The gas valve is the electrically operated valve that controls the flow of natural gas or propane to the furnace burners. When the control board calls for heat, it energizes the valve's coil, which opens and lets a metered amount of gas reach the burners to be ignited; when the call ends, the valve closes and stops the gas. It is both a metering device and a primary safety shutoff.
When a gas valve fails you typically get no heat: the igniter glows but the burners never light because no gas arrives. Other signs are intermittent ignition, an audible buzz or hum from the valve body, repeated clicking, or a faint gas odor. Failures come from a burned-out coil, internal debris, or simple age. Because gas is involved, this is not a part to leave to guesswork — diagnosis and replacement belong with a licensed technician who leak-tests the connections afterward.
Cost varies more than most furnace parts. A standard single-stage valve runs about $200 to $800 installed, with a national average near $415; the part alone is $75 to $300, and OEM units for high-efficiency or two-stage furnaces can push the total toward $850. The job itself is short — usually well under an hour. Because 'no heat, burners won't light' is also the symptom of a far cheaper flame sensor or igniter problem, a good tech rules those out first rather than jumping straight to the most expensive part in the ignition chain.