How do you set gas pressure on a gas furnace?

Answer 1

1. Determine "manifold pressure" from furnace rating plate (often 3.5" W.C.) 2. Identify the manifold pressure plug. This is typically found on the outlet side of the gas valve; occasionally, it is a tap on the manifold itself.
3. With the furnace shut off, disconnect the pressure plug and install a barbed tap in its place. Connect this to your manometer with flexible tubing.
4. If the furnace is "direct vent"--it has two vents, one to bring in combustion air from outdoor, and the second to convey products of combustion back out--then you will need to use a "differential manometer" and connect the second pressure tap to the burner box, probably by teeing in to the pressure hose connecting the burner box to the gas valve and the pressure switch. If you skip this step, you will not read the proper manifold pressure, since direct vent equipment sets manifold pressure not to atmosphere, but to the negative pressure inside the burner box.
5. Fire up the furnace and measure the manifold pressure.
6. If it is incorrect, adjust it on the gas valve. There usually is a large cap that can be removed with a slot screwdriver. Under it is the adjustment itself, usually a slot screw. This often adjusts clockwise to increase manifold pressure, but adjust and observe.
6. You may not exceed the rated manifold pressure, although you may under-rate it by up to 10%.
7. Replace the cap over the adjustment screw and confirm your pressure reading.
8. At the same time, I recommend reading the Hot Surface Ignitor current (if present): on most furnaces, this should be 2A or higher.
9. Also simultaneously check flame signal: this should be at least 2 microamps (uA).


The HVAC Veteran

Answer 2

The right gas pressure for a gas furnace to operate well is between 12 to 13 WC. However, it can still operate as low as 10 WC.