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As a homeowner/mechanical engineer who's always trying to understand how things work, it's bugged me over the years that even some heating pros disagree about whether an 85% boiler is really 85% efficient. What does it really mean when the boiler tech sticks his combustion analyzer probe in the flue, and the analyzer spits out the number 85% efficient?

I think I finally understand one source of confusion/disagreement. Heating pros, please feel free to correct me if I go wrong (and I know you will!)

For starters, I'm talking only about "combustion" efficiency. That means, how much energy gets lost up the flue vs. how much gets transmitted into the boiler mass itself and the water that passes through it.

The other efficiency measure, "thermal" efficiency, is a different subject for another time.

Back to combustion efficiency. In doing some online reading, I found this is a useful guide/reference.
https://tsi.com/getmedia/02417ee5-cccc-4dc7-80bc-f7f10924d20a/CA-basic-2980175?ext=.pdf

It seems that a major source of confusion/debate stems from the fact that when oil or natural gas burn, they produce two types of combustion products: "dry gas" (O2, CO, CO2, etc) and water vapor.

Typical combustion analyzers measure the amounts of dry gas (O2, CO, CO2) and the flue gas temperature, from which they can calculate (or use pre-programmed lookup tables to calculate) how much heat energy remains in the exhaust gas stream, as a percentage of the total available energy in the fuel. That's energy that gets lost up the chimney.

For example, in my 85% efficient oil boiler, the tech sticks his probe into the flue, and the analyzer measures the amount of O2, CO, CO2, and the flue temperature, and deduces that the exhaust gas stream still has 15% of the heat energy that was in the oil before it burned.

However, the combustion analyzer does NOT measure how much water vapor is also in the flue gas, and how much energy that vapor is carrying away. This is also called "latent heat." And the percentage of latent heat lost in water vapor varies by fuel. In heating oil, it's about 6-7%. It's higher in natural gas, because gas combustion produces more water.

So the REAL combustion efficiency of my oil boiler is 100-15-7=78%. 15% is lost to "dry gas" in the flue, and 7% is lost to "latent heat," or water vapor.

So if I want to calculate how many useful BTU's my boiler is outputting per gallon, I can multiply the BTU value of oil (138,490 BTU/gal) times 0.78 = 108,022 BTU useful heat per gallon. From my 85% (but really only 78%) efficient boiler.

Now to complicate things further, the heating industry is well aware of the fact that boilers like mine cannot condense and recover that water vapor that's costing me an extra 7% efficiency loss, because condensation would ruin my boiler. So in recognition of that fact, the heating industry gives me two DIFFERENT BTU values for heating oil: a "high heating value" (HHV) which I used in the above example, and a "low heating value" (LHV) which already has the latent heat of the water vapor subtracted from it. I found slightly different values of HHV and LHV for heating oil, but the two that I'm using here are:

HHV: 138,490 BTU/gal
LHV: 129,488 BTU/gal

Again, the HHV includes the latent heat of water vapor, and the LHV does not.

So, another way to calculate the useful heat output of my 85% (?) efficient boiler is to use the low heating value of 129,488 BTU/gal, knowing that the BTU's lost as latent heat of water vapor have already been subtracted.

So now I don't have to subtract that 7% for the water vapor. So my 85% boiler really is 85% efficient, if I use the low heating value for oil that already has the latent heat of water vapor subtracted from it.

So 0.85 x 129,488 = 110,065 BTU/gal useful heat output. (Note that this is about 2,000 BTU different from the previous calculation because I used a round number of 7% for the latent heat earlier, where the difference here between HHV and LHV is 6.5%)

So the truth is that my 85% efficient boiler is really 85% efficient ONLY if I start with the low heating value (LHV) of oil, which is about 130,000 BTU/gal.

But if I start with the high heating value (HHV) of oil, which is about 139,000 BTU/gal, then my 85% efficient boiler is really only 85-7=78% efficient.

That's my current understanding as a non-pro homeowner/engineer, and I welcome correction if I have gone astray. But this example helps me understand one of the sources of confusion about combustion efficiency and what it does (and does not) measure.

(Please also note that this does NOT get into combustion efficiency measurements for condensing boilers, which try to recover most or all of that latent heat by condensing the water vapor. Since I do not have a condensing boiler, that's a different discussion. But the principles are the same, ie that some heat is lost as "dry gas," and some (but hopefully much less, and ideally none) is lost as the latent heat of the water vapor.)