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Calculating furnace efficiency

Peter_Rogers | Posted in Mechanicals on

I know (or at least I think I know) that to figure steady state efficiency, divide the BTU output by the input. But many furnace spec plates don’t have a BTU input listed. Instead they’ll have Gallons (or Litres) Per Hour. Now, if 1 gallon of residential fuel oil has about 138,500 BTUs, can you do a straight calculation using these numbers?

The example I’ve got in front of me right now is a Kerr Gemini furnace (K5RE-115) with a Riello Burner. 96,000 BTU/hr output, 0.85 USGPH input. Nozzle size is 0.75. Can somebody help me figure the efficiency of this model with this info?

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  1. Peter_Rogers | | #1

    my thought was to do the following: 96,000BTU/hr / (138,500BTU/hr x 0.85) but this results in an efficiency of 81.5%, which is much lower than this energy star model furnace should be....

  2. GBA Editor
    Martin Holladay | | #2

    I can't locate any specs on a Kerr Gemini furnace that has an output of 96,000 BTU/h. But a furnace with an output of 91,000 BTU/h has an input of 105,000 BTU/h -- so that's an efficiency of 86.6%.

    If the model number of your furnace ends in 115, there's in a good chance that it has an input of 115,000 BTU/h. If the output is 96,000 BTU/h, those numbers give you an efficiency of 83.5%.


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