Gentlemen,
I have stumbled upon a Lennox high-efficiency furnace model G32V5-100-3 that has started to lock out intermittently, usually at night during the past 2 months or so. The customer says that this furnace is 3-4 years old.
The diagnostic flashing lights indicate the same problem every time: pressure switch problems. According to the chart, this could be the pressure switch opening 5 times during one heat cycle, the switch closing before the call to start the combustion venter motor, a plugged condensate drain, a blocked inlet or outlet vent pipe.
I have spoken with Lennox tech support twice and they seem adamant that the problem is a water drain problem.
I have variously:
-replaced the pressure switch
-removed and inspected the venter assembly and the rectangular plastic drain trap box to which it is attached - no visible problem
-inspected & flushed all the water drain tubes, etc
-measured the pressure differential across the switch - OK
-watched the venter motor coast for minutes after the heat cycle ends, no problem with bearings in that motor
-checked the inlet & discharge vent pipes - no evident blockage
-climbed into the area directly above the heat exchanger and visually scrutinized the heat-x as thoroughly as possible from that angle - nothing out-of-the-ordinary was seen
I think I have eliminated the following as possible factors in the problem:
-night set-back - it still fails with setback disabled
-outdoor temperature - it has failed at various temperatures
I disconnected the inlet vent pipe from the furnace so that the unit can draw air either from the connection to the outdoors or from the building and left it for a few days like that to double check the the inlet pipe - it still locked out occasionally. I have changed the main blower from continuous to intermittent without solving the problem.
Lennox stated that these furnaces may have a vent length of "25 equivalent feet" and this one might be near or a little over that. But it has worked well for a few years. Also, the vent pipes are pitched properly.
The furnace room is unheated and was 50F on a cold day. I have wondered if this might be a problem re:possible condensation in the pressure switch but the other 5 similar furnaces in the room are fine.
I have considered swapping components between furnaces to see if the problem travels or stays. But this furnace is a G32V5-100-3 and all the others are dash-4. I do not know if this is a significant difference but some of the part numbers that are stencilled on some components are different.
I think I am near the end of the list, at least my list. Lennox didn't really have anything to add.
Can someone help?
Canucklehead Toronto snipped-for-privacy@sympatico.ca