By "purge blower" I presume you mean the blower
which blows air into the combustion chamber. What
happens when it "tries to fire"? Does the hot
surface ignitor glow, and the gas valve open?

So, you say that when you turn the thermostat up
hard and far, the furnace runs?

Of course, there are two fans. One to supply air
to the fire. And one to blow heat into the
building. Some of the more complicated furnace use
a timer, to turn on the blower fan that blows the
heat into the building. I can't comment on the
Lennox, never worked with that brand. Others use a
heat sensor, or other device to know when to turn
on the hot air blower.

A tech with Lennox experience would be more help.
Too much electronics.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"zimpzampzormp"
wrote in message
...
1995 Lennox Pulse 21 furnace. Ironically I did
everything from welding
the heat exchanger to assembling it to installing
it in my house. I am
not an HVAC tech or an engineer.
The other day I noticed my second problem. A
couple of years ago the
blower motor failed. Had to replace it with an
upgraded motor as the OEM
was unavailable at the factory. Very successful
operation. I also
replaced the air flapper as the old one was caked
with dirt.
Last week I noticed it was short cycling. The
purge blower would run and
the for about 5 seconds it would try to fire. Then
it would go silent,
then the purge blower would run. Then the cycle
would repeat. Anyway, I
found that if I have the thermostat set at 75 deg
instead of the 68 deg
I usually run it at, it fires and cycles just
fine.
Weird.
Before I call a real tech, does anyone think the
problem could lie in
the electronic thermostat?

Thanks.