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AFUE vs. Combustion Efficiency vs. ??? Please make me smarter!!!

I'm looking for help understanding the different efficiency ratings.  I've got a 40 year old boiler that was serviced this am and the Tech rated it at 79% efficient.  What does this mean?  I can't believe that it equates to an AFUE of 79%...

What is the best way to compare and or calculate savings when looking to upgrade to a new boiler?




  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Combustion Efficiency vs. AFUE

    Combustion efficiency and AFUE are both calculated from measurements of the heat value of the fuel used, and the temperature and chemical composition of the flue exhaust gas. The AFUE also takes into consideration other factors such as standby and cycling loss, which more closely represent actual operation.A while back,I did a little searching, and found the DOE regulations for testing AFUE. Their tests are almost completely based on the ASHRAE 103 Standard. Although not all the info is there, I was able to get some details. As far as I can tell, the AFUE is based on an analysis of flue gas to determine steady state efficiency, so it is basically combustion efficiency test. The boiler is operated at rated input, and the input BTUs are calculated from the gas flow rate and the higher heating value of the gas used. Water is circulated through the boiler at 140 F out, 120 F return, plus or minus a few degrees. There is no mention of measuring the water flow rate, only that you want to maintain the 20 F delta T. Thermocouples are placed in the flue pipe to measure gas temp, and flue gas mass flow rate and CO2 are measured. These measurements are plugged into a complex formula to determine heat loss up the vent pipe, and the heat loss up the vent is calculated. The flue heat loss is subtracted from the BTUs in to give the output of the boiler. There is no mention of actually measuring the flow rate of the boiler water and delta T, and calculating how much heat gets to the water. It appears that AFUE considers anything that doesnt go up the flue useful output, jacket losses included. This is a simplification because many other factors are taken into consideration, cycling, standby heat loss, pilot energy use, etc, The formulas for calculation are complicated, and many assumptions are made. All in all, It appears that AFUE is basically a combustion efficiency test, and may or may not actually represent the efficiency you will achieve in a particular installation.
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