Lennox Natural Gas Furnace not lighting

MattaWQFZ

I have an eight year old Lennox High Efficiency Furnace (Model ML193UH090XP36C-05) which is equipped with a hot surface ignition system.
A few days ago it suddenly stopped heating.
The startup cycle runs normally until the 4-sec test for ignition where it resets and start over.
I have checked the voltage at the Honeywell Gas Valve (Model VR8215S1248) terminals during Ignitor warmup phase and during test for ignition and it is always zero volts.
Whenever I set the Gas Valve Switch to OFF, the voltage at the Gas Valve is 12V during Ignitor warmup phase and 24V during the 4-sec test for ignition.
I disconnected the 24V wire from the Gas Valve, turned the Gas Valve Switch back to ON and restarted. At the start of the test for ignition, I connected the 24V directly from the Transformer for a few seconds and the Burners light up.
So there is 24V output from the Control Board to the Gas Valve only when the Valve Switch is OFF but will not energize the Valve.
Also, the Gas Valve will energize when connected directly to the Transformer.
Does anyone have a suggestion where to go from here?

EBEBRATT-Ed

Sounds like you have a safety control open not allowing the gas valve to open. Is the ignitor powering up normally when things are hooked up as they should be? If not I would be looking for a safety switch to be open like a high limit. flame roll out or pressure switch

HomerJSmith

Ya! I had the same problem. The tech that came out from another company diagnosed it as a bad gas valve. He replaced the gas valve, of course he did it wrong and you couldn't put the door cover on. I changed the gas valve orientation, a professional courtesy.

The problem was the multi-pin molex connector. The pin connecting to the gas valve on the back of the board had a solder break.

HVACNUT

Flipping the gas valve switch on and off has no effect on whether or not 24v gets to the gas valve. As mentioned, you bypassed a tripped safety, or there's a poor electrical connection somewhere. 

mattmia2

How does this model prove flame? Sounds like it isn't. Does it have a pilot and main valve connection on the valve or only main valve? if it only has a main valve it isn't getting as far as proving flame. if it has an intermittent pilot burner it sounds like it isn't proving flame with the pilot.

MattaWQFZ

The Furnace has only a main Gas Valve and a 120V Hot Surface Igniter. The ignition control board Model is 94W83.
After the 15sec pre-purge period, the ignitor warms up for 20sec after which the gas valve opens for a 4sec trial for ignition. The ignitor remains energized during the first 3sec of trial for ignition.
I can see the igniter as it glows bright orange. 4sec later, there is trial for ignition but the Gas Valve does not open. Also, there is 0V across the Gas Valve terminals during trial for ignition.

I have checked all the switches (Combustion Air Pressure SW, 2 Rollout SW, Primary Limit SW) and they are all good. Also, the system does not continue to the trial for ignition whenever I open each SW.

I have tested the scenario of flipping the Gas Valve SW to OFF and it receives 24V during test for ignition.
I have tested the scenario of flipping the Gas Valve SW to ON and it receives 0V during test for ignition.

Could the Gas Valve be faulty even though it energizes directly from the 24V transformer?
During the Trial for Ignition, why would there be an output of 24V to the Gas Valve whenever its SW is OFF and 0V whenever its SW is ON?

HomerJSmith

The Molex plug. Re-seat the Molex plug and see if that helps. You can also check the back of the board at the Molex plug for a solder break. You can follow the wires from the gas valve to the Molex plug to see which jack it is plugged into.
If I was a betting man, I would bet you $*** that the gas valve is not your problem.

Big Ed_4

After it shuts down what does the LED flash ?

HVACNUT

I would say it's an internal short in the gas valve. You would get a 0 volt reading at TFI if that we're the case. And by turning the switch off and testing again, there's 24 volts at TFI because you manually opened the short.
What kills that diagnosis is how the valve energizes and opens when powered directly from the transformer. 
Check all the grounds.

HomerJSmith

Could the Gas Valve be faulty even though it energizes directly from the 24V transformer?

Ya! That's a good question. The fact that you have 12Vac to the gas valve and a direct connection from a transformer opens the gas valve tells you everything. The gas valve's OK. It is suppose to be 24VOLTS from the board's MOLEX CONNECTOR. I guess, I'm just not getting this.

HomerJSmith

During the Trial for Ignition, why would there be an output of 24V to the Gas Valve whenever its SW is OFF and 0V whenever its SW is ON?

For you to measure 24Vac there has to be a potential difference. You're probably measuring it wrong. So, where are you placing your probes? You can measure the resistance or continuity on the 24V gas valve connections. That would give you an idea whether the coil is intact.

mattmia2

MattaWQFZ said:

Could the Gas Valve be faulty even though it energizes directly from the 24V transformer?
During the Trial for Ignition, why would there be an output of 24V to the Gas Valve whenever its SW is OFF and 0V whenever its SW is ON?

It is probably just a small current passed through a rfi suppression cap or it is a solid state switch, if you put like a 10k resistor across it or used a conventional multimeter with 50k or so impedance it would be 0. there is a voltage with virtually no current if it is open circuit.

MattaWQFZ

Problem Solved.
I disconnected both wires from the Gas Valve and measured the output voltage across both wires from the Control Board.
At call for heat, the output measured approx. 14VAC. During TFI, the output measured approx. 27VAC.
I tried measuring the Solenoid Coil Resistance on the GV with the wires still disconnected but it measured open regardless of the GV ON/OFF SW position.
I had only one option left and that was to remove the GV cap housing the Solenoid. Surprise, there is a Bridge Rectifier Circuit Board converting the transformer AC voltage to DC. That's why the resistance read Open.
I tested the four rectifiers in the bridge and one was shorted.
I have replaced all four rectifiers on the Circuit Board and the Furnace is now up and running.





Thanks Guys for opening my eyes to numerous scenarios.
Maybe I should hunt down a new Bridge Rectifier PS CB to replace the one I repaired.
Does anyone know where I could purchase this CB? It is for a Honeywell Gas Valve (Model VR8215S1248).

HomerJSmith

Good job. Sometimes the junction in a diode does blow, but the diode will fail when shorted out. I would have tested the electrolytic filter cap as to value and leakage. If you can't test it, I would have just replaced it, too. I would have reformed the new cap. I don't think that board is available. Honeywell doesn't want you to replace it, but to buy a new valve. I was wrong on the cause, but glad you figured it out.

mattmia2

There was a time when field repair of gas valves was allowed by the code but it no longer is so that part would not be available.

ratio

Replace the gas valve. It's not worth the risk of the liability.