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My old HVAC system suffered damage due to a lightning strike. I called up a local service co. and they sold me on a Trane split-system that was slightly oversized compared to what we had. The new equipment went into the same space and they adapted the existing duct work.

On the second or third day of work, one of the installers offered to add a new return and he said it would be free. I wasn't sure why it was necessary, but I figured he must know what he's doing and he's doing it for free, so why not? He cut a big return about 14x18" low in the wall directly behind the HVAC, opening up the plenum box to the living space so that intake air feeds right into the bottom of the air handler, just "ahead" of where the filter is mounted.

It seems to me this modification will severely unbalance the pressure throughout the house. Was this a sensible thing to do? If so, why?

More details:

  • Previous air filter was 16x25x1 new one is 20x22x1
  • 2-story single-zone house with thermostat upstairs
  • HVAC is centrally located in a closet downstairs
  • Ducted registers and returns in every room (no returns in 2 bathrooms)
  • In-ceiling ducting downstairs, floor registers and wall returns upstairs
  • High vaulted ceilings upstairs
  • Temperature difference between up/downstairs can reach 11F (chest height)
  • Location is Southern Missouri, heating with heat pump Oct-Apr
  • No dampers or other seasonal adjustment practices
  • My assessment of the home's insulation is poor-to-very-poor
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    Photos might help Apr 17, 2023 at 10:18
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    It's not unusual having a return right at the plenum box. Where was the original return?
    – RMDman
    Apr 17, 2023 at 11:56
  • Does "ahead" mean air from the new intake is filtered, or unfiltered? Assuming it's filtered (ahead means before?) there's nothing odd about the location of it. What's odd is adding such a big new return without a detailed discussion about summer and winter balancing needs. In the summer this will reduce cooling to the upstairs where it's needed more. Is that what you wanted? Or is the system zoned so that's not an issue? Or did you in fact discuss that in detail?
    – jay613
    Apr 17, 2023 at 13:50
  • @RMDman each room has its own return, 1st floor returns are in the ceiling, 2nd floor are in the walls. There is an existing large-size return for the upstairs directly above where they cut the new one downstairs-- they share the same wall cavity.
    – Jamesfo
    Apr 17, 2023 at 17:18
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    @PhilipNgai I'm in southern Missouri on the Arkansas state line. I will try to post photos.
    – Jamesfo
    Apr 18, 2023 at 23:03

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There is nothing wrong with merely having a return vent integral to the plenum. In fact, in a simple configuration you can have supply vents in each of several rooms and one central return vent, and putting that in the plenum is just easy and efficient.

But once you have multiple returns the way to think about balancing is to think of air flow. The more of the HVAC-treated air that flows through a room or space, the faster that room will be conditioned (heated or cooled). Having a return vent in each room is fantastic because you can really tune things to your liking. When you add one, large, new central return it will pull air from where it can ... from the closest rooms to it, with open doors most likely. If it's downstairs and all the downstairs doors are closed and all the upstairs doors are open it may pull air from the upstairs rooms, which would be great for cooling but not for heating. But that situation (all the doors the way I said) is contrived .... you need to think about which rooms if any got cooled or heated too quickly or slowly before, and whether this new return vent will increase the flow of air THROUGH those rooms.

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  • A return at the lowest level is the easiest place for the furnace to draw from and can result in it being harder to push air to the top floor as the return from the top floor can be under utilized. The more the returns at the top pull the easier it is for the furnace to push into those rooms. Apr 17, 2023 at 21:01
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    @FreshCodemonger If all the doors are open so air flow can be somewhat arbitrary, that's the right scenario as you say. More air will circulate around downstairs and less through the upstairs for the reason you describe. And in cooling season this will be disastrous for upstairs.
    – jay613
    Apr 17, 2023 at 21:27

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