Gas Valves, Manifolds, Burners, and Orifices
What the exam tests
Gas valve construction and adjustment, manifold pressure, burner/orifice matching, flame quality, and primary/secondary air.
Core rules
- A combination gas valve contains a manual valve, redundant safety shutoff, pressure regulator, and operator(s). The redundant valve prevents raw gas flow if the main valve leaks.
- Manifold pressure: natural gas 3.5 in. w.c.; LP 10–11 in. w.c. Adjust only with a manometer and the burner operating.
- Orifices are sized by drill number or Btu rating. Converting natural gas to propane requires smaller orifices and a different regulator spring; altitude derating is typically 4% per 1,000 ft above sea level for natural gas.
- Primary air mixes at the burner; secondary air surrounds the flame. Too little primary air gives yellow tips and soot; too much lifts or blows out the flame.
- Burner flames should be stable blue with a well-defined inner cone; impingement, floating, or roaring indicates misalignment or overpressure.
Worked example
A natural-gas furnace rated 100,000 Btu/hr with four burners uses 25,000 Btu/hr per burner. A #43 orifice at 3.5 in. w.c. is rated near 25,000 Btu/hr; a #45 would be too small and cause incomplete combustion. If converted to propane, install the manufacturer's LP orifices and set manifold to 10 in. w.c.
Traps
Do not adjust manifold pressure to fix airflow; do not mix orifice sizes; do not use a standard drill to enlarge an orifice. A gas valve that opens but does not regulate may show correct lockup pressure but wrong flowing pressure.