Commissioning an 80% Furnace With Conflicting Data
An 80% AFUE furnace with a nameplate input of 110,000 BTU/hr and a rated temperature rise of 40-70°F is being commissioned. The technician records: manifold pressure 3.2 in. w.c. (nameplate 3.5), gas meter clocked 2 cubic feet in 60 seconds, supply plenum 128°F, return 68°F, external static pressure 0.82 in. w.c. against a 0.5 in. w.c. rating, blower on the highest speed tap, and combustion readings of 9.1% CO2 and 60 ppm CO air-free. The installer says the temperature rise is 'in range' and signs off.
1. Calculate the actual input, output, and the airflow implied by the temperature rise. Is the unit firing correctly?
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Input from clocking: 2 cu ft in 60 s = 120 cu ft/hr. At 1,000 BTU per cu ft that is 120,000 BTU/hr, about 9% above the 110,000 nameplate. If the local heating value is 1,030 BTU per cu ft, it is higher still.
Output: 120,000 x 0.80 = 96,000 BTU/hr.
Temperature rise: 128 - 68 = 60°F.
Implied airflow: CFM = output / (1.08 x rise) = 96,000 / (1.08 x 60) = about 1,480 CFM.
Is it firing correctly? No. The unit is overfired by roughly 9%, and the data are internally contradictory: 3.2 in. w.c. manifold is below the 3.5 nameplate, which on the same orifices should produce less input, not more. The plausible explanations are oversized or drilled orifices, a higher-BTU gas supply than the clocking assumed, or a meter-timing error. Each has a different fix, so the next step is to verify the gas heating value with the utility and inspect the orifice sizing before touching manifold pressure. Do not simply raise pressure to 3.5 - that would increase the overfire.
2. Evaluate the claim that the temperature rise is 'in range' and therefore acceptable.
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'In range' is a derived result, not an independent pass. Temperature rise equals input divided by airflow, so the same 60°F can be produced by a correct input with correct airflow or by an overfired unit with excess airflow.
Here it is the second case. The blower is on its highest tap against 0.82 in. w.c. external static, far above the 0.5 in. w.c. rating. That means the duct system is restrictive and the blower is near the edge of its curve. Two consequences follow:
- Delivered airflow will fall as the filter loads and as static rises further, and the temperature rise will climb with it, potentially toward the high limit.
- The apparent pass depends on the highest blower speed, which is not a condition the system can be expected to maintain reliably.
The 9.1% CO2 and 60 ppm CO air-free are acceptable, but they were taken at the overfired condition and must be repeated after the input is corrected. A rise reading taken once, at one blower speed, on an overfired appliance, does not close out commissioning.
3. What is your recommendation, and what would you record to support it?
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Recommendation, in order:
- Verify the gas heating value and confirm the clocking method and timer accuracy.
- Inspect orifice sizing; correct input to nameplate by changing orifices if needed, then set manifold pressure to the nameplate value.
- Re-check temperature rise at the corrected input; if airflow must be increased, address the duct restriction and filter rather than defaulting to the highest blower tap.
- Re-run combustion analysis and confirm CO air-free stays low after the input correction.
Record: meter clocking volume and time, heating value used, supply and manifold gas pressure, orifice size, temperature rise before and after, supply and return static pressures separately, filter size and measured pressure drop, blower speed and tap, combustion readings before and after, and the equipment model and serial numbers. A one-line note that the unit was left overfired, or left on the highest tap to keep the rise in range, is the kind of documentation that protects both the homeowner and the technician when the high limit trips next winter.