CHASE DESIGN
Old school rule indeed as tutored by the best-in-town HVAC guys & buddies ... 1st CLEAR & MIN per ton .... plus give a bit more to avoid potential wind noises in those chases ... don't forget sheetrock thicknesses.

We did not design filter sizes which we witness as larger than chase sf
and only sometimes insisted the Bid was for a min. tonnage ... until Manuel-J/etc. proved otherwise.

Prof. Kellett, et al. https://texags.com/forums/61/topics/3390287

I had this problem in the last place I rented in Houston. Landlord got way oversold for a tiny little 800sqare foot garage apartment. Probably about 4x bigger than it should have been, and would take the place from 90 -> 70 in about 5-10 minutes, leaving everything feeling damp and water droplets on the walls.

What I did was re-do the speed settings. I to opened up the controller panel inside the air handler unit and swap the speed wires around to make cooling run on the lowest possible fan setting. It made the air come out of the vents even colder than before, but stopped the short cycling and got my humidity under control.

George P Burdell said:

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I had this problem in the last place I rented in Houston. Landlord got way oversold for a tiny little 800sqare foot garage apartment. Probably about 4x bigger than it should have been, and would take the place from 90 -> 70 in about 5-10 minutes, leaving everything feeling damp and water droplets on the walls.

What I did was re-do the speed settings. I to opened up the controller panel inside the air handler unit and swap the speed wires around to make cooling run on the lowest possible fan setting. It made the air come out of the vents even colder than before, but stopped the short cycling and got my humidity under control.

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Right, but not the OP's problem

My system seems to working better. It's actually catching up and shutting off during the day. It still isn't cooling fast, and I think that has to do with not enough return. But with the extreme temperatures it's hard to tell if I have an actual issue or not. Once we have a break in this heat wave I should get a better feel for it.

I feel for you. I'm currently on week SEVEN of a brand new unit not working correctly. Actually, second brand new unit it 7 weeks because of issue after issue.

Our house is 2400 sq ft and we have two return air ducts (2 story house). I think they're both 20"?

The return ducts were sized right, but supply ducts too small. So pressure was building in the system bc air had nowhere to go. And because the system is "smart", it would turn itself off when the pressure builds to save itself. So we got bigger supply ducts installed yesterday. Was hopeful that would fix it. We couldn't get below 78 upstairs or 74 downstairs. And it'd take about 8 hours for the temp to drop 4 degrees.

Well, now we have a new issue. Now I'm getting LOW pressure thats causing the system to shut off for 4 hours at a time.

GhostWipe15 said:

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So far this has dropped the humidity in the house from 56% down to 50% but hasn't really effect the temperature. Verrrry slowly gone from 74.9 to 74.1 (over a three hour period) but maybes that's the heat outside… 102 right now.

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Well earlier you stated it would not go under 77 so it seems like an improvement. Also, you said it worked before the duct changes. You sure it's not leaking or kinked or blocked somewhere cutting off airflow?

Too much to do about "nothing", noooo but in this case rather moot or shuold be put on back burner ... this focus on RA filter sizes.
Agreed: The 200/ ton rule of thumb is for filter grilles. And only if they have merv1 fiberglass filters in them.
but me thinks your 2150sf isn't getting all it should out of your 5&4 tons system ... filter sized right or not.

Agreed: too small ducts, chases & filters hinder ideal results (esp. in heat pump systems) but since there's been NO evidence of rushing air, whistling, other forced air reports ... let's move on and save all that for later tweaking.

ATTIC INSULATION
If not a DIY, then call the Pro Desk @ Home Depot (or a better installer) to get much more insulation in your attic ASAP. I prefer mineral wool's superior properties but ANYthing will do in this case.
https://texags.com/forums/61/topics/3392693


Have already posted TMI prior of which exterior shading is also key, EZ & fast to do.