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Mitsubishi 2.5 ton whole house system - 1000 sq. feet home

Terry3
Terry3 Member Posts: 6
edited May 25 in THE MAIN WALL
Good evening,

A month ago I had the Mitsubishi system installed in my home, with an added minisplit in the family room (approx 350 sf). My home was built in 1954 and s one level and sits on a slab. It has the one heat pump outside for both systems. Afternoon and all night long, in addition to the sound of the air flow, there is a high pitched whine that comes from all of the ceiling vents, most loud in the 2 back bedrooms to the point that it is difficult to sleep in either of these 2 rooms. The sound is not recordable over the air flow at least not with my phone.

The loudness also seems to be affected by the drop in temperature at night -whether warm or cold weather, if there is a 20 degree dip in temperature, it appears to get louder.

Anyone have a similar experience? Any ideas? The HVAC company has been out a few times to check the system and can't find a cause.

Suggestions? air flow issue? Flex issue? Insulation issue? The system itself is not insulated in the attic.

Appreciate your thoughts. Many thanks in advance.

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,104
    Sounds like undersized ducts and diffusers. Any duct work in the attic certainly needs insulation. In addition flex runs should be kept to a minimum as flex reduces air flow. Good practice is to take a 25'fox of flex and cut it into 3 pieces so no more than 8' or so in a run. shorter is better
    Dave T_2Terry3mikeapolis
  • pecmsg
    pecmsg Member Posts: 2,017
    edited May 12
    Start with Static Pressure Readings!

    Im confused you have a ducted model and a single zone covering 350 Sq Ft. Where is this house that requires 2.5-Tons for 1000 Sq Ft?

    Need Model #'s of the indoor and outdoor units.
  • GW
    GW Member Posts: 4,124
    2 1/2 seems big

    sometimes noise is the ducts, or the diffusers/grilles, but in just about every case the air pressure is too high (delivery system is too small). I've never installed a Mits air handler--- If you/they can turn the cfm down and the noise lowers, then you know the answer.

    Another very fast trick- pull the filter out for a minute and see if the noise drops. It's all related
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
    Brent H.Terry3
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,104
    My house that I sold was stone and block construction about 1000 sq feet. Windows were ok, Attic insulation was 6' walls had no insulation.

    When I did the heat gain it came out at 27,000 btu. I debated 2 or 2 1/2 ton. I went with the 2 1/2 and it worked pretty hard on a hot day. 2 ton would have done it if it ran all the time like they do down south
  • Terry3
    Terry3 Member Posts: 6
    To answer various questions: The house is located in Alexandria, VA. Our outside temperatures are fluctuating approx. 30 degree between daytime and night time - 70 during day and 40 at night. Noise is loudest at night.

    My attic is full of flex, some sections are 20+ feet long with bends. Some sections are flat and other are suspended from the attic ceiling. I currently have a 20x20x1 return with a 16 inch flex over top. It has been suggested that I add an additional return approximately 7 - 14 inches in size.

    How do we insulate the ducts? and how do we make only 8 ft sections of flex if the ceiling vent is about 15 feet from the main line?

    Thank you again everyone, truly appreciate the assist in getting this situation resolved.

    Terry
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,104
    Starting from the air handler both the supply and return should be metal duct wrapped with 2" insulation. The branch takeoffs from the main metal duct should also be metal duct also insulated with 2" duct wrap. Any short branches less than 8' can be all flex. A 12' branch for instance should be no more than 8' of flex and 4' of metal duct.

    With labor costs the way they are every installer is using too much flex. It adds a lot of air flow resistance.

    You need to get someone in their to troubleshoot the air side of the system and come up with a solution
    Terry3
  • GW
    GW Member Posts: 4,124
    Insulation has nothing to do with noise.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Terry3
    Terry3 Member Posts: 6
    Thank you both for your updated comments. I have an independent HVAC person helping to troubleshoot the situation. Will keep you updated as we progress.
  • Terry3
    Terry3 Member Posts: 6
    Good afternoon, everyone. Here are the Model numbers for the system in my home: MXZ-3C0NAHZ2-U, SVZ-KP18NA, MSZ-FH12NA. My home dimensions are the following: Main house -1170 sf; family room - 274 sf, total sf = 1444 sf. The mini split and the main house unit are connected to the same outdoor heat pump. Apologies for the delay, I only received the warranty info last Thursday (unit was installed March 5), and corresponding model numbers from HVAC company. Please let me know if you would like photos of the attic installation - the unit itself and the flex vents. We appreciate your continuing input.
  • GW
    GW Member Posts: 4,124
    Ahhh makes more sense- 1 1/2 ton air handler. 

    Still, as mentioned, static pressure readings tell the truth. If your install people don’t know how to do that, super bummer 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Terry3
    Terry3 Member Posts: 6
    Thanks, Gary, not sure I understand your comment. Is the air handler too small or too big? Static pressure same as cfm? Sorry, want to understand for my own edification.
  • GW
    GW Member Posts: 4,124
    I was just saying 1 1/2 ton is better——at first we all thought you had a 2 1/2 ton air handler. 

    Static pressure is a measurement of how much air is getting stuffed into the ducting. It’s very easy but many hvac guys have no idea. For the most proper readings you need a “pressure tip” and a decent manometer. 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Terry3
    Terry3 Member Posts: 6
    Thank you Gary. Truly appreciate the explanation. We are getting there. Found out that the original installer had placed damper in the vent to the back bedroom to push air to the living room is the contractor's opinion. Now waiting for independent licensed contractor to come and check the rest of the ducts for dampers. Also called the county inspector who failed the install because company failed to provide a duct layout since he replaced the old one. Unfortunately the company has 6 months to comply and I can't wait that long for relief. Will be contacting original company to see if I can push this along.
    mikeapolis
  • GW
    GW Member Posts: 4,124
    Ok glad you’re making progress 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
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