Boiler efficiency comparisons from really old boiler to AFUE

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Old 01-04-11, 08:46 PM
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Boiler efficiency comparisons from really old boiler to AFUE

First, let me say that this forum is great, I have learned a lot about how to maintain and repair my homes hydronic heat system from reading threads. I finally had to register in order to post this question though as I couldn't find the answer in previous posts.

My relatively newly purchased home still has the original Federal Boiler that was installed in the mid 60's when the house was build. The model is a TC-140 if that means anything to anybody. It is still working fine, thanks to threads I read on this board, but I am trying to figure out the payback period/cost savings of upgrading the boiler. The problem is that I have no idea what the efficiency of the installed boiler is. I found a plate that says the following:

A.G.A. Input BTU/Hr: 140000
A.G.A. Output BTU/Hr: 112000
Net Output BTU/Hr: 84000

This data implies that my simple output/input efficiency is either 60% or 80% depending on which output number I use. Also, new boilers are rated in AFUE. Any comments on how to compare simple output/input efficiency and AFUE ratings? Thanks.

WyoBoy - Exiled to CO
 
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Old 01-04-11, 11:59 PM
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Any comments on how to compare simple output/input efficiency and AFUE ratings?
You can't.

AFUE is a laboratory test and while it is useful for comparing similar boilers it cannot be directly compared to any field testing. AFUE is as much calculated as it is empirical in that it attempts to make a determination of the ANNUAL fuel consumption of a hypothetical home during an entire year.

As for your Federal boiler, if it is what I think it is I can guarantee it isn't all that efficient when compared to any modern boiler. Under the best of circumstances I think you would get somewhere around 60% overall efficiency and generally much less. Post a few pictures of the boiler and I can probably tell you more.
 
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Old 01-05-11, 05:20 AM
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You could do a heat loss calculation on the house, and then find your fuel used in BTU and local Heating Degree Days value (HDD) to do a BTU/HDD/sqft calculation.
The heat loss number would allow you to compare to other structures (and you'll probably find some issues with a 1960's house).
The BTU/HDD/sqft will show a combination of the efficiency of your structure and the heating system.
 
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Old 01-05-11, 10:27 PM
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Thanks for the replys, Furd and DaveC72. It is completely understandable that AFUE ratings and simple steady state thermal efficiencies are different. But one would think that there would be some general (rough) rule of thumb for comparing the two. But to begin, does anybody know how the efficiency of boilers was calculated before AFUE came along? For instance, given the BTU/hr numbers I posted originally what is the thermal efficiency of my old beast of a boiler? I don't understand the difference between A.G.A. Output and Net Output.

WyoBoy
 
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