Gas Heating: Combustion, Pressures and the Safety Chain
Combustion, gas pressure and the safety chain
Gas heat questions center on combustion and the sequence of operation. Know the sequence: call for heat, inducer proves draft through the pressure switch, the ignition source energizes, the gas valve opens, flame proves through the flame sensor, the blower starts on a time/temperature delay, and the high limit protects the heat exchanger. Any failed proof shuts the valve.
Know the pressures: natural gas manifold typically 3.5 in. wc. with about 7 in. wc. supply; LP manifold 10-11 in. wc. with 11-14 in. wc. supply. Measure manifold pressure with the burner firing, using a manometer on the valve outlet tap. A classic trap is reading it with the burner off or with the hose on the inlet side. Temperature rise (return to supply) must fall inside the rating-plate range, commonly 35-65°F.
Other traps: assuming a hot surface igniter is good because it glows (check amp draw and position), ignoring combustion air and venting category, overlooking altitude derating (about 4% per 1,000 ft above 2,000 ft), and misreading CO and CO2 readings.
Worked example (clocking the meter): the dial moves 2 ft³ in 60 seconds with natural gas at 1,000 Btu/ft³. Input = (3,600 ÷ 60) x 2 x 1,000 = 120,000 Btu/hr. Compare that to the rating plate; a large mismatch points to overfiring, wrong orifices or incorrect manifold pressure.