Lennox Pulse Furnace
Posted 08 October 2006 - 07:21 PM
ago when the unit came on for about 30-45 seconds and than shut off. I called for
heat about 3 hours later and the unit came on and ran ok with no problems. The
next day a call for heat was needed and the furnace came on and then shut off after
about 45 seconds but the blower remained on while the furnace made a second
attempt to start which was successful. The third day the furnace came on and then
repeated the same problem as before - on for about 45 seconds then shut off,
except for the blower, and then re-started and stayed on for the complete cycle. I
decide it was time to check it over. I have 24 VAC between R and C and C and W
when calling for heat. I also have 24 VAC at the gas valve after pre-purge and the
diff. pressure switch is closing. I did find and repair a leak in the exhaust
tubing where the diff. pressure switch hose connects to the 90' fitting on the tube. I
restarted the unit and it shut off only once after making and breaking the cycle about
three times. My temp rise is also good from 70'F - 135'F before the blower comes
on. The factory blower setting is 45 sec. on and 180 sec. off. I have not yet checked
the gas pressure or the spark plug to see if I have good spark. The air diaphragm
has not been replaced in 10 years and the manufacturer states that it should be
replaced every four years regardless of condition so I think I will do that as well. I
have also looked at the vent termination and it is ok according to manufacturers
install instructions. Any suggestions or info reagarding this problem would be
Posted 09 October 2006 - 03:53 PM
When U say that the unit is "staying on 4 about 45 second and then shuts off", are U saying that the gas is lit all this time and it goers out after 45 seconds?
If yes, then at 1st glance it sounds like maybe your flame rod (it senses that the gas has lit off) is possibly dirty or is not in the path of the flame well. Some manufactures put them right next to the spark ignitor, some put the ignitor on one end of the manifold, and the flame rod at the other end. I've seen this problem. Burner lights and rod is just enough in the path to keep it lit for awhile, but maybe the flame mores alittle and the gas valve shuts down. In the mean time the rod has heated up and may just bend enough to be in a better position for the 2nd lite off, and the furnace stays on.
Find your flame rod and then start the burner. Watch and C if its in the path of the flame. A good portion of the rod should be engulfed by the flame, not just the tip.
Hope this helps:)
Posted 09 October 2006 - 05:32 PM
spark ignitor or gas burner in this unit, it is an actual spark plug like an internal
combustion chamber. The term Pulse comes from the combustion process where gas
and air mix in the combustion chamber. A spark ignites the initial combustion - this is
the first pulse. Pressure then forces combustion products to travel down the tailpipe.
Air and gas re-enter the combustion chamber as the exhaust gases leave the
chamber. A new mixture is ignited and continues the cycle to 60 pulses every
When the furnace starts you can hear the pulse process taking place. Then the unit
dies out after 45 seconds but the blower will remain on and than the furnace will
begin the pulse process again and stays on for the rest of the cycle. I have tried the
unit again today after fixing the leak at the diff. pressure switch tubing and it ran
without a problem. However I did not keep it on for more than 5-6 minutes because
it was already 74' F inside the house. I hope this will provide a little more
background at how the system operates and the problem at hand. Again thank -
you for the help.
Posted 10 October 2006 - 01:31 PM
If it sounds like it is bogging down verify that no exhaust gases are recirculating.
If there is not a noticable noise change check in this order
1 - verify that the pressure differential switch stays closed - if it opens the likely cause is a blocked condensate line, or partial blockage of the air intake pipe (ie - wasp nest in pipe)
2 - Check & monitor flame signal - Normal signals are:
Watsco Primary Control - 3 to 5 microamps DC
Gas Energy Primary Control - 25 to 35 microamps DC
Lennox GC-1 Primary Control - 18 to 35 microamps DC
Since this is a Pulse furnace you may not get a steady reading, if your meter has an averaging feature it helps.
The most common problems I have had on these units are blocked condensate lines, exhaust recirculating into the intake, and Primary Control failures.
Normal manifold pressures are 2.0" w.c. for natural and 9.0" w.c. for LP
Normal exhaust temps are 110-125F - should not exceed 130F
You are correct on the Air Flapper material replacement every 4 years per Lennox but I know of several that have never been changed.
Hope this helps
Posted 10 October 2006 - 04:23 PM
into my sump hole and it appears to be normal, I will still check it at the tee fitting for
any partial blockages. As stated in my initial post about the leakage at the diff.
pressure hose, is it possible that this was the primary problem? I did not state
originally that we had a problem about 6-7 years ago where the furnace would shut
off even before the blower would come on and it would happen until the unit locked
out. That was when we had insurance through Union Energy and their technician
changed the primary control and some other parts that did not fix the problem.
Finally Union sent out another tech. from a local heating company. He came and
looked it over and found that the exhaust and intake were installed wrong and,
under the right weather conditions, was causing the exhaust to be recirculated and
stop combustion. About a month later we received a notice that Union Energy was
cancelling our insurance because the furnace was a high risk maintenance
issue. But after that one and only problem the furnace has run good until now. Let
me know what you think about the leak problem.
Posted 10 October 2006 - 05:01 PM
If there is any question if this is the problem you could wire a low amp fuse(1/100a) in parallel with the contacts, if the fuse blows at any time then you know the switch opened at some point. Keep me posted and I'll try to dig up some other information if you need.
Posted 29 October 2006 - 04:15 AM
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