Odd voltage to main gas valve.

adh99

Lennox G20 with electronic pilot. Not induced exhaust. Main problem is that the main burner will not ignite. All systems check out until I get to the voltage to the gas valve. Pilot voltage is 27 VAC and pilot works as expected; sensor heats up and turns off the spark; pilot continues. The main valve, which does not turn on as set up will not operate so the main burner never gets gas. Here is where it gets interesting. Take the main gas lead off of the valve and it shows 27 VAC, when it should. However, when the lead is connected to the valve and I take a parallel measurement of the main and common terminals I only read 14 volts AC and the valve does not open. Also, if I jump from the pilot terminal to the main terminal, the main valve will work, the main burner lights and the furnace heats up as it should. All grounds check out.

Any ideas? My guess is the ignition control unit (which does not have a status LED.)

Tim McElwain

Try here https://www.questargas.com/ForEmployees/qgcOperationsTraining/Furnaces/Lennox_G20_IOM.pdf

ratio

Correct voltage with no load (gas valve unplugged) & low voltage with load attached (gas valve plugged in) is the hallmark of high impedance in the circuit. A poor solder joint, crummy connection, burned relay contacts: any one of those could cause it. Check any plugs between the gas valve & the control board, then replace the board.

adh99

Thanks! I have combed this and found nothing that explains the difference in voltages.

Ironman

Safeties are in series. Whatever you're jumping out when the valve opens is what's impeding current from going to the valve.

As ratio said: you may get 27v with no load, but something may be impeding it with the load (gas valve) connected.

Ironman

I'm not sure of the board's internal circuitry, but check for continuity through these switches before condemning the board.

That's an old furnace, but well built. I installed many in the late '80's - early '90's.

Alan (California Radiant) Forbes

There's something internal to the ignition control module that's bleeding off current to the main gas valve terminal. Replace the S8610U.

mattmia2

You can measure the voltage at the different points in that circuit to find where the bad connection/contact is. One of them won't be the same voltage to the xfmr on both sides.

JUGHNE

The flue damper contacts are often a culprit.
If you check voltage across each safety switch it should be zero if the switch is closed. If the voltage rises when gas valve tries then that one is most likely failing intermittently.

Alan (California Radiant) Forbes

@adh99 said:

"Take the main gas lead off of the valve and it shows 27 VAC........"

and

"However, when the lead is connected to the valve and I take a parallel measurement of the main and common terminals I only read 14 volts AC and the valve does not open."

@mattmia2: Are you saying to test the voltage at each safety until you find the drop in voltage on a call for heat? Makes sense.

@JUGHNE: I don't understand. The voltage should rise when there's a call for heat, no?

Maybe you're both saying the same thing.

ratio

There's an unwanted resistance in the circuit. When the gas valve is disconnected, your meter reads full voltage; but when the valve is connected, the resistance reduces the voltage (& therefor power) & the valve can't open. You can find it by measuring across the resistance (probably coil contacts, connector, or bad solder joint)—it should measure right around 13 volts (27-14). Contacts that are closed should measure ≈0 volts across them.

Google "Kirchhoff's voltage law" for a better explanation than I could offer.

mattmia2

@JUGHNE, @ratio and I are saying the same thing in different ways. You are looking for a contact that has a voltage drop across it. It should be nearly 0 vac if the contact is good. There are 2 approaches to measuring it. You can stick both probes of a voltmeter across it and look for a reading of nearly 0 or you can connect one probe to one side of the voltage source and you can hold the other probe on each side of the contact and see if the voltage is the same. Since you are seeing about a 15 volt drop it will be obvious when you find the right one.

I think the second one is faster to get through all the points, you will have to make your tests while it is trying to open the valve so you will have to either wait for it to retry or reset it if you don't get all the way through the main valve circuit before it gives up.

scottnjr

You’re over thinking the voltage. You know the main valve works because you sent it voltage. The control knows the pilot is lit because it stopped the spark. Replace the control

adh99

Appreciate all of your comments. After rechecking all safety switches, I replaced the ignition controller with a Robertshaw 780-845. Gas valve now works flawlessly, but the blower, after coming on properly in 45 secs., will not turn off. The fan will also come on if there is power and no call for heat. Fan is off only during the 45 sec power-up sequence. Checked all of the normal thermostat, high limit etc. problems to no avail.

I have seen other inquiries with a similar issue when the ignition controller is replaced with an old blower control. (Mine is a BCC2.) One questioner on this forum from years back found that switching the blower start from the PV terminal to the MV terminal made the blower run and stop as expected. I tried that and it worked for me too!

Is this a minor Ignition control to BCC compatibility glitch? Is the above solution reasonable?