I have a 2 ton and a 2.5 ton unit and they can't keep up on my 2880 sqft one story when the temp is 95+. Keeping t-stat on 76* it creeps up to 78-79* in the afternoons inside. Yesturday was a mild day and with outside temp at 87* the 2 ton pressures were 225/75 and 2.5 ton were 225/55. What's your opinion on what's up?

 

Too much house... trade me

 

How old is the house? How good is it insulated?

 

you seem to be low on gas, the 55 is near freezing the a-coil...needs to be up around 75, just fixed mine 10 min. ago, i was low on gas, still not sure were it went.

 

did you try cleaning the inside a coils and both ouside units

 

Check and see if 2nd AC is iced up at indoor coils.

 

Thanks for the input guys. The outside coils were cleaned last year but never the inside. They are in the attic. I'll check them for ice.
The house was built in 1958 and has insulation in the walls, cellulose and other insulation in the attic, new windows and Injectaseal injection foam in the walls and behind the brick. The 2.5 ton is the one that seems to not keep up as well.

 

Well some more details on the units would deffinaetly help.

Such as do you have a txv style metering device? Or fixed piston. Also I'm assuming you are using r22 refigerent or r410a?

There is absolutely no way for anyone here to guess or assume what your pressures should without knowing some basics facts.

A few that would help us would be

Metering device?
Txv or piston?
R22 or r410a
Your indoor return air wet bulb and dry bulb?
Your supply dry bulb?
Superheat measurement and sub cooling measurement?
Also the data sheet most likely found in the service department of the condenssor.

Also you did mention the units are in the attic. I would spend some time inspecting the return air duct for sucking itself together and obviously leaks of any sort, this is the most crucial aspect of a horizontial hvac system. If the unit is pulling in unconditioned air trying to diagnose will be a guessing game.

Hope this helps, I'll check back in tomorrow after work.

 

if the ;units were pulling in hot air the suction would be higher( if on a Fixed orfice). a 55psi suction is low and is low on charge. the suction like should be around 55 to 60*if the suction line pressure is 75. this would be a 10 to 15* super heat. which is ideal.

the low suction pressure could also be caused by low air flow due to dirty air filter, weak blower motor, small return duct work.

the liuqued line pressure is great and the condesning coil isnt dirty.