CSLB C-20 HVAC ContractorHeating SystemsMedium
A technician is adjusting manifold gas pressure on a new natural gas furnace with a single-stage gas valve. What is the typical factory-specified manifold pressure for natural gas at sea level?
- A7.0 inches W.C.
- B3.5 inches W.C.
- C11.0 inches W.C.
- D1.5 inches W.C.
Show answer & explanationAnswer & explanation
Correct answer: B. 3.5 inches W.C.
Standard manifold gas pressure for natural gas furnaces is typically set to 3.5 inches water column, while propane (LP) systems are commonly set to 10 to 11 inches water column due to propane's higher energy density requiring a smaller orifice and different regulation.
Why the other options are wrong
- A. 7.0 in. W.C. is excessive for natural gas and would overfire the burners.
- C. 11.0 in. W.C. is the typical setting for propane, not natural gas.
- D. 1.5 in. W.C. is too low and would cause underfiring and poor combustion.
Manifold Gas Pressure
The regulated gas pressure at the burner manifold, typically 3.5 in. W.C. for natural gas and 10-11 in. W.C. for propane.
- Measured with a manometer at the gas valve tap
- Incorrect pressure causes under/overfiring and poor combustion
- Set per manufacturer nameplate specifications
Memory trick: Natural gas: three-point-five; propane: eleven.