If you have a 5 ton AC that return is too small even without the 6x6 blocking it. You would need to at least double the size. For a 5 ton configuration it is common practice to have two returns entering the air handler, one on each side when it is an upright position.

Have you turned the air handler on yet? I imagine the air rushing through the return is very noisy. You need to discuss this with the HVAC contractor ASAP.

Thanks for the response Mike! The contractor said that he turned it on and it was quiet. I haven't heard it personally since it just got wired yesterday.

So is there an additional intake for the air handler besides the one at the bottom? Or would a second return have to brought into the return "box" (which would probably be a little hard since it's sorta a wall cavity)? I looked at the installation guide and couldn't make any sense out of it.

Thanks!

Usually furnaces and air handlers will have openings on both sides in order to allow the return to enter from the left or right if it were in an upright position. I assume you would have access to one of the sides in order to add a second return.

Even if the air flow is quiet, in my opinion this is a problem. A 5 ton unit needs 2000 CFM of air flow to operate correctly. This configuration is starving for air and will not be able to provide sufficient air for the supply ducts.

One more question. Since the air handler intake is pretty much directly connected to the grill vent cover (minus the 6x6), would it be as simple as making the grill vent cover larger?

Thanks!

I found this note on page 11 of the installation manual under upflow units:

NOTE: Where the maximum airflow is 1800 CFM or more, both sides or the bottom must be used for return air.

This is saying the bottom return by itself will be enough. There was note under the upflow unit section, but I assume it also applies to a horizontal installation.

You want the filter and grill cover to be at least the size of the opening to the furnace. Making the grille bigger is not going to help if return itself is smaller. I don't know how this is going to work with the filter size you quoted. I would think you would need two 14X24 filters. It would be nice if you get get a 4 inch filter instead of the 1 inch. Otherwise you will be changing the filter every month.

The 6x6 stud obstruction should be removed and you want a tightly sealed return duct. The return duct should not have a right angle. It should be a curved elbow to make the 90 degree transition. Sharp angles create more air resistance.

How big was the original house and AC, and much area are you adding? Most houses don't have the properly sized duct work to handle a 5 ton AC. Do you have confidence in this HVAC contractor?

Thanks for the detailed response Mike! I talked to my GC about it and he's going to push the HVAC contractor. We'll see what he says, but the bottom opening of the air handler is about 24x20, so I'm hoping that they can move handler further back into the attic so that they can have a bigger return box (and not have the 6x6 obstruction). So that should allow at least a 24x20 and maybe a 20x30 grill size.

The grill allows a 1-inch filter, but I've seen some Honeywell filters that have a 1-inch "inset" but are 4-in filters so I'll take a look at that.

I'm not totally thrilled with HVAC guy. I know he didn't do a Manual-J and I didn't know enough about all of this to insist that a Manual-J be done.

I believe our previous A/C was a 4 ton, don't remember exactly. We are adding about 900 sq ft and will have a total of 2300 sq ft after we're done. We have some pretty large cathedral ceilings and the rest of our ceilings are all at least 9 ft, so perhaps that factored into his decision. Anyways, I really like our GC and all of his other subs have been great so far, so perhaps I deferred a bit too much to this guy.

A 5 ton unit for 2300 sq. ft. sounds oversized. What are the average highs during the summer?

You need to push to have this done correctly. You want the biggest filter possible. The general rule is you 2 CFM of air flow for every square inch of filter. Your 5 ton unit requiers 1750 - 2000 CFM of air flow to operate properly. This requires a filter of 875 - 1000 sq. inches. A 20 x 30 filter is 600 sq. inches. A 30 X 30 filter would be sufficient. It would be great if you can accomodate a 4 inch media filter.

Here is a link that might be useful: How to Size Return Filter Grilles

Mike, sorry for the late reply. I was out this weekend. I think you are right, 2300 sounds like it's oversized. Our highs in the summer are in the 70s/80s with a touch of the 90s, maybe a week or 2 every year. Anyways, I talked with our GC and then our HVAC guy. They are going to build a return box that removes the 6x6 obstruction and comes out to a 30x30 grill. We'll see how it goes over the next few days after he's done with the update.

I'm so thankful for your advice. I can't believe you took the time to look through the installation manual to help!

I'll update the thread with the results later.

Well for best airflow and comfort, you need more than one large return. Best to have 2-3 strategically located in your home. You should discuss size of the new condenser with your dealer. Unless you have large open areas with high ceilings and lots of windows, I would say you are oversized.

IMO