Measuring supply gas pressure

What is going on

Supply gas pressure is what the utility or the piping hands to the furnace gas valve, and you measure it with the furnace firing at maximum rate. If it is too low, the furnace may light erratically or underfire, and you will see weak flame or lockouts. If it is too high, you can permanently damage the gas valve or overfire the unit. So it is worth measuring before you blame the control board.

Likely causes

• Low supply pressure from undersized piping or a restricted regulator
• High supply pressure from a utility regulator problem
• Other appliances on the same line dropping pressure when they run
• Gauge connected to the wrong tap or a loose connection giving a false reading

Check in this order

1. Use the 1/8 in. NPT plugged tap or pressure post on the gas valve for the gauge connection.
2. Check with the unit at maximum firing rate, since that is when pressure sags the most.
3. On multiple unit installations, check each unit separately, with and without the others running.
4. Confirm supply pressure is within the range in the Supply Line and Manifold Pressure table.
5. If pressure is outside the range, have the gas utility or a licensed gas fitter correct it. The furnace cannot fix it.
6. When finished, replace the plug and leak-check it with bubble solution.

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