Use which rating, DOE or I=B=R?


Old 08-07-11, 09:02 AM
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Use which rating, DOE or I=B=R?

To size a new boiler, should the heat loss calculation match the DOE or I=B=R rating? Also, if the output and efficiency ratings are based on a certain return water temperature, how would these numbers be affected if the return water temperature is 145 F? Thanks
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Old 08-07-11, 10:41 AM
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DOE. Efficiency of the system will be better with lower water temperatures. The output should be maintained with flow rates. Someone else with more experience will be a long and can probably provide a more solid explanation. I've never seen this angle discussed before. A properly sized system is based on the heatloss. Once that is determined, you calculate your water temperature based on the amount of radiation installed.
Old 08-07-11, 09:23 PM
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I just found this info: From Weil-Mclain's Boiler Replacement Guide page 6 "The boiler should be selected based on its Net I-B-R Rating in Btu/Hr." This raises the question of which is an more accurate efficiency rating, the AFUE rating (which seems to be based on the DOE output) or a ratio of the I=B=R output to the input? It seems to me that the more realistic efficiency rating is the latter and that, for example, a boiler listed with a AFUE rating of 84% could be performing at closer to 73%. Is there a similarity here with the difference between the EPA gas mileage for cars and real world driving?
Old 08-08-11, 09:25 AM
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Size to DOE. IBR is just DOE less 15% for 'piping and pick-up losses' or similar official-sounding thing. If the boiler and piping are in the envelope of the building, you don't need to account for that, and certainly not by 15% in a reasonably modern structure.

Actually, if you have a well-insulated structure and/or lots of heat emitter output potential and can keep the water temperatures in the condensing range, then it is perfectly reasonable to size a boiler to the gross input, as you are up around 95% efficiency and +/- 5% is in the slop of all the calculations anyway.

But getting back to the original question....

I assume you're looking at gas-fired, modulating-condensing boilers. These will do about 93-96% efficiency when they are in condensing mode. Condensing requires return water temperatures around 130F or lower. This will allow the combustion gases to condense in the heat exchanger. The latent heat of condensation that is released is what provides that additional 10-16% efficiency over a more standard non-condensing boiler.

Now, if you run that boiler outside of the condensing range, with return water temperatures well above 130F, then you are no longer getting that additional efficiency from latent heat of condensation. At that point, the boiler efficiency drops and is probably in the 85% range.

So, what you want to do is a heat loss calculation for the building. Then you want to determine how much heat emitter capacity you have at different water temperatures. For example, fin-tube baseboard puts out around 550 BTU/hr per foot with 180F water. If you have 100 feet of baseboard, that's 55,000 BTU/hr output. Now say that your worst case heat loss is only 30,000 BTU/hr. You can look at rating tables to determine output for fin-tube at lower water temperatures, like here:

From that, you see that you can use 130-140F supply water to meet the building heat loss. Assuming a 10-20F temperature drop from supply to return, you have 110-130F water coming back to the boiler. It will condense and you get that higher efficiency.

Now consider that you rarely ever have that coldest situation. By definition, it's only a few hours per winter. The rest of the year, your heat loss is less and you can use even lower water temperatures and continue to reap the higher efficiency.

AFUE is somewhat like EPA mileage ratings. Hard to measure in real life. But it is a standard....

What your personal AFUE will be depends on a large number of factors, like boiler sizing relative to heat loss, emitter type, control system, boiler type, etc. But AFUE is not to be confused with combustion efficiency, which is a different beast entirely.
Old 08-08-11, 08:11 PM
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I agree that you size the boiler off DOE. The latent heat recovery from condensate is 9%. It will drop when the entire boiler is not in condensing mode. By that I mean it is possible that only half more or less may be condensing. The efficiency of the mod/cons are affected much more than cast iron boilers as water temperature is increased above the AFUE testing standard of 120 return and 140 supply temp.
Once you get the heat loss done and calculate water temperature you will be able to make an educated decision to use a mod/con or cast iron boiler. If you have multiple zones you need to calculate the water temp in each zone and set the boiler limit to the hottest temp zone requirement.
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