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-   -   Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too (

Muzzle127 09-25-2012 06:52 PM

Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too
I have been trying to fix this problem for 2 days. Here is the sequence. Demand for heat. Small fan turns for a couple seconds, pauses then runs again. I believe the ignitor is supposed to heat during this. Ignitor never heats and fan turns off. Stays off for a couple minutes and then repeats sequence. I tried cleaning ignitor with emery cloth but it wasn't that dirty. Still no heat. Board shows slow flashing right LED. Suggestions??

how 09-25-2012 07:05 PM

The inducer motor would need to run long enough to activate the pressure switch before the igniter would operate.
It isn't. How have you tested that the pressure switch is OK?

That code indicates possible.....

Pressure switch open/ inlet or exhaust restriction/ blocked condensate line or pressure switch stuck closed before ID motor starts.

Muzzle127 09-25-2012 07:35 PM

I removed the pressure switch and connected an ohm meter across the switch contacts. Then with my mouth pulled air thru the hose and heard the switch click and the resistance went to ~0 ohm. I also re connected switch and measured voltage across the contacts. Voltage was ~26 volts

how 09-25-2012 07:42 PM

Is your condensate line clear?
& since you've got a meter and can use it...
Can you test if the Inducer motor is pausing while it's still getting 120V?

Muzzle127 09-25-2012 07:53 PM

This is a standard efficiency. Condensate isn't used for heat. I do have central AC which ran fine through the summer. Could the condensate for AC affect it ? I have to climb back in my attic to test the inducer voltage. I will also check the ignitor voltage, and see if the switch changes state when the inducer stops running. Be back in 30 minutes. Oh and I just switched back to AC and that runs fine. The blower motor and control stuff would also be ruled out by this check. (I think)

techpappy 09-25-2012 08:13 PM

check to ensure that the switch makes when the inducer fab runs..if not..could be the inducer fan is dirty/defective. vent clear? Some times get birds, leaves etc. blocking vent line.

Muzzle127 09-25-2012 08:50 PM

ok the switch appears not to be making as it stays at 26V across the contacts regardless of state of inducer motor. I measured the voltage across inducer leads it is 120V AC when active. The brief pause causes meter to range (0V) then back to 120V when motor turns on again. The device I was measuring is the flame sensor not the ignitor. That is at 64V always. Earlier, I disconnected the B Vent inside to look inside for blockage. It looked clear as far as I could see. I can feel some air coming out of the side of inducer motor. Is this normal?

Doc Holliday 09-25-2012 09:45 PM

Continually suck on that hose, manually keeping the pressure switch closed. This can be done as I do it all the time when diagnosing heaters. You breathe through your nose and keep suction on the hose. If the furnace lights up then you need to make certain the ports where the hose connects to are both clear (straightened out paper clip) and that the hose has no cracks, that the flu pipe is unobstructed (remove from furnace and tap it hard a few times to see if any sediment falls out. if it does tap it many times more until nothing falls out and then check your roof cap as well, birds nest and such) and then finally check the amperage draw of the inducer motor as well, make sure it's still strong enough to pull that pressure switch closed. You can sometimes get away with oiling it. There might be a hole specifically to drop oil into the motor.

techpappy 09-25-2012 09:47 PM

the voltage to inducer is being interrupted therefore the inducer fan can not stay on long enough to make the air switch..not sure what is causing interruption..could be as simple as loose wiring connections, control board? I would start by tightening all connections first. Try just supplying separate 120 V to inducer motor to see if it will stay running.

Not sure if you should get air from ID motor housing/venting..could be internal cooling fan? OR sign of blockage..can you remove ID fan to check operation/signs of blockage ..some recent threads have found dead birds inside fans ..

Doc Holliday 09-25-2012 09:53 PM

I completely missed the voltage dropping on the inducer. Is it doing that all the time or only the first time (or once) and then staying on full without a voltage drop and still not firing?

Muzzle127 09-25-2012 10:53 PM

The hesitation happens on the first spin of the ID motor. It's very repeatable as far as when it happens and the duration of the first spin. After the small delay, then the ID runs for a good 20-30 seconds. Shouldn't that pull the switch closed ? I will try the manual suction to check if I get ignition before I start to dismantle the ID. I want to be very cautious what I take on before I get in too deep. I may pull the cap off the flue and look down into it for blockage. It's a very short and straight pipe(6-7 feet long). There's one elbow up from the furnace and then straight thru the roof but I will check this. I was on my roof and looked at the cap and didn't see any signs of a nest. The cap is pretty well screened off.

FYI I found this site thru Google search

Muzzle127 09-25-2012 11:08 PM

Maybe I need to clarify.

Here is what happens every time from power off. Turn power on. Thermostat demands heat. ID motor gets power and runs for approx 2 seconds. ID motor powers off for about 1 sec then gets power back and runs for 20 - 30 seconds. This is not intermittent. It happens repeatably every time. I observed it at least 10 times since Ive been trying to fix this.

Something else I forgot is there is a short 4" piece of red hose between switch and ID motor. It is not blocked and I didn't see any signs of cracking on it. There's a 90 degree bend in to make the connection.

how 09-26-2012 12:23 AM

What you don't know is why the PS is not being activated. A manometer that measures the ID suction, as well as what it takes to activate your PS is the usual test. It tells you if the ID is pulling what it's supposed to and if the PS is tripping when it's supposed to.

The ID may not be running fast enough, the capaciter may be failing (if it has one), the ID fan cage may be slipping, dirty or missing fins, your vent may have something dead in it, that bend in your suction line may be too restrictive, you may have a restricter plate on the exhaust of the ID housing that is rusting out, or a broken mounting seal on the back of the ID housing, a plugged exchanger, a leaking PS ..etc..

The momentary ID voltage drop so early into the ID start up is weird. I'd still be interested to make sure it still doesn't pause when the ID is hooked up to and powered through an extension cord.

Doc Holliday 09-26-2012 12:48 AM


Originally Posted by Muzzle127 (Post 1017495)
The hesitation happens on the first spin of the ID motor. It's very repeatable as far as when it happens and the duration of the first spin. After the small delay, then the ID runs for a good 20-30 seconds. Shouldn't that pull the switch closed?

Yes it should, why I asked.

ben's plumbing 09-26-2012 06:48 AM

check small port on inducer motor where hose from pressure switch connects this clogs sometime....

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