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Currently two long duct banks (one on each side of the house) are used to heat/cool both the 1st floor and the 2nd floor. Vents for the 2nd floor are installed in the top of the duct and vents for the 1st floor are installed in the bottom of the duct. The vents in the 2nd floor bedrooms are installed nearly directly over the vents below in the first floor. I'm thinking this is contributing to our problem of the 2nd floor being uncomfortably hot in the summer and the 1st floor being colder in the winter. During an upcoming remodel, new flooring will be installed. I'm looking at moving the two vents in the bedrooms, and need to know the how far away (horizontally) from the vents below they need to be spaced. Also should a single duct even be used to heat/cool two separate floors? (aka having vents directly coming off the top and bottom of a duct, specifically right on top of each other?)HVAC Vent and Duct Locations

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  • how far away (horizontally) from the vents below they need to be spaced? - As far as possible, but it has nothing to do with air flow. It's so crap can't fall from the 2nd fl to the 1st, and so you can't hear the other rooms.
    – Mazura
    Sep 16, 2022 at 16:06

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There shouldn't be a problem as far as horizontal distancing as long as the duct work decreases in size as it gets further away from the air handler. Where the ducts are above and below each other, permanent dampers or diverters should be installed to divide the air between the two. Have you checked all the duct sizes? Would you ever consider running a separate duct in the attic for the second floor and keeping the other duct just for the first floor?

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  • While running a separate duct in the attic is feasible, it will require significant insulation as the attic temperature will be the opposite of the current heating or cooling mode resulting in a loss of performance. In the summer, the attic will be hot and will heat up the cooled air in the duct. During the winter the attic will be cold and will cool down the heated air in the duct. The extent of this will depend on location.
    – pdd
    Aug 16, 2022 at 23:13
  • @JACK "duct work in attics ... with no loss in performance"? Duct work is R6 or R8 at best, in a 120 degree attic this will certainly effect performance. Every quality builder recommends against this, and that all ac equipment and ductwork should be within the conditioned envelop of the house.
    – Glen Yates
    Sep 16, 2022 at 0:35
  • @GlenYates Not sure where you're located at but South Florida's had a building boom for years and all the duct work in single story homes is in attics and and second story homes utilize the attic for the second floor. I've been involved with hundreds of subdivisions constructed by quality builders that have had no problems doing this. There are R values higher than R8.
    – JACK
    Sep 16, 2022 at 1:31
  • @JACK "building boom" and "quality builder" is an oxymoron! Just because a lot of houses are designed poorly does not make them good. Look up resources such as Green Building Advisor or watch some of Matt Reisinger's videos on youtube. Also, I challenge you to find flex duct with greater than R8 insulation. And, sure the builders have no problem putting ductwork in uninsulated attic space, it is the easiest cheapest thing to do, it is the homeowner that suffers with higher than necessary bills for years thereafter.
    – Glen Yates
    Sep 16, 2022 at 16:22
  • @GlenYates It's Risinger. Link to R-12 ... lambro.ca/en/products/conduits-flexibles-isoles/type-gr12-r12 ..... Good luck selling whatever it is you're selling.
    – JACK
    Sep 16, 2022 at 18:06
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This sounds like an issue with air balancing. (Assuming that your air handling unit and the distribution ductwork is sized and installed correctly).

Do your registers have integral dampers?

If so, you can start by opening all of the dampers and allowing the system to run for a bit. Let's say that you are balancing the system when in cooling mode. Start slowly closing the dampers in the rooms that are too cold and letting the system run for a bit more. Repeat this process unit all areas are at a similar temperature. It will be fiddly and will take a bit of time.

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