High Temperature Rise, Tripping Limit, and CO in the Supply Air
An upflow furnace rated 100,000 BTU/h input and 80,000 BTU/h output is rated for a 35–65°F temperature rise and 1,600 CFM at 0.5 in. w.c. external static. The technician measures return air at 68°F, supply air at 145°F, and total external static pressure of 0.95 in. w.c. (0.38 in. return plus 0.57 in. supply). The high-limit switch trips after about 12 minutes on nearly every cycle. The system has a 4 in. pleated filter ahead of a dirty A-coil, a single 14 in. return, and long 6 in. supply runs. A flue combustion test shows CO of 60 ppm air-free and O2 of 6.5%, both acceptable. A CO reading taken in a supply register shows 12 ppm, while the return reads 0 ppm. The customer has no CO alarms and reports the house "heats fine, just slow."
1. Calculate the airflow from the temperature rise and explain the limit trips.
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Use the sensible heat equation solved for airflow:
CFM = output BTU/h ÷ (1.08 × ΔT)
ΔT = 145 − 68 = 77°F
CFM = 80,000 ÷ (1.08 × 77) = 80,000 ÷ 83.2 ≈ 962 CFM
That is only about 60% of the rated 1,600 CFM. The rise of 77°F is also 12°F above the top of the 35–65°F range.
The static reading explains why: 0.95 in. w.c. total against a 0.5 in. w.c. design - nearly double - with 0.57 in. on the supply side alone, indicating a restrictive duct system, and a dirty coil plus a 4 in. pleated filter on the return side. Less air across the heat exchanger means each cubic foot absorbs more heat: the rise climbs until the high limit opens. The limit is doing its job; it is not the failed part.
Note the consequences of low airflow beyond the limit: higher flue temperature, higher heat exchanger metal temperature, and greater thermal stress on the primary and secondary sections. A unit running at 77°F rise is being cooked, which is exactly the condition that cracks heat exchangers.
2. Interpret the 12 ppm CO in the supply register. What does it indicate, what limits apply, and what should the technician do right now?
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A repeatable 12 ppm in the supply airstream against 0 ppm in the return means combustion products are entering the circulated air. That is a heat exchanger integrity finding until proven otherwise, and it is not something to 'watch.'
Important caveats before acting on a single number: confirm the measurement with a calibrated analyzer, sample several registers (not one), let readings stabilize, and make sure the probe is not picking up ambient CO from the room, a nearby vehicle, or the technician's own tools. A single 12 ppm reading can be an artifact - but a repeatable increase over return air cannot be ignored.
There is no universal 'safe' supply-air CO number; the practical rule is that any consistent rise above return/ambient warrants shutdown and inspection. Do not use the acceptable flue numbers (60 ppm air-free, 6.5% O2) to argue the unit is fine - good combustion at the burner says nothing about whether the flue products stay in the flue.
Immediate action: shut off the gas, tag the appliance, inform the occupant in writing, and do not restore operation until the heat exchanger is inspected and either passed or replaced. Prohibit the use of the furnace as a temporary measure without a documented decision by the responsible party.
3. Describe the inspection sequence you would use to confirm or clear the heat exchanger, and the order in which the airflow and heat exchanger issues should be resolved.
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Airflow first, because a heat exchanger cannot be fairly evaluated at 77°F rise - but safety first if the CO finding holds.
If the supply CO rise is confirmed, shut down and inspect before anything else. Otherwise:
- Reduce the restriction - replace the 4 in. pleated filter with a low-pressure-drop filter of the correct size, clean the A-coil, and measure static again. Address obvious duct deficiencies (undersized return, excessive equivalent length of 6 in. runs).
- Re-measure temperature rise with a calibrated thermometer in the return and supply plenums, burner door in place, at steady state, and confirm the rise falls inside 35–65°F and static is near 0.5 in. w.c.
- Inspect the heat exchanger visually with a bright light, then with a camera through the limit or burner openings; check the secondary section and the collector box gaskets on a condensing unit. Perform a blower-on/blower-off combustion comparison: a rise in flue CO or O2 shift when the blower starts indicates a breach into the airstream.
- Re-test the supply airstream after correction. If any CO rise over return persists, the heat exchanger is condemned regardless of how it looks.
Document all readings and the customer notification. A heat exchanger that has run at a 77°F rise for an extended period should be treated as suspect even if the immediate CO test is clean.