Lennox ML 193DF Short Cycling

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Old 12-07-13, 03:08 PM
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Lennox ML 193DF Short Cycling

I have a 2-year-old Lennox 90% gas furnace that is not working properly. The specific problem is that the burner ignites for 10-15 seconds and then shuts off when the blower motor comes on. This will happen over and over again and never raise the temperature in the house. The diagnostic LEDs are showing normal operation most of the time, but sometimes will show this:

Pressure prove switch open
OR: Blocked Inlet/exhaust vent
OR: Pressure switch closed prior to activation of combustion air inducer


Things I've checked:
I inspected the roof exhaust vent (PVC) and heard gurgling while the furnace was running. There is very little exhaust coming out of the pipe. I'm assuming that the inlet air is fine since there is burner flame for more than several seconds.

I've also measured the air temp coming from the vents and it is about 80 degrees F. Seems mighty low

Thermostat is working fine, but just in case, I replaced it.

What exactly does the pressure switch do? I've measured with my DVM and it seems random when it's tripping. Any help here would be appreciated!
 
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Old 12-07-13, 03:47 PM
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Welcome to the forums.

You appear to know what the pressure switch is..... that's a plus. There may be two on your furnace if it's a two stage model.

Look for the thin rubber tubing connecting the pressure switch to the draft inducer blower. Carefully remove it and make sure it's clear. Check the tiny hole into the inducer to see if it's clear.

The pressure switch monitors the pressure in the exhaust system but if its connecting line is blocked it can not sense the pressure.
 
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Old 12-07-13, 05:21 PM
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The fact there is little exhaust coming out makes me think there is a restriction somewhere or the exhaust blower is not coming up to speed.
If there is any horizontal part in the exhaust, make sure it slopes steadilly upward away from the furnace.
 
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Old 12-07-13, 06:54 PM
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Condensate

Thanks to the both of you for the advice, I'll check the pressure switch tubing first. Does the fact that I heard gurgling coming from the top of the exhaust vent mean that a drain might be plugged? As little as I know about modern furnaces, I'm also assuming that there is typically condensate build up at the bottom of the exhaust fan and it must go to drain.

Also, if the exhaust fan isn't coming up to speed, would that typically be a control board problem or a motor problem (maybe a capacitor)? It seems that since the unit is only 2 years old, I should expect either of these problems, but Murphy's Law is in effect I suppose.
 
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Old 12-07-13, 07:05 PM
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Drain problems are quite common. In the unlikely case the motor is not coming up to speed, the culprit is more probably the motor rather than the control. If the problem is with the furnace, rather than installation or lack of maintenence, parts should be under warranty.
 
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Old 12-09-13, 09:24 PM
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Fixed!

As it turns out, the condensate line was plugged. This probably explained the gurgling sound that I heard in the exhaust vent when the furnace was running. Consequently, the pressure switch was shutting everything down because of the exhaust vent being partially blocked by the accumulated condensate.
 
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Old 12-10-13, 01:58 AM
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Great news... Now you'll know what to keep an eye open for in the future.
 
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