HVAC unit upgraded, contractor cut a new return in the oddest spot

Question

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

Answer 1

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.