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
-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
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
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?