Furnace Gas Valve Not Opening: Igniter Glows but No Flame

By HVAC Cheat Sheet Team · 4 min read

A hot surface igniter glows, the furnace waits, and nothing lights. After a few tries the board locks out. The gas valve gets blamed, but it's only bad if it gets 24 volts and gas and still won't open.

The sequence, so you know where it stops

Most modern furnaces follow the same order: call for heat, inducer starts, pressure switch closes, igniter warms up, then the board energizes the gas valve for a few seconds of trial for ignition, then the flame sensor has to prove flame. See furnace sequence of operation.

If the igniter glows but the valve never opens, either the board never sent 24 V to the valve, or it did and the valve didn't open.

1. The easy stuff

  • Valve switch or knob ON. It gets bumped during filter changes and service.
  • Gas supply open: the appliance shutoff, the meter, and on LP, gas in the tank.
  • Gas at other appliances? If the range and water heater work, the supply is probably fine.
  • Air in the line after gas work or a new valve: the first few tries can fail until gas reaches the burner.

2. Check for 24 volts at the valve

Put your meter across the valve's coil terminals (often marked MV and C, or with a two-wire plug) and watch it through the cycle. You're looking for 24 VAC right after the igniter warm-up, during the trial for ignition. It only lasts a few seconds, so be ready.

  • No 24 V at the valve: the board isn't calling for gas. Check the board's fault code (furnace LED codes). The usual reasons are an open safety in the valve circuit (rollout switch, limit), a pressure switch that isn't proving, or a failed board. Check for 24 V leaving the board's valve terminals; if it leaves the board but doesn't reach the valve, look at the wiring and any rollout switch wired in series.
  • 24 V at the valve and no gas: keep going.

Don't jumper the valve terminals or short the board's valve output to test. White-Rodgers warns not to short gas valve or control terminals to test, and Honeywell warns that shorting valve coil terminals, even briefly, can burn out the thermostat or transformer.

3. Check inlet and outlet pressure

Connect a manometer to the valve's inlet pressure tap (gas off while you open the port). With the valve energized:

  • Low or no inlet pressure: a supply problem, not the valve.
  • Good inlet pressure, 24 V at the coil, no outlet pressure: the valve isn't opening. Replace it.
  • Outlet pressure present but no flame: look at the burner orifices, igniter position, and wrong-gas setup (natural vs. LP).

Typical outlet (manifold) pressure ranges on a White-Rodgers 36J single-stage valve are 2.5 to 5.0 in. w.c. on natural gas and 7.0 to 12.0 on LP. Always set to the furnace rating plate. See checking furnace gas pressure.

4. If it lights, then shuts off

If gas flows and the burner lights for a few seconds, then goes out and retries, the valve is working. That's a flame-proving problem: see flame sensor microamps and cleaning and furnace ignition lockout.

Replacing a gas valve

  • Turn off gas and power. Match the replacement: 24 V coil, single or two-stage, standard or slow opening, inlet/outlet size, and natural vs. LP. White-Rodgers says not to use a natural gas control for LP or the reverse without the proper conversion.
  • Use the right pipe sealant, don't overtorque the valve body, and make sure a sediment trap is installed ahead of the valve.
  • Don't use a valve that's been underwater.
  • Leak test every joint with soap solution, never a flame.
  • Set and record the manifold pressure, then check temperature rise: furnace temperature rise.

FAQ

How do I know if a furnace gas valve is bad?

It has 24 V across its coil during the trial for ignition, there's good inlet gas pressure, the switch is on, and no gas comes out. All three conditions, then the valve is bad.

Can I test a gas valve with an ohmmeter?

An open coil confirms a bad valve, but a coil that reads normal can still have a stuck valve. The live test (24 V in, pressure out) is the real answer.

Why would the board not send 24 V to the gas valve?

The board only opens the valve after the inducer, pressure switch and igniter steps are satisfied, and with every safety in the valve circuit closed. A fault code usually points to which step failed.

Can you jump a gas valve to test it?

No. Gas valve and control makers warn against shorting terminals to test; it can burn out the transformer or thermostat and bypasses safety controls.

Sources

Parts:Gas ValveIgniterControl Board
Symptoms:No HeatFurnace Lockout

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