re: Adding another return air vent

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don’t get putting in a return air at the furthest part of your house. That doesn’t make sense to me. Logically you’d want it near the other return air or at close to the supply plenum. If it were me, assuming you have the room, I’d put another return air very close to the original then tie into the plenum. If you’ve relatively hand it would be a pretty quick job. Whoever put in your unit with only a 20x20 return air screwed up big time.

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In order for the HVAC system to function properly and effectively cool all areas of the house equally, air must be pulled from all areas of the house to the return plenum - sometimes that is difficult if the air return plenum is not centrally located in the house (which it is often not as people don’t want a return plenum grill located in a central living room, dining area, etc), or if hallway, bedroom, home office or utility room doors are kept closed. So more than one return air duct might be located in different areas of the house to more effectively pull return air from all areas of the home to the air handler to be cooled or heated.

Also, when supply registers provide cool air into a closed room without a return air register in the same room, the room is under + air pressure and some cool conditioned is forced outside - the leakier the building envelope the more cool air is forced outside. If a return register is placed in that same room, sized at an equal cfm to the supply register, the room is under neutral pressure and you don’t lose conditioned air to the outside or attic. And the opposite is true, a room with a return register, without a supply register, will be under negative air pressure and thus some non-conditioned air from outside or the attic will be drawn into the house. That is the reason some houses are built with with balanced (cfm) supply and return air ducting/registers in each conditioned room. That is the ideal scenario, but more costly, which is probably the main reason it’s not done more often.

But I’ll agree, if the existing return air supply is under-sized and insufficient return air is not being supplied to the HVAC it would better to add an additional return air register to the unit even if located right next to the exiting return register than to not add one at all.

And I’ll close by saying all this assumes that only the return air ducting is undersized and not the supply ducts. If the supply air ducting is also undersized for the HVAC tonnage, then adding additional return air ducting to the system may not make any difference. This is pretty easily checked by HVAC technicians by measuring static pressure of the system at the air handler on both the return and supply side, getting a total external static pressure and comparing the cfm air flow to the HVAC manufacturers air handler blower performance charts. I’ll bet dollars to donuts the OPs HVAC tech didn’t do this, but it’s one of the first things that that should be checked when a homeowner is experiencing an air flow related cooling/heating issue.