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Reducing Manifold Gas Pressure

GMR Member Posts: 3
Hello all,

Just wanted to reach out to the gas heating experts out there.  What is your feeling about reducing the manifold pressure and adjusting the fan speed to achieve the proper temperature rise in order to reduce cycling?


  • Jamie Hall
    Jamie Hall Member Posts: 23,419
    Don't do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 4,881
    So, your unit is seriously oversized?
    How old of a system?
    LP or NG?
    Moving more air will give you a cool heat and noised registers.
    Without a combustion analyzer and the knowledge to use it I wouldn't do it!
  • GMR
    GMR Member Posts: 3
    I have a combustion analyser.  I would not be increasing the airflow, I would decrease it in order to maintain the minimum recommended temperature rise.
  • GMR
    GMR Member Posts: 3
    The system is Natural Gas
  • HomerJSmith
    HomerJSmith Member Posts: 2,467
    What is your altitude? I have come across furnaces that needed to be de-rated and weren't. Lennox had furnaces that the service instructions suggesting altitude de-rating being done by reducing the manifold pressure.

    All the registers should be open and temperature rise checked.

    Sometimes the temperature thermodiscs that are in the safety circuit need replacing.
    EBEBRATT-Ed Member Posts: 15,607
    You can usually go from 3.5" wc down to 3" wc without issue but not without a combustion test and make sure it lights properly