How to Adjust Manifold Pressure

The manifold pressure gauge on a furnace measures the amount of pressure the furnace can handle. Most furnaces have a factory pressure setting of 3.5 inches water column (WC) and is a measurement of pressure, such as pounds per square inch (PSI). The pressure limits are stated on the furnace rating plate. If the manifold setting on the home furnace is not reading properly, it is possible to adjust it. The manifold pressure must be operating correctly or it could cause improper combustion of gas or fuel, which could cause carbon monoxide to be produced.

Manometer
Flexible tubing
Flat-head screwdriver

Instructions

- 1
  Figure out the manifold pressure from the rating plate on the furnace, which is usually 3.5 inches WC.
- 2
  Find the manifold pressure plug, which is usually found on the outlet side of the gas valve and is a tap located on the manifold.
- 3
  Turn off the furnace before testing.
- 4
  Connect the plug to the manometer with flexible tubing. Turn on the furnace and check the manifold pressure. If it is below or above 3.5 inches WC, the gas valve will need to be adjusted.
- 5
  Find the large cap on the manifold pressure and pop it off with a flat-head screwdriver. Beneath the cap is a slot screw where you adjust the pressure. Turn the screw in a clockwise direction to increase manifold pressure in in a counterclockwise direction to decrease it. You cannot go above the recommendations stated on the furnace and you can undercut it by 10 percent.
- 6
  Check the manifold pressure again with the manometer to make sure it is correct. Readjust the screw again if necessary.

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