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  1. #1
    I have a Goodman 92,000 Btu 4 speed single stage high efficiency furnace. It's aparently too big for this house. My house natural gas pressure is 4 inches of water ( a little low). The manifold output is supposed to be no higher than 3.5 according to the nameplate. The trouble is that, even with the CFMs set to max, at 3.5 or even 3.2 inches of water on the manifold, the temperature rise goes to about 75 degrees which is 10 degrees over max. Can I reduce the manifold pressure with the set screw on the valve with this type of furnace (it's not a draft furnace)to cut the flames back a bit?. I would like to set it at 2.25 inches w.c. which gives a temp rise of 60-65 degrees. Even at 1 inch, there is a very strong flame with no delay in lighting although I wouldn't be comfortable with this much of change. No, there are no closed vents or blockages. The CFM table says the vents are big enough to support the CFMs. There are lots of (22) registers.

  2. #2
    Join Date
    Oct 2004
    Posts
    3,400
    Don't do it.

  3. #3

    More info

    I spoke with a friend of mine a few minutes ago who is licensed. He told me that you can safely reduce the pressure to the manifold in this furnace but you lose efficiency. What happens, he tells me, is that the velocity of the gas entering the combustion tubes is reduced and the flame is not as "long". He tells me - off the record - that it is done all the time but the proper way to make this type of adjustment is to change out the orifices on the manifold (smaller) and keep the pressure the same. This keeps the velocity of the gas up and lets the flames reach more of the combustion tubes.

    Any comments?

  4. #4
    Join Date
    Oct 2004
    Posts
    3,400
    You're getting closer...

  5. #5
    I've decided not to monkey with my installer's installation and leave it alone.

    How is that?

  6. #6

    Stupid

    My installer pointed out that I was measuring the output temperature too close to the heat exchanger. I'm going to take his advice and not try to second guess the installer. That's what he's paid for.

  7. #7
    Join Date
    Oct 2004
    Posts
    3,400
    You know, that is probably a good idea.

    Years ago, it was fairly common to "de-rate" a furnace by changing the orifice to a smaller size.
    However, with today's high efficiency furnaces, that practice has fallen out of vogue.
    The concern is over condensation in the heat exchanger.

    Do you have adequate airflow?

    Sometimes a filter is the culprit.
    Those marketed as high efficiency usually restrict airflow considerably.

    Do you have any other reason, besides temperature rise, to believe the furnace is oversized?

  8. #8
    Join Date
    Oct 2004
    Posts
    3,400

    Re: Stupid

    Originally posted by readywolf
    My installer pointed out that I was measuring the output temperature too close to the heat exchanger. I'm going to take his advice and not try to second guess the installer. That's what he's paid for.
    There's a difference between "stupid" and uninformed.
    I don't think you are a bit stupid.

    I'm glad your installer was there to give guidance.

    It always makes me nervous when pressures are adjusted. Especially when the criteria for success include "strong flame".

    Good luck.

    Get it serviced every year, so it can serve you well.

    [Edited by bwal2 on 02-28-2005 at 10:42 PM]

  9. #9
    Join Date
    Feb 2005
    Location
    Barrie, Ontario
    Posts
    4,622
    On any oil, natural gas or propane furnace with a force vent, the air/gas mixture through the venturi is set by the manufacturer and has to remain close to the rating plate. If you back off on the gas alone you will end up with an extremely lean flame. Know anything about oxy/acet torches? What happens when you add too much oxygen?. Your flame becomes short and hot, right?. You will burn out(crack) the newer heat exchangers if you don't have the proper gas/air ratio information from the manufacturer. many manufacturers use one size vent assembly for their furnaces and adjust the size of the air restrictors. A furnace should only be derated with orifices AND proper sized manufacturers air restrictors for the vent assembly. I agree with bwal2. Best left alone unless the manufacturer is involved. The approvals depend on it.

    [Edited by gruntly on 02-28-2005 at 11:18 PM]
    Is this a Fabreze moment? C.Y.D. I'm voting white elephant. 2.
    My competition are my best salespeople!

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