In general, grilles reduce free air flow 50% or more so I'd aim to transition to a standard size at least twice as large as what's there. Maybe 20x25 if that fits.

Thanks for your reply. So what size duct would it need from the 14x14 opening to the wall? Same size all the way? Thanks again. Mark

I would make a sealed box from the duct to the wall. It doesn't matter what size as long as it's over 14x14. Caulk all seams. Then place a grille in a framed opening in the wall.

our new furnace and central air was not drawing through the registers like it should

Did you have this equipment recently installed? Is this the way the installer left the main return?

A 14 X 14 return may be too small for the required air flow. Do you know the size of your furnace and AC? Is there a filter installed at the furnace and if so what are the dimensions?

mike_home this was also listed in DIY. To those who come here unaware mike_home is a homeowner that lives in New Jersey. He doesn't do HVAC for a living, to my knowledge he never has.

An elaborate example why you should stop endorsing these kinds of things.

To the OP: DIY means you possess the knowledge needed to complete the job yourself. That accronym does not imply running to the internet to look at those showing you things in exchange for clicks, notoriety & fame. The health of your family, the value of your property as well as your own health can be greatly effected by things you do to your HVAC system without the proper knowledge applied. There is more to making an HVAC work than just clicking on an off.

You won't like what I say here, but it's for your own good. There is no excuse good enough...

click to enlarge manufacturer rules.

Ray,

How to you go from me asking if the HVAC contractor left a poor return installation to directing the poster to fix it as a DIY project?

HVAC isn't rocket science, Ray. Your home state of Texas, for example, specifically allows homeowners to install or replace their own HVAC system. That doesn't guarantee they have the knowledge and skills, but they're allowed to do it. Consulting resources--including those on the internet is perfectly appropriate to a DIY task.

Fabricating and installing proper return air ductwork in the OP's specific case makes use of 8th grade metal shop skills. There's nothing electrical involved, they won't need to mess with a furnace or air handler or controls, and they aren't risking life, limb or their family's health and safety. Posting an electrical shock warning is most inappropriate to the specific situation.

It is listed in DIY mike as well as heating and air conditioning.

Do I need to explain to you how the internet works now too?

What does improper installation imply to you? Oh look who I am asking --- a home owner who doesn't have an ounce of liability here.

What is liability? causing the destruction of property or the death of another by negligence, incompetence etc.

No offering advice here is helping --- regardless what the out come is. Is that right Mike?

You should be lecturing the person who installed an HVAC system with no air flow.

I have a electronic filter they installed. I had another company do the central air 3 ton unit because they were so poor installing the furance.

My furance is a Rheem 2 stage