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Thread: Two zone system, first floor AC works great, second floor inconsistent

  1. #1
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    Apr 2013
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    Two zone system, first floor AC works great, second floor inconsistent

    I've got a basic two zone system with a Honeywell HZ322 controller connected to two dampers (one for each floor of the house) and have Venstar T7900 thermostats installed. The first floor seems to always be fine and works great. The second floor works fine as well but when it gets >100F outside, which happens often in the summer here in Las Vegas, the air blowing out of the vents is at least 83F vs ~53F and the second floor rapidly gets very hot. The interesting thing is if I power cycle the 2nd floor thermostat, everything starts up fine again within a few minutes with the air blowing around ~53F again and slowly cools things down. And as I said, the first floor works fine and never has a problem at all.

    When inspecting the HZ322, zone 2 is green and the damper open. I have watched the damper open/close and didn't hear anything odd indicating a broken gear or anything. I've had three technicians out here and spent a ton of money for their diagnostics so far and no one can seem to even point me in the right direction to fix this (even though 2 of the 3 technicians were here while it was faulty). The only thing I can think of is that the damper motor could in fact need to be replaced for the 2nd floor or the HZ322 itself could be faulty? And to top it off, this ONLY happens when it's over 100F outside and only the 2nd floor zone. The attic is insulated on the roof line (Pulte construction) so it never really gets more than 10-15F hotter there than the house. Last summer it happened multiple times each week and hasn't happened since. Today we broke 100F and it happened about an hour ago for the first time this year unfortunately so I am trying to figure this out before it gets too far into the summer.

    Thanks in advance for any guidance!

  2. #2
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    A quick update, I did some inspections this evening and it looks like the 2nd floor damper motor may actually be failed this time compared to the other times which may have been intermittent. The dampers are M847D1004 units and the 1st floor one opens/closes easily but the 2nd floor one seems to be stuck in the closed position unless I jiggle the bolt slightly and then it opens. When I confirm it is open, the air flows as expected, nice and cold!

    So I guess it's time now to get that one replaced and be hopeful thats all this is. Do these fail often? Should I proactively replace the second damper motor for the 1st floor (which is currently working fine)? The house was built in 2015 so they are about 7 years old.

  3. #3
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    Quote Originally Posted by smokey7722 View Post

    A quick update, I did some inspections this evening and
    it looks like the 2nd floor damper motor may actually be failed this time
    compared to the other times which may have been intermittent.

    The dampers are M847D1004 units and the 1st floor one opens/closes easily
    but the 2nd floor one seems to be stuck in the closed position
    unless I jiggle the bolt slightly and then it opens.

    When I confirm it is open, the air flows as expected, nice and cold!

    So I guess it's time now to get that one replaced and be hopeful thats all this is.
    Do these fail often?

    Should I proactively replace
    the second damper motor for the 1st floor (which is currently working fine)?

    The house was built in 2015 so they are about 7 years old.
    Problem only occurring at > 100'F really leads to a more complex situation.

    Source of ANY Intermittent electrical issue may persist due to
    the inherent difficulty in identifying the ROOT CAUSE.

    7 Years SEEMS to be a relatively short period IMO.

    ??? Wiring connection.
    ??? Motor / Damper Actuator Alignment
    ??? Damper mechanical action
    _ _ or the HZ322 itself could be faulty? ___

    https://s1.img-b.com/build.com/media...tion-sheet.pdf

    I would not proactively replace 1st floor motor at-this-time.
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  4. #4
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    Apr 2013
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    I agree it seems odd, especially given the attic really doesn't get too hot and the HVAC on first floor continues to always work well. Last year we thought the HZ322 was defective as everything else seemed ok but since it was the end of the summer I was curious if it would continue to exhibit issues over the winter (without replacement) and it did not. Obviously the system needs to run more often during the heat of the summer though.

  5. #5
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    Quote Originally Posted by dan sw fl View Post
    Problem only occurring at > 100'F really leads to a more complex situation.

    Source of ANY Intermittent electrical issue may persist due to
    the inherent difficulty in identifying the ROOT CAUSE.

    7 Years SEEMS to be a relatively short period IMO.

    ??? Wiring connection.
    ??? Motor / Damper Actuator Alignment
    ??? Damper mechanical action
    _ _ or the HZ322 itself could be faulty? ___

    I would not proactively replace 1st floor motor at-this-time.
    So I had the second floor damper motor replaced and things seem to be working well now for the past 2 weeks. Ironically 2 days ago I started seeing the same problem on the first floor zone so that one is being replaced ASAP as I am not bothering to mess with it. Symptoms are the same so assuming it's the same issue. My neighbor commented the other day about his house having similar issues (we were all built around 2015) and so his will get done when my 1st floor is done. Hopefully that's all it is and we're done with this issue!

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