Lennox gas furnace flame goes out reignites goes out etc. New circuit board installed. Exhaust vent clean. Exhaust blower clean working. Will only continue to burn if vacuum line is disconnected from sensor and clamped off. What is probable cause?

Blockage in condensate trap? Restricted air intake?

The sensor you are referring to-could it possibly be a pressure switch? (round disc shaped thingamabob with 2 wires and 1 or 2 rubber hoses connected).

Yes, It is the pressure switch. A New one was tested same problem so that's not it. The vent blower was taken off and checked for blockage all Ok. Air does not appear the problem. What is the Condensate trap??

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Originally Posted by Den53nis

Yes, It is the pressure switch. A New one was tested same problem so thats not it. The vent blower was taken off and checked for blockage all Ok. Air does not appear the problem. What is the Condensate trap????

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A condensated trap is used in high efficiency furnaces to allow the water in the secondary heat exchanger and collector box to drain properly. It's usually plastic, sits in the blower compartment, or right outside the furnace. It typically drains out of 1/2" CPVC pipe or some such into a condensate pump.

They are not found on 80% efficiency furnaces-just the higher efficiency models. Higher efficiency furnaces extract so much heat out of the vent gasses that the gasses actually reach a temp below dew point in many cases, and condensate is extremely heavy.

Did the pressure switch have 2 rubber hoses on it or just 1?

Check for a split in the hose and make sure the unit is sealed tight (for example if the cover is off the burner enclosure, put it back on with its proper seal. If the seal is destroyed, seal it back up with Hi Temp RTV-a red silicone based caulk. It is extremely important to have ALL seals intact on a sealed combustion unit, otherwise pressure switch failure can occur as the negative pressure it needs to engage is not developed when the inducer comes on... it just sucks air through the leak (air, like water, will seek the path of least resistance)

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Originally Posted by Den53nis

The vent blower was taken off and checked for blockage all Ok.

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When the inducer blower was taken apart to inspect, was it sealed properly when being reassembled? If not, you maybe adding to your problems. You have to be sure the seal between the inducer and its mounting is totally sealed as well. Not getting a good seal can cause the pressure differential issue you now have and can allow condensate to leak out of the inducer halves or mounting.

How old is this furnace? Has the exchanges been inspected?

Eric D

The blower is sealed. The hose is not leaking. The furnace is 13 years old. Not high efficiency. Only one hose to the pressure switch. Everything mentioned has been checked, and the same problem exists. The flame is full for 10 seconds with vacuum hose attached and then full shutdown of flame. With hose disconnected and plugged works fine.

Sounds like you have the wrong pressure switch for the job. If you had the right pressure switch for the system and the pressure switch tube was not hooked up the furnace would not work.

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Originally Posted by T-Top

Sounds like you have the wrong pressure switch for the job. If you had the right pressure switch for the system and the pressure switch tube was not hooked up the furnace would not work.

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I agree-sort of. It sounds like the pressure switch is NormallyClosed and proving, but should be NO, and proving on the draft? But all models I've worked on wouldn't fire at all if pressure switch failed on call OR reset (returning to open state). The old carriers with the little inducer board up top... those can be finnicky-but this is a Lennox.

It's not high efficiency, but has a pressure switch? Hmmm... that's a bit odd in itself. Must be a "chimney friendly" type furnace.

Honestly, this is starting to get a bit drawn on... and I'm a bit concerned. It may be nothing, but I would certainly have this checked out by a technician. I like to err on the side of safety when it comes to gas and electric. Bad things tend to happen if you don't...

I have had two separate technicians both of which are stumped. The factory rep has been called. Thanks for all of the posts

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Originally Posted by Den53nis

I have had two separate technicians both of which are stumped. The factory rep has been called. Thanks for all of the posts

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Dennis,

When you do get it figured out how about a follow-up reply to let us know what you found?

Thanks,

Eric D