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HVAC: Lennox Hot Rod Ignition Furnace.

184 Views 10 Replies 5 Participants Last post by  yuri
I have a Lennox furnace with hot rod ignition. The gas valve do not open. The sequence of operations are Ok, like turning on fan, Hot surface ignitor begins getting hot, a click sound. However, the Gas Valve do not open. lights on the control board shows normal operation. The output on the transformer is 28 volts instead of 24 volts. Have tested two gas valves.
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I have a Lennox furnace with hot rod ignition. The gas valve do not open. The sequence of operations are Ok, like turning on fan, Hot surface ignitor begins getting hot, a click sound. However, the Gas Valve do not open. lights on the control board shows normal operation. The output on the transformer is 28 volts instead of 24 volts. Have tested two gas valves.
Model number?

Have you checked to see if the board is applying power to the valve after the igniter warmup period?

If you're sure you have gas, power is being applied to the valve but it doesn't open, valve is bad. Normally, verified with a manometer - see if there's any pressure output when power is applied, inlet pressure checked to verify there's normal gas pressure from the meter's regulator.

If no power is being applied to the valve, you have a problem in the electrical connection between valve and board, a safety like a pressure switch is cutting it off, or board is bad.

Does it throw any error codes?
 

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Discussion Starter · #5 ·
Model number?

Have you checked to see if the board is applying power to the valve after the igniter warmup period?

If you're sure you have gas, power is being applied to the valve but it doesn't open, valve is bad. Normally, verified with a manometer - see if there's any pressure output when power is applied, inlet pressure checked to verify there's normal gas pressure from the meter's regulator.

If no power is being applied to the valve, you have a problem in the electrical connection between valve and board, a safety like a pressure switch is cutting it off, or board is bad.

Does it throw any error codes?
Thank you for your quick response.
- Lennox Merit Series M/N: G40UH-24A-045-03
S/N: 5802F 78542

  • I checked the output from the transformer to the valve; it shows 28 volts for a SECOND on the voltmeter.
  • The original valve number was: 31L8201 VR8205M2476
The new valve number : VR8345M4302
According to the information on the new valve box, both valves are compatible.
  • I will check the input/output pressure on the meter's regulator.
  • The two green lights on the board shows normal operation.
  • The furnace is not burning. I am assuming the gas valve did not open. I cannot judge if the click sound coming from the valve. But the click sound is there for sure.
  • I don't know the input pressure. I need to figure out how to do it.
 

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The gas valve should get 24 volts and maintain that voltage while the burner tries to start. It will remain energized for about 5 seconds or maybe longer and then if the flame sensor has not sensed a flame it will drop out the gas valve and retry 4 more times and then lockout for 1 hr and retry again.

I think your circuit board may be faulty and the gas valve relay is not staying closed. It should get 24 volts at the valve for a lot longer than 1 second.
 

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A pressure switch momentarily dropping out/opening can cause the same symptom and if fast enough the board may not catch it and throw and error code?

Is there any error code? If so, what is it?

1 second is too short, as posted above, normal trial is a few seconds so something else is going on, the valve is probably okay.
 

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You can get a weak pressure switch but that is VERY rare. Would have to jumper it once the igniter (Hot surface igniter aka HSI ) ( hot rod ) starts glowing to be sure that is not the issue. That is a 20 yr old furnace and likely the gas valve relay is weak and fluttering and dropping out.
 

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Discussion Starter · #10 ·
Thank you, jmon, user_12345a, roughneck, and yuri. I really appreciate you guys helping me out. I learned a lot from your suggestions. So, the final disgnose was the faulty control board. I changed it and is working fine. The lessons learned for me.
a) Don't trust error codes from control board especially when it is signalling normal operations.
b) Normal sequence of operations until the opening of valve do not mean valve is bad.
c) The indication of 24 volts to the valve was for a fraction of a second. It should be longer.
 

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Thank you, jmon, user_12345a, roughneck, and yuri. I really appreciate you guys helping me out. I learned a lot from your suggestions. So, the final disgnose was the faulty control board. I changed it and is working fine. The lessons learned for me.
a) Don't trust error codes from control board especially when it is signalling normal operations.
b) Normal sequence of operations until the opening of valve do not mean valve is bad.
c) The indication of 24 volts to the valve was for a fraction of a second. It should be longer.
Glad you got it going. Circuit boards have a response time and cannot react as fast as a fluttering pressure switch or some other problems but they do the best they can.
 
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