Combustion Analysis help on boiler

mx6er2587

I have an old 195k btu input American Standard boiler. Model GPM-7. Cast iron heat exchanger, cast iron burners. VR8304 gas valve adjusted to 3.5 in w.c.

I rented a testo 300 to perform a combustion analysis on it to see how it was performing and am a bit perplexed by the results.

O2, CO2, and CO all look ok to my eye, but the T-stack temperature looks high. 550-575F. I expected it to be 450-475F

Also CO seemed to improve a bit with the vestibule cover in place. Not sure what that indicates.

Is this temperature too high? Any place to start looking for issues? I had brushed out the heat exchanger earlier in the fall and it looks pretty clean.

HVACNUT

Switch the mode to check draft.

Burners are clean? Smooth crossover?

If the readings are at steady state, it's about what I'd expect. CO isn't high. <50 ppm is ok.

EBEBRATT-Ed

Your #s for that boiler look pretty good to me. In fact I used to have that same boiler but with less sections and lower BTUs. It probably dates to the late 60s-early 70s.

I changed the boiler when I sold my house a few years back although it ran fine because the new owners wouldn't bite on a 40-50 year old boiler.

Keep in mind that your basement ambient temperature is usually deducted from the actual stack temperature to get the "net stack temperature" so that would give you a net stack temp of 499 or so.

I don't know if the TESTO 300 does this deduction automatically for you or not' You could take the actual stack temp with a thermometer to figure this out.

Steamhead

I agree with @EBEBRATT-Ed - those are good numbers for a 1960s boiler. Fuel was cheap back then. There wasn't much incentive to make them efficient. Also, the sections in GPM boilers are stacked vertically, and I don't think they had baffles, so the GPM is pretty much "up and out". That helps account for the stack temp.

mx6er2587

Burners are clean and the draft was .08 in wc. Readings were taken after running for about 20-30 minutes and were steady.

@Steamhead is right the sections are stacked vertically and there are no baffles.

If 575 F isn't concerning for the Stack temperature, I'll let it keep soldiering on. Thanks all.

HVACNUT

-.08 is some pretty good draft. Like doubly good. Not much to do about it without replacing the flue.

pedmec

You're a little high with your O2 but that is most likely related to the high draft which would account for a higher flue gas temperature. The excess O2 is "just passing by" and stealing some of the heat with it. Other than that very minor issue I would leave it as is because safety comes before efficiency and trying to tweek it might cause your CO to spike.

ch4man

so the scale was cleaned on the fire side of the chambers, don't forget scale on the water side can also hinder heat transfer.