National Comfort Institute (NCI) training covers as many combustion problems as possible on equipment in the field. NCI created diagnostic charts for many different types of equipment. However, there will always be situations in the field that don’t conform to these charts.

Jim Davis, NCI
Jim Davis, Senior Instructor, National Comfort Institute

When the measurements we get don’t fall into charted areas, what should we do? It is important to note that we are talking about measurements outside NCI and possibly industry standards.

The following problem arises in this area.

The Scenario

An HVAC contractor finds the following after servicing a condensing furnace and doing a combustion test on a two-pipe installation – The combustion test measurements are as shown in Chart 1:

The NCI Combustion charts show that:

Combustion Air measurements - test in
Chart 1.
  • O2 should be between 6% to 9%
  • Flue temperature between 100 and 140°F
  • CO should be less than 100 ppm.

The O2 and CO should be stable, as show above.

The O2 and flue temperature indicate an underfired condition. The NCI diagnostic sheets show that the CO reading cannot specifically identify an underfired problem. This is because underfiring can cause CO to be high, over 100 ppm or 0 ppm, or anywhere in between. So the contractor called me for help.

Because the CO is above 100 ppm in this case, one way to determine if underfiring is causing the situation is to raise the gas pressure slightly. I asked the contractors to do that, and the new combustion readings are shown in Chart 2:

Second combustion air test results
Chart 2

These results show that it wasn’t underfiring, causing increased CO. The CO is over 100 ppm and stable. That only leaves a DAM problem – Dirt/Alignment/Mechanical.

What is the DAM Problem?

The contractor had just cleaned the furnace. He removed the burners, cleaned them, and carefully replaced them. But is it possible one of them might be off just a little?

In this scenario, the furnace had a sealed burner box, so the cover needed to be removed to see the burners and flames. I asked the contractor to remove the cover and do a visual inspection. What happened next might be considered weird. Here are his next set of measurements with the cover off. The results are in Chart 3:

Combustion air final testing
Chart 3

The O2 reading remained the same. CO dropped to an acceptable range and was stable. No change in the O2 reading indicates the amount of air entering the furnace stayed the same and eliminates combustion air as the problem.

But wait! The NCI Diagnostic Chart already eliminates combustion air as a problem on a condensing furnace. How can removing the burner box door fix a DAM problem?

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