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Crouching Tiger

Lennox Pulse Furnace

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Crouching Tiger

Fired up the furnace 2 weeks ago and everything was running good  until three days

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


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Need appliance parts? Call 877-803-7957 now!

Guest ahammer48


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:)

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Crouching Tiger

Thank-you for your response in regards to my Pulse problem.  However there is no

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.


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After start up listen for a change in sound before it shuts down and retries ignition.

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

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Crouching Tiger

Thanks for the new information hvacdrd.  I checked the condensate line which runs

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.

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Odd thing about the pressure differential switch on the Pulse. The contacts are normally closed and only open if there is a problem. Most units use the switch in a normally open state and close to prove that the inducer is running. Always seemed backwards to me but it works.

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.

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I had the same condition last fall with my G21 pulse. The condensate line appeared to be o.k. (water coming out of tubing) But the bottom of the drip leg on my unit was plugged with rust and sediment to the point that the furnace was producing more condensate than the pipe could discharge. I cut the cap off of the end of the pipe and cleaned it out and put a rubber Fernco cap on it with a stainless clamp. I had  my original Pulse replaced with the G21 under warranty and nobody ever checked for this condition in the 20 plus years I have had a Pulse furnace. Hope this helps.



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Crouching Tiger

I forgot to post back about my furnace issue.  I broke down and called a local service company to see what was goin' on.  He checked and adjusted the gas pressure and talked about the possibility of the condensate line being plugged and a few other things.  In the end he really didn't do a goddam thing but charge me 100 bucks for the call.  So I check the drain line which had a removable plug and I cleaned out some junk but the problem still went on.  Then I kept on focusing when the unit shut off - before the blower came on - like it was tripping on the hi-limit.  So I sit in front of the damn thing for about an hour with my amp probe on the blower and finally 9-10 amps on the motor.  The start cap. was no good!  Duh!  So I slam another cap. on it and she runs like a dream.  Thanks for all your suggestions guys. 

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That Guy

Change out the air diaphragm if you haven't already. They get kind of rotten after a while. The pluse is kind of like a one cylinder engine and the air diaphram is its valve. Its a pretty important part.

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I concur with ahammer48.  in my experiences i find the flame sensor being dirty or just needing replacement. replacement is easy, just remember to disconnect unit before doing any electrical work.



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Crouching Tiger

Perhaps I was not clear on my last post.  The blower motor start capacitor had failed causing the delayed start in the blower which would then trip the hi limit thermostat within approx. 45 seconds.  Everything else with the unit is good - diaphragm, sensor etc.  I replaced the start capacitor about 3 months ago and I haven's had a problem since.


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