I live in a 2-story 2500 sq. ft. house with 2 split R-22 units that are over 8 years old. The downstairs 2-1/2 ton cools (and cycles) fine. The upstairs is a 3-1/2 ton unit for approx 1350 sq ft. The upstairs runs all day long here in Houston. Here are some specifics

Outside Temp: 90deg
WB temp at return: 65
Dry bulb temp at register: 75
temp at first register after evap: 61
Temp of suction line just before service valve: 54
Low pressure gauge reads: 75 corresponding temp-43
High pressure liquid line reads: 260
Return air grill is 20x30 and I use the Filtrete Ultra (Purple) filters, and always have.

my unit will only cool to 78-79 in heat of the day.

Will installing another 20x30 return help this problem? The return air rectangular duct that runs into the furnace is approx 12x16. I would be tying into this with another 16-inch return flex duct.

First you should be looking at a <18> degree TD (total differential) between the return and supply duct (dry bulb on both).

Dry bulb temp at register: 75

Is this supposed to be dry bulb at return?

I see you have a temp at the first register after the evaporator at 61 degrees. Is this dry bulb? What is the dry bulb at the return?

Need these answers first.

I would stop using that filter... they are already wound tight and once they start to get dirty thet make your system work even harder. Try the lesser 3m filter or go to home depot and buy a three pack of natrualaire filters for like $8. be sure to change that filter every 30 to 60 days.

First you should be looking at a <18> degree TD (total differential) between the return and supply duct (dry bulb on both).

Dry bulb temp at register: 75

Is this supposed to be dry bulb at return??

I see you have a temp at the first register after the evaporator at 61 degrees. Is this dry bulb? What is the dry bulb at the return?

Need these answers first.

Sorry, here it is edited


Outside Temp: 90deg
WB temp at return: 65
Dry bulb temp at return: 75
dry bulb temp at first register after evap (10 ft run from evap): 61
Temp of suction line just before service valve: 54
Low pressure gauge reads: 75 corresponding temp-43
High pressure liquid line reads: 260
Return air grill is 20x30 and I use the Filtrete Ultra (Purple) filters, and always have.

61 from 75 = 14 degrees not quite the 18 degrees usually found on a proper operating system but not to bad.

12X16 does not quite cut the mustard for as 3 and 1/2 ton unit. Not enough square inches especially after you knock off 20 to 25&#37; for the return air grill louver blockage.

The duct size is more important than the grill size. You can have a monster grill but small duct and have problems. The system only can draw the smallest denominator.

One other problem with attics above the cooling floor level. Heat soak passes from the attic space through the ceiling drywall and causes many heat problems. Make sure your attic is ventilated properly both high vents and low vents. We used to be able to save aprox 1 ton of cooling in this area with the use of a powered attic roof ventilator.

61 from 75 = 14 degrees not quite the 18 degrees usually found on a proper operating system but not to bad.

12X16 does not quite cut the mustard for as 3 and 1/2 ton unit. Not enough square inches especially after you knock off 20 to 25% for the return air grill louver blockage.

The duct size is more important than the grill size. You can have a monster grill but small duct and have problems. The system only can draw the smallest denominator.

One other problem with attics above the cooling floor level. Heat soak passes from the attic space through the ceiling drywall and causes many heat problems. Make sure your attic is ventilated properly both high vents and low vents. We used to be able to save aprox 1 ton of cooling in this area with the use of a powered attic roof ventilator.

I could change out the 12 x 16 to a 12 x 20 rectangular duct. Would that be enough, along with another return? The new filter really slaps hard when loaded into the register.

Also when my house was built in 94', they used the el Cheapo plastic registers that open and close by pulling out and pushing in. Even fully open, the air wasn't coming out very well. When I removed the register, the air really started pouring out. I have since removed all registers upstairs and it seem to be cooling better. I plan on buying the steel, higher quality registers that allow for better flow.

A 12X20 is better than a 12X16 for sure. Those plastic registers are junk.

UPDATE...

An additional 20x30 return air filter grill with 16-inch flex duct to a 20x12 return lowered the indoor temp during 100 deg dayh, 2 deg to 77 from 79 upstairs.

You can never have too much return air but it's the location that will tell the tail and make a difference.