Measuring and adjusting manifold gas pressure

What is going on

Manifold pressure is the pressure at the gas valve outlet that actually feeds the burners. I check it with a heat call active, and I set it on high fire first. If the flame looks wrong or the furnace is over or under fired, this is where you look right after you have confirmed supply pressure is good. Take it slow when you turn the regulator screw, because small turns move the number a lot. Gas work is for the licensed tech.

Likely causes

• Flame is lazy, yellow or lifting from the burners
• Furnace is over or under fired after a conversion or altitude change
• Regulator out of adjustment
• Supply pressure not within limits

Check in this order

1. Shut off gas at the manual gas valve and remove the outlet pressure tap plug
2. Loosen (do not remove) the outlet tap screw with a 3/32 allen wrench
3. Connect the manometer with 5/16 in. I.D. hose
4. Turn gas on and apply a heat call
5. Set manifold pressure to 3.5 in. w.c. (natural gas) or 10.0 in. w.c. (LP) on high fire
6. Remove the regulator cap and turn the adjustment screw to set the pressure
7. Replace the regulator cap securely
8. Shut off gas, remove manometer and hose, and tighten the inlet tap screw
9. Turn gas on, apply a heat call and check for leaks with an approved detector

More furnace guides

• Annual and start-of-season inspection
• 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