What the Gas Valve on a Furnace Does

The gas valve sits in the gas line inside the furnace, just upstream of the burners. When it is closed, no gas reaches the burners. It opens only after the thermostat has called for heat and the furnace has checked the draft and the safety switches. It is a control, not a manual gas cock. The manual shutoff is the handle on the pipe outside the cabinet, and that one stays where the utility and the installer left it.

What a normal start looks like

You hear the inducer, a pause while the pressure switch proves draft, a click as ignition starts, then the burners light and the blower comes on after the heat exchanger warms. If the sequence stops before the burners, the valve may be staying closed because a switch told it to. A failed pressure switch, a blocked vent, a flame sensor that is dirty, or a limit that is open will all keep the valve shut. Replacing the valve first misses those.

What not to do

• Do not jumper the valve, push the solenoid, or bypass a rollout switch or a limit. Those switches are closed because the furnace thinks combustion is unsafe.
• Do not take the valve apart. Spring, seats, and the correct replacement are a tech's job, and the joint is a gas joint.
• If you smell gas, do not look for the valve. Do not flip lights. Leave the house and call the gas utility or emergency services from outside.

A furnace that will not light, with no gas odor, is a heating repair. The diagnosis includes the safeties that tell the valve whether it is allowed to open. A new furnace is a later conversation, after those parts have been tested.

Common questions