A lot of utility companies or municipalities are asking for furnace efficiencies based on combustion analysis. Combustion analyzers are okay to use for the purpose of determining if there is excessive CO or CO² in the flue products and to see if the furnace is burning correctly, but they are not acceptable for trying to verify efficiency on 90%+ units. You need to let them know this.
The true efficiency of a condensing furnace takes into account the heat being recovered from the condensation process, which cannot be measured by a flue gas analyzer. So a combustion analyzer can never get an efficiency reading above about 89% even though the furnace has a “tested” AFUE of 96 to 98%.t
You also need to know that even on an 80% furnace, the efficiency readings obtained from a combustion analyzer will be only an approximation of the efficiency of the furnace.
The combustion analyzer can only measure flue loss, it can’t measure AFUE, which is what is publish in the any manufacturer’s literature.
Combustion analyzers are a great tool when servicing a furnace to see how it is working. This can be done as part of a clean and check of the unit.
As for what the CO and CO² readings you should be getting, the answer is: it depends. The readings depend on the model of furnace, the firing rate of the furnace, the length of vent and air intake pipe, the altitude, the type of gas as well as other lesser factors. So, in general, the CO² for furnaces should be between 5% and 9%. The CO should always be less than 400 ppm. This shows that the furnace is operating within the design parameters and that it has proper combustion.
So if you have a utility or municipality asking for combustion analysis efficiency, you will never show what has been design certified by AGA/GAMA for true AFUE by your manufacturer.
Combustion analysis is a good tool to use because it can show if there are problems with the combustion side of the furnace but the analyzer should not be use to determine a furnaces efficiency.
If you want to do combustion analysis of a furnace, there are any number of excellent meter for doing this.
Even though they claim they can calculate combustion efficiency, they can only get close since they cannot look at the total combustion heat exchange process.
So, if you have a utility or municipality pushing for efficiency of a furnace based on combustion analysis, refer them to the manufacturer’s literature or to AGA/GAMA for that information.