Lennox G40UH(X) Series, Heat does not turn on


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Old 01-21-08, 10:49 PM
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Cool Lennox G40UH(X) Series, Heat does not turn on

We have a G40UH(X) by Lennox which is about 7 years old. The heat does not turn on. We know a bit about the circuit board. So, we find out that the CIB is burned and we have replaced it with a new CIB. It seems like the furnes does not lit up. can anyone help us to better trouble shoot?
 
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Old 01-22-08, 12:19 AM
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Take a look at the chart attached. It has the codes (for your furnace, as per M/N provided) that indicate which failure has occurred. See, compare and post what you saw.
 
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Old 01-22-08, 09:06 PM
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Thanks a bunch for the chart. Ok, so we just came out of the addict and here is what we saw:
The LED 1 and 2 has simultaneous fast flash and does not change i's flashing status as it goes through operation. I mean, that the fan turns on but the ignitor doesn't lit and the gas valve opens. We measured the resistance value of the ignitor and it was 46.3 Kohm and the voltage level at the pins 2&4 reached 90 Volt AC. But under these conditions the ignitor didn't lit. Do you know why ?

Thanks
Minoo
 
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Old 01-23-08, 02:39 AM
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isn' t it funny how these codes could be tough to interpret?

Well...simultaneous fast flashing means all is normal there, which is obviously not the case. I will assume that what you really saw was alternating fast flashing.

Imagine you're a drummer but tap the drum with the two drumsticks wrapped with duct-tape; that will be simultaneous tapping. Now imagine removing the duct-tape and tapping the drum as a drummer would do, that's alternating tapping.
Now imagine flashes instead of tappings. I hope what you saw was really "alternating" and not "simultaneous" flashing b/c

To know what exactly the "alternating slow flashing" on the LEDs means, you need to tell me the board's M/N. There are 4 of them: 56L84** (that's the one on the pic below, see red arrows), 10M93, 78M47*** & 100973-01...but bottom line is that it means one of the following:
(a) improper ground, (b) broken ignitor, (c) open ignitor circuit, or (d) line voltage to ignitor below 75V [for boards: 10M93 or 56L84] or below 90V [for boards: 78M47 or 100973-01]

The control board for your unit has a feature by which on successive calls for heat the board gives the ignitor less and less power, sort of ramping down" the voltage, to the minimum that would still light-up the burners. The board could go all the way down to 90V. Since you tell me below that you could get an ohm's reading from the ignitor and that the voltage read was 90V..."from your readings" and from your information that you've replaced the board by yourself I will presume that you forgot to properly ground the new board. But if the 90V reading you report was at the wire terminals and with the ignitor removed, maybe the connection (plug to ignitor terminals) was not proper (misconnection...prongs not properly inserted into each other, etc.)

I'm taking my students to NYC to the AHR expo today, so I will be offline most of the day and unable to see further postings of yours until late in the PM...hope by then you've got your problem solved, else I'll see you in the forum tonight

Ah, Ah! on the light side, one more thing: I take it your unit is in the "attic"...not "addict", as in "drug addict"...I don't suppose your Lennox is on drugs or something like that...
 
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Old 01-23-08, 02:58 AM
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Lightbulb Bad Ignitor

Please disregard my previous posting where I suggested an improper grounding of the board. [but read it anyway since it has some good info from Lennox...not me...pertaining to your unit...besides of course some personal input of mine]

I just got to the part in the manual that shows the specs for the ingitor. Lennox says that a good ignitor must have a resistance value in the range of 10.9 to 19,7 OHMS (yes, "ohms" and not "Kohms" which is what you report having read). So the reading of 46.3Kohm you indicate tells that the ignitor is MUERTO (dead)...replace and have a WARM day. This place sells original Lennox components:
http://www.cozyparts.com/lennox-parts/locator_form.aspx
Ask for the "silicon nitride ignitor" for a Lennox G40UH
 
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Old 02-01-09, 08:53 AM
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Lennox won't send me the PDF service manual

I built two rental houses in 2003, using Lennox G40UH(X) gas furnaces.
One of them has a problem; the Surelight controller board LEDs do not light up. My guess is that the board is fried, or the transformer feeding it power is bad. I called Lennox customer support to request a PDF copy of the service manual (I have the paper copy in the attic). The customer support guy said he could only send the PDF version of the manual to a certified HVAC technician. This policy seems overly paranoid, since as an owner I have a paper copy (in the darn attic!).

Can anybody provide me with a PDF copy? Thanks.
 
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Old 02-01-09, 10:27 AM
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