Boiler efficiency comparison: non-AFUE rated versus AFUE rated.


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Old 07-11-11, 09:02 PM
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Boiler efficiency comparison: non-AFUE rated versus AFUE rated.

I am considering replacing my oil-fired boiler with, I hope, a more efficient natural gas boiler. But what is the efficiency of the current boiler? After my last annual service, the following test data was obtained: "Flue draft inches -.04, over fire draft -.00, comb CO2 ppm 8, net temp F 340, EFF % 81." Does this data refer only to the combustion or the overall efficiency of the boiler? How does my existing boiler compare with a replacement boiler of AFUE 84 % ? I am hoping for a more specific answer than " Your heater is 71 years old, and therefore must be inefficient."
Also, the above data was obtained after the annual service and cleaning. Would the efficiency be less before the cleaning? How much does the efficiency of an oil-fired boiler decrease over the heating season? Would a gas-fired boiler be cleaner and operate closer to its AFUE rating throughout the heating season?
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Old 07-12-11, 04:27 PM
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A gas boiler will be cleaner and cheaper to run. If your boiler is 71 years old, chances are it is way over sized for your home. You need to get a heat loss done on your house to properly size the new boiler. A properly sized boiler will cost less to run. You can also add some other controls to it that will improve your efficiency.
 
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Old 07-12-11, 10:42 PM
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After my last annual service, the following test data was obtained: "Flue draft inches -.04, over fire draft -.00, comb CO2 ppm 8, net temp F 340, EFF % 81." Does this data refer only to the combustion or the overall efficiency of the boiler?
This is only the combustion efficiency. AFUE is a laboratory test that cannot be replicated in the field and in my opinion is only useful for comparing different boilers, not in predicting fuel consumption in actual service situations.

AFUE takes several things into consideration and the standby losses of the entire heating system is one of them. Your current boiler probably has a considerably larger amount of water contained than a modern boiler of similar heating capacity. It probably has a fairly lousy insulation resulting in a high standby loss to the basement or boiler room. The larger water capacity means it will take longer to heat up on a call from the thermostat and that much of the heat input will eventually be lost due to the standby losses. The design of the gas passages in your existing boiler will mean that you have a higher stack temperature while firing and also high draft losses when not firing. I think that going to a modern boiler WILL result in a significant fuel savings from just these facts.

Yes, the combustion efficiency would be less at the end of the heating season (before cleaning) because the soot on the heat exchange surfaces acts as an insulator with corresponding higher stack temperatures. Every BTU that escapes with the stack gases is a BTU less to do useful work in heating the water. How much efficiency is lost over the heating season is dependent upon how clean (or dirty) the flame is and how much it burns. Tuning an oil burner for maximum efficiency WILL cause the gas passages to soot up faster, at least in an older boiler, than will tuning for slightly less than maximum initial efficiency. Since gas is inherently cleaner burning than oil the loss of efficiency over the course of the heating season is less than with oil.
 
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Old 07-13-11, 09:05 AM
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Properly sized boilers far outweighs the efficiency of the boiler when looking at fuel usage. The efficiency is not as important as it is changing as the water temperature changes.
Properly sized equipment keeps the boiler operating at a more steady efficiency as the water temp is not changing as rapidly.
 
 

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