HVAC/Furnace gas valve.. how to test wiring?

[rattlsnak]

My upstairs gas furnace seems to be working half of the time.. I went up and checked it out and the gas valve is not opening. (no "click" or initial light off) I know the sequence of events is.. thermostat calls for heat, control board send signals to start the inducer fan motor, then the pressure sensor verifies that, then the ignitor heats up, then the gas valve is supposed to open and light the burners. But mine will not light the burners.. so then everything shuts off after @ 5 seconds.. no gas is coming through the valve so it's down to two things. the control board not sending the signal to the valve, or the valve itself is bad.. Every video I watched on how to test that, shows a 2 or 3 wire gas valve, but mine has four wires, so not sure which ones to check to see if the 24v is there or not. Anybody know??

[unclebilly]

Take the hose off the pressure sensor and blow into it when the inducer fan is running, see if it lights.

[masraum]

Originally Posted by unclebilly

Take the hose off the pressure sensor and blow into it when the inducer fan is running, see if it lights.

Many years ago on this board I did a TON of troubleshooting for a similar issue. It's been 10+ years, so I am not confident that I would point you in the right direction, but there is an old thread of mine. Just in case that helps...
HVAC - Furnace and fan question

In my case, it was the pressure switch that failed, and then the cheap ($16) plastic pressure switch failed a year later, so I bought a slightly more expensive ($26) metal pressure switch and that fixed the issue permanently (until I sold the house).

My old thread may or may not help. Good luck.

[rattlsnak]

Originally Posted by unclebilly

Take the hose off the pressure sensor and blow into it when the inducer fan is running, see if it lights.

I will try that tomorrow, but it is my understanding that the ignitor will not light if the pressure sensor does not sense that the fan is running and mine is working..

[Cajundaddy]

If the combustion blower runs and the ignitor glows then you are almost home. Failed pressure switch, damaged pressure sensor tubing, failed ignition control board, or gas valve.

Mouse chewed sensor tubing is a common problem.

[Baz]

This probably will not help, but I always have to bleed off air when I start mine up at the beginning of each season, before it will work. Does that every year. I only use my natural gas in the winter.

Sounds like yours is already working - but inconsistently - which is different.

[Arizona_928]

Get out your multimeter and test for some voltage... or if you have the one that goes over a wire to measure current...?

[billybek]

Hot surface ignitor? May be cracked. If it appears to have a white spot on it, it is probably done.
Can you take a photo of the wiring diagram?
Check the hoses on the pressure switches for cracking.

[ltusler]

There should also be a flame sensor, it may be corroded, pull it and sand it with some emery cloth and try again. Then order a new one.

[rattlsnak]

Originally Posted by ltusler

There should also be a flame sensor, it may be corroded, pull it and sand it with some emery cloth and try again. Then order a new one.

I never get a flame to get that far..

This is how it is supposed to work:

1. Heat call from t-stat
2. The inducer fan comes on
3. pressure switch checks to see if fan came on..
4. the igniter heats up
5. the gas valve opens
6. all the burners light
7. the flame sensor detects a flame
8. the plenum heats up
9. blower fan comes on.

I dont make it to step 5. Everything up to that point works, so either my gas valve is bad, or the control board is bad. Trying to figure out which wires to test for the 24v to which one is the problem. Voltage there, bad valve, no voltage, bad control board. The wiring schematic just shows all four wires going from the board to the valve. I think there may be markings on the valve. Ill check that tomorrow.

[fatima mehru]

To test the wiring on your four-wire gas valve in the HVAC/furnace system, start by understanding the sequence of events in the heating process. With the furnace calling for heat, check for 24V signals at the designated wires leading to the gas valve during each stage—inducer fan motor start, pressure sensor verification, and ignitor heating up.

If the gas valve is not opening and there's no "click" or initial ignition, focus on these 24V signals. Using a multimeter, measure voltage across the relevant wires. If the 24V signal is absent when it should be present, it suggests an issue with the control board. If the signal is present, the problem may lie with the gas valve itself. This process helps you identify whether the wiring or the valve is causing the malfunction in your furnace's heating sequence.