Gas Pressure on Residential Furnaces

Improper gas pressure can cause issues with your furnace if it is not properly set at the time of the installation. Overheating or under-heating can result in problems, ranging from damage to the furnace to not properly heating the space as intended.

Typical gas manifold pressure on natural gas is usually set between 3.2”WC – 3.7”WC. Manifold pressure is the gas pressure that the gas valve delivers to the burners, to supply the BTU’s.

manometerHow do you set the pressure? Before firing the furnace, you must connect a manometer to the gas valve, so that you have a reference point to check the pressure or change it if you need to. The manometer must be able to read in inches of WC (water column.)

Gas valves usually have one of two possible reference locations to take these readings. The most common is a 1/8” MPT plug located on the outlet portion of the valve, usually on the side of the valve. You remove the plug and insert a 1/8” MPT barb fitting. This fitting will allow you to attach a hose from your manometer to the barb fitting.

The other type is a small tower connection (nipple) on the top of the valve. It usually has a hex type screw that is inserted in the tower. You backseat the hex screw one turn and install your hose directly over the tower and connect the other end to your manometer. At this time, you can start the furnace and make your adjustments by turning the gas pressure adjustment screw clockwise to increase the pressure or counterclockwise to decrease the pressure.

If you have a two-stage furnace, you will have to set gas pressure on both the low-fire and high-fire operation. The valve settings are typically listed on the furnace’s rating plate, along with the desired temperature rise of the incoming air and outgoing air.

When all settings are done, be sure to reverse the procedure and make sure all reference points are closed, so as not to leak any gas.

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