How to Diagnose a Bad Furnace Gas Valve (10 Things To Check in 2021)
Channel: Fox Family Heating and Air Conditioning
Date: 2021-01-02

If you're trying to figure out how to diagnose a bad furnace gas valve, this is a video that explains what the gas valve does, why it's important, how to figure out what's wrong with it, and 10 other things you should check before condemning the gas valve.

If 24 volts is coming from the board to the gas valve terminals and you don’t hear that little clicking noise the internal valve makes, you could have a bad gas valve.  To double-check, take the leads off to the gas valve and check there.  Got 24 volts?  Then something downstream of that 24 volts is not working.  

Here are 10 things we can check when we think we have a bad gas valve, before condemning it:

1. Check the wires to the gas valve.  Are they cracked or frayed?  That could mean a couple things.  You have a REALY old furnace, or something could have scorched the wires.  Things like that.  Replace the wire and continue your diagnostic.

2. Check the coil at the gas valve.  If you check the resistance of the coil, by putting your two meter leads on each terminal, and it reads OL, you have a bad coil. There are more involved things here but let’s keep this straightforward.  

3. The gas coming into the valve should be at utility line standards.  In my neck of the world, it’s around 7”-10” wc for natural gas.  There’s a port on the inlet side to check it. 

4. The burner orifices could be plugged.  A furnace that has sat for the summer without being run can be the victim of a spider spinning a web inside the burner orifices.  Now, that’s a tiny spider, I know, but I promise, it happens!  Take a small piece of thermostat wire and gently poke inside the holes of the orifices attached to the manifold and try to fire up the system again.

5. The flame might be coming on for a few seconds but then shuts off.  Is there a dropout of voltage or gas pressure to the gas valve?  That’s something to check for sure.  And you can do that by putting a “T” fitting in line with the hose to hook your manometer up to.  Check the inlet and the outlet side to see if the pressure is dropping on either side of the valve.  

6. Another reason the flame could drop out after only a few seconds of burning is the flame sensor.  If the sensor doesn’t detect the flame, the gas valve will be told to shut down by the control board. 

7. If the flame does anything but shoot directly into the hollow metal heat exchanger, a safety can trip.  One safety is the rollout switch.  Sometimes you’ll get a little part of the flame that drifts off to the left or right.  This will set the switch off.  That doesn’t mean remove the switch, that means fix the problem.  Clean the end of the burner assembly nearest the heat exchanger.  Rust will sometimes build up on the crossover channels.  Use a wire brush to clean and see if that solves it.  Then place the burner correctly into the channel.

8. The other safety that can cause the system to cut the gas off to the valve is the high limit switch.  If the furnace runs for a few minutes, then shuts off, something could be causing the inside of the furnace to get too hot. The first thing I would check is to see is if the evaporator coil is dirty.   I have a great video that shows what a dirty evap coil looks like and what it takes to clean it.

9. The other reason the high limit could open is the blower motor speed could be set too low.  Check your installation guide as a reference for where the settings should be.

10. Check the ductwork too.  These last three have all dealt with airflow.  If the return duct is crushed, then we’ll have low airflow again.  Visually check the return duct and feel around it if it looks questionable.  If the duct is not perfectly round, then this could be the problem. The furnace is suffocating. 

00:00 Intro
0:40  Sequence of operation of the furnace
3:51 Troubleshooting a furnace gas valve
5:22 Ten Things to check on a furnace gas valve
8:38  Installing a furnace gas valve
10:59  Recap

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