Gas & Oil Appliance Inspection

Gas Valve Types and Operation

The gas valve is the primary control device on a gas-fired heating appliance. It controls gas flow to the burners in response to the thermostat signal. BPI Heating Professionals must be able to identify valve types and verify proper operation.

Standing pilot valve - Found on older equipment. A small pilot flame burns continuously. When the thermostat calls for heat, the valve opens the main gas port. The pilot lights the main burner. Standing pilot valves waste energy because the pilot burns year-round (approximately 500-800 BTU/h, or $30-50 per year in gas).

Intermittent pilot valve - The pilot only lights when heat is called for. An electronic ignition module generates a spark to light the pilot, then the pilot lights the main burner. More efficient than standing pilot because no gas is consumed when heat is not needed.

Direct spark ignition (DSI) - Eliminates the pilot entirely. An electronic spark ignites the main burner directly. Most common on modern furnaces and boilers.

Hot surface ignition (HSI) - A silicon carbide or silicon nitride element heats to approximately 1,800-2,500 F and ignites the gas directly. The most common ignition method on modern residential furnaces. HSI elements are fragile and have a limited lifespan (3-7 years).

Manifold Pressure Testing

Manifold pressure (also called outlet pressure) is the gas pressure at the burner manifold with the burners firing. Correct manifold pressure ensures the burners receive the right amount of gas for proper combustion.

Standard manifold pressures:

• Natural gas: 3.5 inches water column (IWC) - this is the standard for virtually all residential natural gas appliances
• Propane (LP): 10.0-11.0 inches water column

Measuring manifold pressure:

1. Locate the pressure tap on the gas valve (usually a 1/8-inch NPT fitting)
2. Connect a manometer or digital pressure gauge
3. Fire the burners
4. Read the pressure at the manifold tap
5. Compare to the appliance nameplate specification

High manifold pressure causes overfiring (too much gas), producing higher CO, overheating, and potential heat exchanger damage. Low manifold pressure causes underfiring, poor flame quality, and incomplete combustion.

Gas Burner Inspection

Visually inspect the burner assembly for:

• Rust or corrosion - Corroded burners have degraded orifice sizes, altering the gas-air mixture
• Debris or blockage - Spiders, insects, and lint can block burner ports, causing poor flame distribution
• Flame quality - See Lesson 3 for flame quality assessment
• Burner alignment - Misaligned burners can cause flame impingement on the heat exchanger
• Crossover tubes - Must be clear and properly positioned to ensure all burners light from the igniter

Gas valve types range from standing pilot (oldest, least efficient) to hot surface ignition (most modern, most common). Manifold pressure must be 3.5 IWC for natural gas and 10.0-11.0 IWC for propane. High pressure causes overfiring and increased CO; low pressure causes poor combustion. Inspect burners for corrosion, blockage, misalignment, and debris.