Gas Furnace Overfired at Nameplate Manifold Pressure
A residential natural gas furnace is rated 90,000 Btu/hr input with an 80% AFUE and a nameplate temperature rise of 35–65°F. The technician clocks the meter's 1 ft³ dial: two revolutions in 72 seconds. Gas heating value is 1,020 Btu/ft³ and manifold pressure is 3.4 in. w.c. against a nameplate 3.5 in. w.c. Supply air is 128°F and return air is 56°F at the plenum, measured with a calibrated probe after 15 minutes of run time. Total external static is 0.78 in. w.c. against a blower table listing 1,200 CFM at 0.50 in. w.c. The filter is new; the evaporator coil is upstream and visibly matted. The combustion analyzer shows 120 ppm CO air-free and 7.5% O2.
1. Calculate the firing rate from the clocking data and compare it with the rating. What does the manifold pressure tell you?
Read the worked answer
Clock the input first - this unit is overfired.
- 2 ft³ in 72 s = (2 ÷ 72) × 3,600 = 100 ft³/hr.
- 100 ft³/hr × 1,020 Btu/ft³ = 102,000 Btu/hr.
- Nameplate is 90,000 Btu/hr, so the unit is firing about 13% high.
- Useful output at 80% AFUE is roughly 102,000 × 0.80 = 81,600 Btu/hr.
Manifold pressure at 3.4 in. w.c. does not explain the overfire, so do not simply turn the regulator down. Check orifice size against the nameplate and any altitude derate, verify the gas heating value, and re-check the clocking method (dial size, revolutions, exact seconds) before adjusting anything.
2. Use the temperature rise to estimate airflow, then decide whether the high rise is caused by airflow or by firing rate.
Read the worked answer
Separate the two effects with arithmetic.
- ΔT = 128 − 56 = 72°F, above the 35–65°F range.
- CFM ≈ output ÷ (1.08 × ΔT) = 81,600 ÷ (1.08 × 72) ≈ 1,050 CFM - on the low side but not the dominant problem.
- If input were corrected to 90,000 Btu/hr (output 72,000 Btu/hr) and airflow stayed at 1,050 CFM, ΔT ≈ 72,000 ÷ (1.08 × 1,050) ≈ 63°F, inside the range.
So overfiring is driving most of the high rise. The 0.78 in. w.c. static and the matted coil still cost airflow and should be corrected, but do not slow the blower to 'fix' the rise.
3. State the correction sequence and the combustion checks required before sign-off.
Read the worked answer
Correct input, then airflow, then verify combustion.
- Re-verify the clocking before changing anything.
- Compare orifice size to the nameplate and altitude; install correct orifices rather than backing off manifold pressure, which alters flame shape and CO.
- Re-clock after the change; target 90,000 Btu/hr ±2%.
- Clean the evaporator coil and re-check static; confirm airflow matches the blower table for the required rise.
- Re-check temperature rise (should land near 60–65°F), CO air-free, O2, and draft.
120 ppm CO air-free is above the 100 ppm action level many technicians use, so inspect the heat exchanger and venting before signing off. An overfired furnace runs hotter than design and is a heat-exchanger failure risk.