Heating: Combustion Setup, Firing Rate, and Temperature Rise
The Heating domain tests gas-fired warm-air equipment and electric resistance heat: setting the firing rate, proving the temperature rise, and reading combustion to catch problems the flame alone will not show.
Firing rate and manifold pressure
- Set manifold pressure to the nameplate value: about 3.5 in. w.c. for natural gas, about 10 in. w.c. for LP, measured with the burner box closed and the unit running.
- Clock the meter: input Btu/h = (cubic feet timed ÷ seconds) × 3,600 × heating value, using 1,000–1,050 Btu/ft³ for natural gas.
- Derate above 2,000 ft elevation, roughly 4% per 1,000 ft, and change orifices when converting fuels.
Temperature rise
- Rise = supply temperature − return temperature, taken in the unit's own measuring locations after about 15 minutes of steady operation.
- It must land inside the nameplate range, commonly 35–65°F. High rise means low airflow; low rise means over-air or underfire.
Worked example
A furnace rated 100,000 Btu/h input shows one cubic foot of gas in 32 seconds. 3,600 ÷ 32 = 112.5 ft³/h; 112.5 × 1,050 = 118,125 Btu/h - about 18% overfired. Before condemning the valve, confirm the manifold pressure, the orifice size, and whether other appliances were drawing from the same meter during the test. Overfiring shortens the heat exchanger's life and usually shows as a high temperature rise.
Combustion checks
- Steady-state air-free CO in the flue should stay under about 100 ppm; rising CO with stable O2 points to a cracked exchanger, blocked flue, or flame impingement.
- Flue O2 commonly runs 4–9%, with excess air around 25–50%.
Traps that cost marks
- Clocking while a second appliance on the same meter fires.
- Reading temperature rise before the unit reaches steady state.
- Reporting raw flue CO instead of air-free CO.
- Ignoring altitude derate on a high-elevation install.