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  1. #1
    Join Date
    Nov 2002
    Posts
    45

    Question

    My question is very simple. I was working on a mua and taking gas pressure. the rating plate say's 3.5" do I set my manifold pressure to 3.5" or do i take the suctoin on the burner which was -1.5 into consideration and set my manifold to 2.0". I ask this question because I have conflicting answers from the manufacturer and other Techs.
    THANKS FOR THE ADVICE.

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    18,146
    manifold pressure 3.5 w.c.

  3. #3
    Join Date
    Jan 2004
    Location
    Rogers,Minnesota
    Posts
    638
    I agree-and if possible,never hurts to be sure and clock the meter to make sure right pressure/orifice size.Ive been fooled by setting at 3.5"wc before but my co level was high-found out the orifices were oversized and someone reduced the pressure to compensate.

  4. #4
    Join Date
    Jan 2004
    Posts
    1,176
    I beg to differ, if your manifold is in a 11/2 inch vacuum you have to calculate that into the equation or you will overfire the burner so I would set the manifold to 2 inches.

  5. #5
    Join Date
    Nov 2001
    Location
    Moose Jaw, Saskatchewan, Canada Occupation:Interprovincial Plumber, Commercial Gasfitter Interests:
    Posts
    2,414
    If the rating plate says 3.5" it should already take into account any negative pressure.
    I love my job, but paydays Thursday

  6. #6
    Join Date
    Feb 2003
    Location
    Wichita Ks
    Posts
    1,505
    check your temperature rise.dont adjust gas pressure that will over shoot your temp.

  7. #7
    Join Date
    Jan 2004
    Posts
    1,176
    In the case of direct fired make up air units it is not figured into the equation.The manufacturers do not know how much duct etc is on the supply side so therefore they do not know what the velocity will be across the burner plate.The higher the velocity the more negative pressure will be on the manifold.If an air balance is not done and the unit is over design CFM than the burner will be overfired if set to 3-1/2".

  8. #8
    Join Date
    Nov 2002
    Posts
    45
    I tend to agree with AC. but as you see there people who differ. Hackhater how do you go about clocking a meter when your working in a plant using aprox 15 million btu's. I also like the answer of setting unit to temp rise but you might run into problems with that.

  9. #9
    Join Date
    Jun 2004
    Location
    Maryland's Eastern Shore
    Posts
    892
    Carrier requires using a u-gauge manometer to take manifold gas pressures on sealed combustion systems. It involves taping into the suction hose coming from the burner box and going into the gas valve and attaching that t-eed off hose to one side of the u-gauge and attaching the manifold gas pressure tap to the other side. You will then be able to set the required 3.5" WC while taking into consideration the vacuum. If you just set the manifold at 3.5" WC, and you have 1" WC vacuum, you will be over gassing by 1" WC.

  10. #10
    Join Date
    Nov 2002
    Posts
    45
    Sorry i may of led some people astray this is a amu direct fired.

  11. #11
    Join Date
    Jun 2004
    Location
    St Paul, minnesota
    Posts
    1,245
    take manifold pressure & add the negative. 3.5" + (-1.5")= 2.0" All direct fired MUA units are this way. Never adjust gas pressure for temperature rise. Air flow has to be set 1st you should have approx. .5" across the profile plate. You can measure this @ the air flow switch taps. If press too high either slow down the fan by readjusting the motor pulley or by closing down on all of the supply dampers.
    By the way if your air flow is right & you set regulator on high fire to rated manifold minus the negative your temp rise is usually within 10% of unit rating.

  12. #12
    Join Date
    Jan 2003
    Posts
    126
    Always set by temp rise 100-125 that is all that matters if air flow is correct and profiles are set .5o-.60

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