Measuring manifold gas pressure

What is going on

Manifold pressure is measured at the gas valve outlet with a manometer after the furnace reaches steady state. It tells you whether the valve is delivering the right pressure to the burners. Compare it with the Supply Line and Manifold Pressure table.

Likely causes

• Manifold pressure out of range, causing yellow, lifting or unstable flame
• Wrong orifices for the fuel
• Gas valve regulator needing adjustment or replacement
• Bad supply pressure upstream

Check in this order

1. Remove the threaded plug from the outlet side of the gas valve and install a field-provided barbed fitting, with the gas off.
2. Connect a manometer to the barbed fitting.
3. Start the unit and let it run 5 minutes to reach steady state, because pressure shifts as the valve warms up.
4. Check that the flame is stable, does not lift from the burner, and burns blue for natural gas.
5. After 5 minutes, record the manifold pressure and compare it with the Supply Line and Manifold Pressure table.
6. Shut off the unit, remove the manometer and barbed fitting, and replace the threaded plug.
7. Leak-check the plug with bubble solution. This is gas work, so use a licensed technician.

More furnace guides

• Annual planned service inspection
• Annual service checklist
• Blower access panel and carbon monoxide hazard
• Burner assembly cleaning
• Change blower speed tap
• Check temperature rise and adjust blower speed