Combustion Efficiency Test On Old Large Boilers

Joseph_4 · January 2010

Can someone please tell me where the right place to do Co2 test on an old large boiler. Yesterday I worked on an old American Standard A-5 boiler. The stack temp was 400. The co2 was 6.5. I drilled a hole in the flue before the regulator to do test. My chart says thats 76% efficiency. However it hasn't been cleaned in a long time. Last recorded test was in 1990 and then also reads 76% on the oil company's efficiency sticker. Some people told me on old boilers you do all the tests above the fire and not in a drilled hole in the flue pipe before the regulator. Others told me in the flue. The reason I feel unsure again is because on my own Peerless Jot 175 with Riello burner I recently got 575 stack and only after cleaning went down to 450. I feel the A-5 I worked on has its flue pipe coming out of the top of boiler like 5 feet above the flame. (its very tall) and thats why the stack is low temp. I can't believe not being serviced in many years, that its still same efficiency as 20 years ago.

Charlie from wmass · January 2010

Is your tester digital?

Has it been checked as of late for accuracy?

Steamhead · January 2010

The A-5

is an early pin-type boiler. You really have to watch your draft with this one, it doesn't have much internal resistance so too much draft will pull all the heat out of the boiler. If you can't get the flue draft down to -.03 or so, install a draft regulator that is one size larger than the flue pipe.

Also, what burner is on it? If you're using a burner that can develop a medium-to-high static pressure, it can blow the flame off the head and you'll get poor readings and soot. A Beckett AF (up to 3 GPH) or SF (to 5 GPH) will produce all the static pressure you need on this boiler. Even an AFG is too much burner for it. Ask me how I know that.

Also make sure you're using the right nozzle pattern. On the 5-section A-5 we service, I ended up using a semi because a solid caused the flame to slam the target, and a hollow produced very low CO2, likely the result of being a poor match for the air pattern of the AF burner in that boiler. I ended up with about 10.5% CO2 before making smoke- not the best but certainly acceptable for that boiler.

Going to a lower-rated nozzle and a higher pump pressure helps too.

You are definitely taking your readings in the right place. But if the burner is set up properly and you're still seeing too much air, you probably have a leak between the sections or around the flue collector. Compare the CO2 over the fire with that in the flue- if it's lower in the flue you have a leak. You'll have to remove the jacket to find and fix it.

Oh, and make sure it's clean! But you know that.

What size A-5 is this, and is it steam or hot-water?

Combustion Efficiency Test On Old Large Boilers

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