How do you replace the gas valve in your furnace?

Answer 1

Short answer: get a licensed Gas Fitter to do the job.

Long answer: as a 34-year HVAC veteran service technician, here's how I would do it.


1. Confirm that the valve is definitely At Fault by confirming that the gas is on, and that there is "the applied voltage" (typically 24 volts) to the valve, and that the valve will not operate. A gas valve is usually rather expensive.
2. Shut off the gas and electricity to the furnace.
3. Identify all wires connected to the valve, either by drawing a diagram with each lead identified, or by taking photos. Then disconnect the wiring.
4. If there is pilot tubing attached to the valve, disconnect it. Be aware that it can be difficult to properly install pilot lines, and if you cross-thread the fitting, you will have to replace the entire gas valve, again.
5. If there is a gas relief line from the valve to the pilot assembly, disconnect it. In all likelihood, your new valve will not have a connection for a relief line, so you can just leave it disconnected. (The relief line is a relic from the days of "diaphragm gas valves," which used gas pressure to operate the valve itself.)
6. Using 2 pipe wrenches, disconnect the gas piping from the existing valve.
7. If there is room to spin the valve in place in the furnace, disconnect it there. Otherwise, it will be necessary to remove the valve, manifold, and burner assembly to get the valve out.
8. Clean all pipe threads using a wire brush. Apply dope or gas-rated Teflon tape in the approved fashion (set back 2 threads from the end of the pipe, overlapping by 1/2).
9. Tighten all gas connections using 2 wrenches. Reconnect the pilot tubing (if present). Reconnect the wiring per your diagram or photos.
10. Attach a manometer and tubing in order to read and calibrate manifold pressure.
11. Turn on the gas and leak test the connections using a combustible gas leak detector, soap, and/or meter dial test.
12. Turn on the gas valve itself (it is usually shipped in the OFF position). If it is a "standing pilot" valve, light the pilot according to the instructions.
13. Turn on the furnace electrical power, and set the thermostat to call for heat.
14. When main flame lights, confirm manifold pressure. It should not exceed the pressure listed on the furnace nameplate (often 3.5" W.C.), but may under-fire by up to 10%, if desired.
15. Shut down the furnace and remove the manometer and tubing. Reseal the pressure tap.
16. Turn the furnace back on and check all gas connections downstream of the gas valve (using a combustible gas leak detector and/or soap).
A note: when using a combustible gas leak detector, it is not unusual to pick up a small amount of gas at the outlet of a valve (ie, at the burner ports), when the heat is shut off. It's been my experience that somewhere between 10%-25% of all gas valves do not seal off completely.


The HVAC Veteran