheating where a smaller boiler would be on more and off less in cold weather. Tough to put #s on that
• Member Posts: 6,505
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Your steam boiler needs to be sized by the EDR of the connected load/piping for maximum efficiency.

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• Member Posts: 9,546
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Typically heat loss programs are padded by up to 20%. The bottom line is you only need so many btus to meet the load. Whether the boiler is oversized, or right sized it’s a matter of how long it runs to make those btus.

As stated the inefficiency would be in the cycling of the boiler that’s oversized. The cost of that inefficiency of which is usually not enough to justify replacing with a right sized boiler.

The last statement can change, depending on how future fuel prices rise.
• Member Posts: 7,607
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I am not comfortable with the amount of WAG in your math. The heat loss is a scientific estimate, the infiltration rate is a full blown WAG
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 9,546
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The only way to get a good handle on infiltration is by doing a blower door test. Preferably in the winter months as the components of a structure can fit differently from summer to winter.