Gas Boiler: Go Big or Go Small? (how much, house)

[purelygeneric]

My house is on the borderline of needing 140,000 BTU or a 105,000 BTU burner. I figure spending an extra $100 to go bigger makes sense - but I'm not sure. Any pros/cons?

[Kbinspections]

Great question......there is a formula that is very complex to rate how much BTU's you need. No one pays to have it done. I would say 80% of all boilers are too big for the house they are serving. Why? Because the plumber will spend the extra money to ensure you will never call back for lack of heat. What does he care, you will be paying the gas bill....

[LifeInLI]

^^kbinspections hit it right on the head. You really need to have a heat-loss calculation done.

[rocafeller05]

Yes, heat loss calculation is a must. Even if your just doing a replacement. Boilers are the most oversized equipment. Just dont forget to add for your domestic water heating if thats an option.



Home Heat Loss Calculator

[krewat]

Originally Posted by purelygeneric

My house is on the borderline of needing 140,000 BTU or a 105,000 BTU burner. I figure spending an extra $100 to go bigger makes sense - but I'm not sure. Any pros/cons?

Fuel oil, or natural gas (or propane) ?

From my experience with my gas-fired baseboard heat, unprofessional, but having worked on gas/fuel-oil and even gas-fired steam boilers: When my thermostat calls for heat, the burner turns on and the circulator pump starts running. As the burner heats the water, it gets up to 180 or so degrees, and then the BURNER turns off, but the circulator keeps running, heating the house. As the water temp falls back down to a certain point (160?), the burner comes back on and heats the water back up to 180 degrees. This cycle continues until the thermostat turns off the entire system because the house temperature (at the thermostat) has reached it's setting.

A larger burner (all other things being equal) will just cycle more. If the efficiency is the same as the smaller burner, it shouldn't make a difference in fuel costs. There is a small amount of time when the thing first fires that can make a difference, but it's probably negligible. Of course, there is a point where "too big" is just too big, but you get the idea.

A burner that is too small, when it's really cold outside, will run and run and run and not be able to keep the temperature in the loop at optimal temperature, and cause the house to get cold.

If it were me, I'd go for the larger one, since you're on the edge of needing it anyway.

There are various other concerns when talking about fuel costs, and little things like how far the thermostat allows the temperature to fall after turning off can make a BIG difference in comfort, efficiency, etc. Too much of a difference between on-and-off at the thermostat, you'll get chilly before the heat kicks back on. Too LITTLE of a difference, and the end of the loop will be cold, and the burner will be coming on and going off too much. Even opening and closing the flap on the baseboards in different rooms, versus how close those baseboards are to the thermostat, can make a difference.

[rocafeller05]

Nope. A bigger boiler means bigger gun(burner) and more fuel being used everytime the boiler fires. You will be heating more water and using more fuel then really whats needed.

[krewat]

Originally Posted by rocafeller05

Nope. A bigger boiler means bigger gun(burner) and more fuel being used everytime the boiler fires. You will be heating more water and using more fuel then really whats needed.

I am far from an expert, but I think it just means the water heats faster, so it doesn't stay on as long. Talking about natural gas, not oil. I don't play enough with oil burners to remember if they have a high-temp cutoff and cycle on-and-off while the circulator runs, do they?