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Thread: Loud lennox

  1. #1
    Join Date
    May 2003


    I came across a loud lennox pulas unit abot 1987....looked like a 90+ when heat comes on it makes a loud noise ...i throught the combustion motor might be bad i took metal cover off ,removed insulatoin ......and it got real loud.....unit is putting heat out good..tooo the complaint was the loud noise and the unit shacks causing the fan downstairs to shack really bad....any one got any clue what causing the trouble......when i took metal cover off it looked like a muffler..... blower motor is not lose or going out....thxxxxxxxxxxxxxxxxxxx

  2. #2
    Join Date
    Feb 2004
    Could be a weld broken on the heat exchanger. Can be very noisy if that happens. And the finish of the furnace, of course.
    CO tests done?
    Question authority!

  3. #3
    Join Date
    Feb 2005
    Is that the first time you have ever saw a pulse furnace, If so the pulse is not very quiet, It does have a rumbling sound and when they first came out in 1986 or 87 they were really loud because the did not have any mufflers in pvc line. Which should of been put in down the road to help the noise. Also the draft moter is actually a purge moter and only runs to get ignition and then shuts off.

  4. #4
    Join Date
    Jul 2000
    NW IL.


    The Pulse Heat Exchanger needs to be pressure tested every 4 years. In addition the air flapper valve also requires replacement every 4 years as well. The shock mounts on the heat exchanger sag with age. This causes the air flapper valve body to rub against the decoupler box. Excess noise can also be caused by a worn gas diaphragm or incorrect gas valve pressure.

    You can download a service manual at

    MechAcc Dadgum ole furnace.

  5. #5
    Join Date
    Aug 2004
    North Richland Hills, Texas
    As others have said, the air intake diaphragm, part of the "muffler" looking thing you saw when you took the metal cover off, needs to be replaced every 4 years.
    If the unit is running and heating OK, but making a ton of noise, you may have a bad gas intake diaphragm. Generally a Pulse furnace will have a hard time starting, and sputter a lot, if the air intake diaphragm is bad.

    You can diagnose most problems with a Pulse furnace by sound and touch if you are very familiar with them.

    After the gas valve, there is a metal cylinder in line with the gase line before it turns and goes into the furnace. If that is vibrating badly, and a lot of the noise is centered around that area, you likely have a bad gas intake diaphragm.

    If it has been a few years since the furnace was serviced, or if is often the case, it has never been serviced, there are a number of things that need to be done.
    Some of them require special tools and training/knowledge that you won't get from the service manual.

    I would recommend that you refere the customer to a reputable Lennox dealer that has the tools and training to service the thing properly.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Oct 2003
    34.8n 102.4w

    Good advice

    Mark is right, the Pulse is a different creature... call a Lennox guy.... NOT saying a good tech can't fix it tho!!
    Life goes on long after the thrill of living is gone.

  7. #7
    Join Date
    Dec 2004
    Derby City
    Agree with Mark and Sline. A PULSE trained tech should be the one performing the trouble-shooting. Again, not to say someone else couldn't eventually figure it out, but the PULSE furnace is unique! When you removed the heavy lead plate and noticed a major increase in sound, that is WHY the heavy insulated lead plate is there. I installed hundreds of PULSE systems and always took great pains to advise the customer the furnace made a "differnt" sound, not necessarily an objectionable one. I can tell you that I had customers unhappy with the draft inducer motors on 80% furnaces, so sound has to be dealt with like any other concern. I had a PULSE installation that customers were encouraged to visit so they could determine for themselves, whether or not the sound would be objectionable. With changes that took place throughout the life of the PULSE, someone who is knowledgable needs to be looking at it. The big issue and eventually the downfall of the PULSE was the deterioration of the secondary heat exchanger return bends and the subsequent CO issue that accompanied it. The PULSE technology was GREAT! and featured multiple benefits inherent to the design. This single issue, is what contributed to its demise. I would strongly advise a Lennox dealer inspect the furnace thoroughly, especially if it is a first generation unit, as there were field repairs associated withthe heat exchanger that were performed back in the 90's. Your's may need the repair. Also, the venting (intake and exhaust) must not be in contact with any joists, or structural members or you will definitely transfer that sound throughout the entire house. When people had an opportunity to complare side-by-side, the majority had no problem with the PULSE sound. For your benefit, simply because of the "sound" there may not be a problem.

  8. #8
    Join Date
    Dec 2004
    South Central Kansas
    Yup, an '87 model needs to be pressure tested! Has anyone else heard what these do with a bad gas valve after a normal heating cycle? Boom!

  9. #9

    Post Test Kit

    Sounds like a worn Air Intake Flapper... If furance has not been properly service as already mentioned it's time. If this is your first Pulse I would recommend finding a Lennox dealer to complete the call.

    Lennox has a Service Kit tht contains tags, flapper material... But if your not familar with the Pulse I recommend the above.
    Lennox has a Pressure Kit that contains all the require plugs you'll need and a gauge setup for the excahnger but as stated... Get a Lennox dealer or contarctor familar with the Pulse.

    Once your familar with the Pulse have the proper tools having the knowledge is a pulse Plus for your area :-)
    AllTemp Heating & Cooling

  10. #10

    I worked for a Lennox dealer. When we came accross a Pulse. We got the customer to buy a new furnace.

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