Measuring supply gas pressure

What is going on

Supply, or inlet, gas pressure is measured at the gas valve with a calibrated manometer. Do it with the furnace and all the other gas appliances on the line running, since pressure that looks fine at idle can sag under load. Gas work belongs to a licensed technician.

Likely causes

• Undersized or restricted gas line causing pressure drop under load
• Regulator at the meter or tank out of adjustment
• Low supply pressure showing as weak flame or pressure switch problems
• Supply too high for the valve, risking over-firing

Check in this order

1. Shut off gas at the manual gas valve outside the unit
2. Loosen (do not remove) the inlet pressure tap plug with a 3/32 allen wrench
3. Connect the manometer with 5/16 in. I.D. hose to the tap
4. Turn on gas and run the furnace and all other gas appliances on the line
5. Read the line pressure and adjust it to 5 to 10.5 in. w.c. for natural gas or 11 to 13 in. w.c. for LP
6. Shut off gas, remove the manometer and hose
7. Tighten the inlet tap screw with a 3/32 allen wrench and replace the plug before turning gas on
8. Turn gas on and check all joints for leaks with an approved detector

More furnace guides

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• Blower codes 66, 61 and 60 - what to look for
• Blower does not start after burners light
• Burners go out before the heat call ends / short cycling
• Canada only: derate high altitude by lowering manifold pressure
• Cement PVC vent joints