Brand new system that feeds 3 bedrooms, one downstairs and two upstairs. The system is hooked up to 2 electronic dampers for zoning. The downstairs gets good and cold, however there doesn’t seem to be enough air getting to the upstairs.

It’s quite a long run to the upstairs bedrooms, maybe as much as 50’. We don’t see to get the same pressure upstairs as we do downstairs.

From the picture below you can see the ducting and the connection quality. Clearly these connections should be taped, but from brief inspection they don’t feel like they’re leaking air. The returns also seem to lack much sucking power. To even feel the airflow I have to wet my hand and hold it up to the return.

Would a booster fan help? Anything else?

branch in hot as hell attic

EDIT: I noticed that the airflow was worse than usual and noticed that the refrigerant line was frosted over both at the coils and at the condenser. So I clearly have multiple symptoms if not problems with this system. The HVAC company is installing a blower into the supply duct, hopefully with increased airflow it will solve the cooling issue and the freezing of the refrigerant line.

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    Possible to replace the run with a larger diameter? run a second parallel line? Is there a kink? Jul 10, 2018 at 10:13
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    2 electronic dampers for zoning: Upstairs/Downstairs I presume.. Close off the down stairs more (I would try to close it all the way just for a test ) see if that helps. Where is the distribution baffle - is there a proper configuration there or do they simply feed a box and out with the two ducts? Your fan speed might also be set wrong - I would check it while closing off distribution zoning to lower level as well.
    – Ken
    Jul 10, 2018 at 10:57
  • @JimStewart both the supply and return are going through a pretty narrow passage that leads into the garage and down to a sub attic in a new addition on the house. It’s possible the air could be restricted there. If we were to sacrifice the return, I suppose that could help, maybe route the return to a duct outside the house (unfortunately might be the only other way). It’s also possible it’s not restricted there, I would hope the HVAC installer didn’t just leave a huge kink there, but who knows with these things.
    – Matt
    Jul 10, 2018 at 13:18
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    Closing a certain damper entirely could reduce airflow through the A-coil by so much that the A-coil temperature will drop below freezing. Just don't close the damper completely. Be sure that all the dampers in the path you need more cooling are completely open and see what the result is. Jul 14, 2018 at 12:06
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    Post your update as an answer and I'll give you a +1 for it, @MatthewLevine Aug 3, 2018 at 11:39

1 Answer 1


Fixed! Two main issues, one was that the 12” duct was being constricted because it was being squeezed through a small area along with a line for the return.

The other was that they setup a feedback line from the coil directly to the return. Effectively there were three 12” ducts coming out of the supply, 2 went to the various parts of the house and one fed directly back into the return. Removing that feedback line gave us a ton more air. No blowers needed.

  • What was the alleged purpose of the feedback line?
    – mrog
    Aug 3, 2018 at 16:09
  • 1
    The system was originally setup with two electronic dampers. The explanation I was given was that the feedback line was there to prevent issues (freezing/backflow) when one of the dampers was closed while heating/cooling one zone and not the other. We have disabled the dampers in favor of a single zone. But even when one damper was entirely closed we never saw a noticeable increase in pressure
    – Matt
    Aug 8, 2018 at 23:22

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