Allied Furnace: Measuring supply gas pressure

Measuring supply gas pressure on a Allied furnace: 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.

Homeowner with a furnace problem? Call (647) 657-2977 or book a service call — licensed technicians across Vaughan and the GTA.

Applies to: All sizes

Common causes of measuring supply gas pressure

• 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

How to fix measuring supply gas pressure: step by step

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.

If the unit shows an error code, look it up: Allied Furnace codes