Return restriction and Variable speed fan
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  1. #1
    Join Date
    Jul 2004
    Location
    South Alabama
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    24

    Return restriction and Variable speed fan

    I am getting ready for a air handler with a variable speed fan.

    It is a 5-ton unit and my return is a 24x24 grille. I've read some stuff on here that says that too much pressure/restriction could hurt the life of the motor.
    I know most of the pro's recommend certain "tests" to determine capacity, etc.. but I've never seen or heard of any of the companies here doing that... at least on a house that has already been built and has central a/c. Maybe it is an area thing?

    Anyways.. I feel certain the installer will say that we can do whatever I want on the return and leave it at that... Is that a big enough return for this size of a unit. I think my only choice would be to put another return on the other wall if it is not. This unit is in a closet that is on a corner in the central hall.

    The unit was previously in the basement with an even smaller return, but a duct leading to a floor grate in another room to provide additional input.

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Check Alabama Power's website for trained contactors.

    If the return duct is only the closet below the air handler that's not all that bad,however that's the correct size for 3 tons,so increasing it is a good idea.


    Lower static ducts/grilles(think resistance to air flow),will lower your operating costs with the VS motor.

    That space below the air handler is negative pressure,so it should be sealed air tight from the wall cavities,to prevent pulling in hot air from ouside or the attic.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,595
    You will know how under sized it is when it goes to high speed by the noise.
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  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    4,781
    Dash, is right on target!

    Are you sure you need(ed) a 5-ton system?
    It is much easier to get a 3.5 or 4-Ton airflow, than enough airflow for 5-tons.

    Does the evaporator have a TXV metering device?
    If so, with the unit charged to a lower airflow it will deliver that lower BTUH.


    The main thing is that the compressor is properly cooled, the tech should check the compressor discharge line temperature.
    http://www.hvacwebtech.com/1marks.htm
    Better safe, than sorry!
    - udarrell

  5. #5
    Join Date
    Jul 2004
    Location
    South Alabama
    Posts
    24
    My current unit only has one speed... Zoooom & Off and I never thought it was very loud, so maybe its ok then. I'm going to try and do some clean up work in the closet before the new unit is installed, maybe put in a light,etc.

    I built the stand and converted the closet for the old unit to sit on.. It was in the height of summer between Jun/July when we moved in. I didn't install the unit or anything just did the hack carpentry stuff so they could put the unit in the closet... I was a bit stressed/rushed

    I have some questions on improving the register box and the closet layout.. Maybe fix my hack-carpentry work. Are these kinds of DIY questions ok to ask here?? I'd like to post some pictures and get some hints on making the design "neater" when they do the install (possibly next week).

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,595
    Its quite common for a standard blower to have been relatively quit. And then when a VS blower is installed. It is very loud. The VS motor will increase RPM until it either senses the torque it should have to move its rated CFM, or its reaches its MAX RPM.
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  7. #7
    Join Date
    Jul 2004
    Location
    South Alabama
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    24
    Quote Originally Posted by udarrell View Post
    Dash, is right on target!

    Are you sure you need(ed) a 5-ton system?
    It is much easier to get a 3.5 or 4-Ton airflow, than enough airflow for 5-tons.

    Does the evaporator have a TXV metering device?
    If so, with the unit charged to a lower airflow it will deliver that lower BTUH.


    The main thing is that the compressor is properly cooled, the tech should check the compressor discharge line temperature.
    http://www.hvacwebtech.com/1marks.htm
    Better safe, than sorry!
    - udarrell
    I'm not sure about anything... they guys I have had out have just suggested I stay with a 5-ton unit. I've had 5 companies here to give estimates... no one did any kind of calculations or tests or anything, they just wanted to look at the old unit & the install to see how complicated of a job it would be.
    I (enough to be dangerous homeowner) think personally I could get by well with a 4-ton based on how little the old unit would cycle on.

    One guy did look at the heater strip.. I told him it was undersized (I knew this, it was only an 8kw and would run forever) The old unit is not working (Hole in compressor) so no way to do any test that require a working system I guess other than airflow.

    It's the end of June in the Deep south, so business is probably really busy, so I understand if they don't have the time, I'm not complaining.

  8. #8
    Join Date
    Jul 2004
    Location
    South Alabama
    Posts
    24
    Beenthere.. I know this is probably a best guess.. but the old unit is a goodman straight AC 5-ton manufactured in '99, just running on High.

    If I go with a carrier/bryant 5-ton 16 Seer with VS fan.. do you think the VS Fan would push more air when they are at max RPM than my old unit with a single speed? It sounds like the VS fans have a higher rpm and flow rate than the single speeds.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,595
    Most likely it will move more air.
    And yes, VS motors can run at a higher RPM then standard motors.
    Average standard motor is rated ay 1075 RPM. VS max out at I believe 1400 RPM.

    Of course if its at 1400 RPM, it will have a short life span because it will be working against too much pressure.
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  10. #10
    Join Date
    Jun 2008
    Location
    va
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    800
    Is it a heatpump or a/c? 8kw heat is norm for 1 1/2 tons of air. What's the square footage of the home? I wonder if whoever put in the 5 ton last knew what was going on. I agree with a preveous poster on the 3 1/2 or 4 ton unit assuming that is what is needed.

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