When identifying combustion air problems, NCI defines it as a misdirection of combustion air, not the lack of it. NCI also says combustion air affects all burners and equipment in the same room, but that definition goes out the window in the case of sealed combustion.

What is the Explanation for this Condition?

The first time I saw this was also on a two-pipe condensing furnace. The difference was the outside combustion air was piped into the side of the burner box rather than the top and middle. The CO readings were near 1000 ppm!

Adjusting the gas pressure lower diluted the CO readings but did not lower them to an acceptable range. After removing the burner box cover to inspect the burners and the flames, the furnace came back on, and the CO readings were less than 20 ppm and stable.

The contractor placed the cover back on the burner box while the furnace was still running, and the CO rose back up. He took it back off, and the CO went back down. What was happening?

I determined that because of the combustion air location in the side of the furnace, most of the air was going to the closest burners. Not enough air was getting to the farthest burner. How does this happen? When the cover is on, you can’t see all the burners. When the cover is off, everything is open to the same amount of air, and there is no problem.

Okay, it was an educated guess. We know how draft hoods can pull flue gases from some heat exchangers, but not all of them. Why couldn’t a draft inducer create a similar problem with air?

Back to the question as to why there wasn’t rising CO? Answer: There was enough draft through the heat exchanger to remove all the flue gases but not enough to bring in enough combustion air to the burners.

Combustion Air or A Mechanical Problem?

Was this a misdirected combustion air issue or a mechanical problem? I would say it could fall into both categories. On the other hand, I would consider this a design flaw and put it in the DAM category.

So it doesn’t matter why this is happening. What matters is what we do about it. After all the service work, the O2% is above 9%, and CO is above 100 ppm. The measurements are close and definitely within industry standards.

Economically is it worth spending more time trying to get this furnace within NCI standards? The answer is no. We must realize that as contractors, our job is to make the equipment do what it can do based on its own rating. Therefore, in this case, we have done our best.

Is it okay for the O2 to be below 6% or for the gas pressure to be lower than 3.5-in.? Any measurement is acceptable if that is what the equipment requires.