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  1. #1
    Join Date
    Jul 2008
    Posts
    9

    Confused Hissing from new Ruud Modulating Heat Pump?

    In May I replaced the failing and ancient central heat/air unit in my 1940 era house. The house is well built and the ductwork drew compliments from several of the contractors who bid on the job. I picked a well established contractor who completed a Manual J heat gain/loss calc on the house. Since I plane to live here a long time, I choose a high end heat pump unit (93%) & 15 SEER from RUUD, the UPRL series. The installation went fine and the unit is cooling the house well enough.

    But there is this high frequency hissing/static type noise that is still making this unit unsatisfactory. The VS fan is fine on the low setting with no noise, but as the fan comes up to cooling load, heating load or just high fan speed, the hissing that comes from the ducts gets into my ears and won’t go away. The ducts closest to the indoor unit are the loudest and the sound fades the longer the ducts are. The contractor guys have been good to change out the fan, the aux. heat exchanger, and the evap coil, but still the same hissing. They have looked over all the ductwork and don’t find a source of the hissing. The regional distributor calibrated the temp. and air flow and still the hissing is there.

    A factory rep came out and looked things over and asked relevant questions of the tech guys and concluded that the sound is coming from turbulence in the new unit. He tells me that all the makers of the high efficiency units use a similar design and that this noise is part of this type of equipment. His idea was to install some flex duct to the short runs to dampen the sound.

    I am wondering whether to start over with new equipment from another maker, (but will I still end up with similar hissing?) or to try some of the flex ducting or what about duct liners? Can liner be installed in the existing ducts or not and is the liner safe?

    Any informed opinions on this situation would be greatly appreciated. Thank you.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,367
    The flex duct may help.

    But, have your contractor check your systems TESP.
    Most likely its too high.
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    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Jul 2008
    Posts
    9
    What is the TESP? Thanks.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,367
    Total External Static Pressure.

    The static presure in your supply is most likely too high.
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  5. #5
    Join Date
    Jul 2008
    Posts
    9
    Thanks for the ideas... with the filters removed the hissing is still the same... with the unit partially disconnected from the main supply and air coming out all over the place, the hissing appears to be coming from within the fan, exchanger and coil unit... as far as I can tell the noise is not in the ducts...

    Should I go for the equipment change or will a similar unit probably sound the same?

    Thanks again.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,367
    On teh first Yorks with VS blowers. Yhere was a problem with the blower housing. They weren't formed quite right. And they made noise. A new blower, or on some, we reformerd it ourselves, and the noise went away. ( A machine was not set up right at teh factory.)

    You could have a simular problem.

    See if your contractor will swap out the unit.

    VS blowers are quiet. So something is wrong with yours.
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  7. #7
    Join Date
    Jul 2008
    Posts
    9
    Thanks again for the info from this site....! Some of the hissing/vibration noise has been taken away because the refrigerant lines were in contact with the metal hangers that were attached to the metal ducts! (I found those)

    One final question before I insist on a complete unit swap..... The unit is installed in an upflow position and when the fan is pulled out and I look up into the heat exchanger and tap on the outside of the case, there is a long lasting metalic vibration sound inside the aux. heat area. I discovered one sheet metal screw that was loose and a metal piece was loose and vibrating. Can this area be taken apart and all the screws tightened? Or is it time to swap?

    Thanks again....

  8. #8
    Join Date
    Jul 2008
    Location
    Coastal North Carolina
    Posts
    10
    JP, I wouldnt swap the unit out until I had the static pressure tested. With variable speed air handlers, there may be a duct issue going on.

    With a regular motor, the rpm's will turn at 1075 and will stay there. If the duct system is too small, the airlfow will be diminished according to the size of the duct. With a variable speed motor, the motor will actually rev up to ensure the proper amount of airflow. You may have a situation where the velocity through your ductwork is too high, and your motor is turning too fast.

    If you change out your air handler with another variable speed one, if the problem is the ductwork, then youre back at square one. Just because the ductwork looks nice, and was installed well doesnt mean that it was sized correctly.

  9. #9
    Join Date
    Jul 2008
    Posts
    12
    Round or rectangular duct?
    Is the plenum insulated on the outside?
    Think about upsizing the plenum and insulating the inside.

  10. #10
    Join Date
    Jul 2008
    Posts
    9
    TESP has been tested and falls within the range of the equipment.

    Still within the Aux. Heat Exchanger area there is a metalic noise that vibrates for a couple of seconds and fades out when the system is off and you tap on the top of the case. Is this normal? I can't help but think that something is loose, out of place or designed wrong. When all that air starts moving through this part where there is a vibration, I can't help but think this is the source of the hissing noise.

  11. #11
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,339
    Ask the dealer if he removed the shipping stuff from the blower assy. That would cause hissing but does cause other noises and vibration.

  12. #12
    Join Date
    Jul 2008
    Posts
    9
    The hissing noise is coming from the heat exchanger of the gas furnace of the new unit. With the unit off and listening in the duct a light tap on the exterior of the furnace unit leads to a metalic vibration sound that fades away in a couple of seconds. I imagine when the fan is on this vibration is amplified a great deal.

    The contractor says nothing can be done to stop this sound in the heat exchanger, it is normal. Is it? They want to start duct modification to damper the sound, I don't plan to go this route as we can hear the hissing in five to six ducts. Are there quiet high efficiency furnaces out there?

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