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foresthillsjd
Member Posts: **52**

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?

(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?

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## Comments

12,695I enjoyed Andre the Giant. Great guy, great comic, great actor. And perhaps it is perverse of me -- but his performance in

The Princess Brideis really wonderful.Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch

6,9274,1039,523As 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.

5,734Albert Einstein

9,523