Lennox Furnace Won't Heat

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Old 10-24-08, 09:16 AM
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Lennox Furnace Won't Heat

I have a Lennox G4OUH gas furnace. It will not light, just blows cold air. Diagnostic lights indicate open limit switch. I have checked the flame rollouts, and they are fine. I was unable to find any other thermal cut-offs or limit switches. The hot surface glow ignitor is reading 16 volts, and does nothing. The gas valve is reading 0 volts, and the secondary portion of the transformer, is reading 30 volts. Any assistance would be greatly appreciated.
 
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Old 10-24-08, 10:19 AM
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Originally Posted by WEGESE2215 View Post
The hot surface glow ignitor is reading 16 volts, and does nothing.
Is that the voltage output from the control module? It sounds off. See if you can find the specs on your HSI.
 
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Old 10-24-08, 11:03 AM
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Originally Posted by dac122 View Post
Is that the voltage output from the control module? It sounds off. See if you can find the specs on your HSI.
The manufacturer's website says the voltage varies, because the cpu will lower the operating voltage in order to extend the life of the ignitor. I don't know what the lowest operating voltage is supposed to be, though. They did not specify.
 
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Old 10-24-08, 05:30 PM
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If you have an actual HSI glow coil, then it starts out at 120 volts, but drops off immediately after ignition. Maybe that is what he meant. But you need all that voltage at first to get that thing to really glow the way they do.

Every furnace has high limit on it. Either it is going to be a thermodisc small little thing with a couple wires leading to it, or will be 2 wires that plug into terminals, or have a fan-limit control on it. No matter which type, they are located on the vertical sheetmetal above the burners. That circuit is bound to be your problem, so you simply must look and find it. It may have a a small pin-like reset button on it, betwen the two wires.

If no reset for teh high limit, you reset the high limit by turning off the furnace for about 30 seconds and then switch it back on again, and the furnace sequencing will start over. Then you will probably notice the furnace fires up, but then will go out again in minutes. Some causes: Maybe the blower is not coming on. Maybe the blower motor bearings or motor or capacitor for it is shot. Maybe the furnace filter is plugged. Maybe half the registers in the house are shut down.

Let us know what you find and then what happens.
 
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Old 10-24-08, 07:03 PM
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Originally Posted by ecman51` View Post
If you have an actual HSI glow coil, then it starts out at 120 volts, but drops off immediately after ignition. Maybe that is what he meant. But you need all that voltage at first to get that thing to really glow the way they do.

Every furnace has high limit on it. Either it is going to be a thermodisc small little thing with a couple wires leading to it, or will be 2 wires that plug into terminals, or have a fan-limit control on it. No matter which type, they are located on the vertical sheetmetal above the burners. That circuit is bound to be your problem, so you simply must look and find it. It may have a a small pin-like reset button on it, betwen the two wires.

If no reset for teh high limit, you reset the high limit by turning off the furnace for about 30 seconds and then switch it back on again, and the furnace sequencing will start over. Then you will probably notice the furnace fires up, but then will go out again in minutes. Some causes: Maybe the blower is not coming on. Maybe the blower motor bearings or motor or capacitor for it is shot. Maybe the furnace filter is plugged. Maybe half the registers in the house are shut down.

Let us know what you find and then what happens.
I have checked the voltage of the HSI at start up, and it always reads 16 volts. It has never attempted to light. The 2 flame rollouts on the sides of the burners, are both functional. If I remove them from the series, the diagnostic message chages to redeading that the flame rollouts are open. So they must be functioning. They also pass a continuity test. I've already tried shutting the system down, from the circuit breaker, and leaving it off for several minutes. The same thing happens. The unit just blows cold air, and never makes an attempt to ignite. I have been in the attic, with the furnace, and activated it by means of pushinf in the safety cut-off for the access panel, and holding it in. I've tried calling the manufacturers troubleshooting department, but they won't tell me anything.
 
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Old 10-24-08, 08:25 PM
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Well mine runs at 120VAC long enough to walk down the stairs to the unit.

If you discover that HSI can handle 120 VAC and you have an old heavy junk test cord, remove the HSI igniter, wire it up and plug it in. Be careful how you handle it as oil from your hands can be a problem, be careful how you wire it up, etc. We don't want you getting shocked or burning anything. So be careful. Did I mention be careful?

You won't need to plug it in for very long to see if it glows. If so you have a bad control module or something inside that is supposed to ramp that temporary voltage up.
 
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Old 10-25-08, 05:04 AM
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OK Thanks. I'll try doing that, and I'll let you know what happens.
 
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Old 10-25-08, 07:56 AM
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My Lennox G60 does indeed have a circuit that changes the voltage to the ignitor to the lowest value that will successfully light the burner. This feature is to prolong the life of the ignitor element. Lennox states that because of this feature yo CANNOT use voltage at the ignitor element for diagnostic purposes but must instead check the resistance of the ignitor element which should be between 10.9 and 19.7 ohms.

Here is the pdf of your furnace's installation manual:

http://www.hvac-direct.com/pdf_files...on/505011a.pdf

Here is the site for downloading other Lennox information:

http://www.hvac-direct.com/Installat...structions.htm
 
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Old 10-25-08, 01:22 PM
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Originally Posted by furd View Post
My Lennox G60 does indeed have a circuit that changes the voltage to the ignitor to the lowest value that will successfully light the burner. This feature is to prolong the life of the ignitor element.
Interesting. You learn something new all the. Then you die.

I wonder how they manage to make it glow hot enough with lesser volts? I wonder that if say it glows hot, what the difference would be for the life of the ignitor, if it still has to glow real hot?, which you would think it would have to.

I would like to understand the principle. Some basic principle of resistance and physics must apply. Like, can this be because they use a way smaller ignitor? Or, is the material different, that it can glow as hot, with less current? But even if so, why would the voltage make it last longer if it still glows cherry hot? Do they stick their neck out and guarantee that ignitor to last 10 years or anything? And was one of their reasons for doing this becasue they put the ignitor in some spot real hard to get to, so they wanted to make extra certain it lasts?
 
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Old 10-25-08, 04:15 PM
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Gee, Dave, you ask all the easy questions. Quite honestly, I don't know. I did read the instructions for the ignitor board a year or two ago, maybe I'll look it up for you.
 
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