Air Conditioning - central a/c cooling questions (3ton miller)

i live in central florida in a house that was buildt in about 1940ish also it is 1400sq.ft with no insulation in the walls it's also a one story. yes i know i need to insulate i am going to cut holes in the top of the walls and fill with blow insulation.just before i raised the ceiling the livingroom(which is the one in question) the a/c use to cool good and not run all day to keep house cool with an inside temp.of 78 and the outside be 96 or higher.i just raised the ceiling to a cathederal '8.5 to center room size '20by'20 '6.5 wallsand now the a/c will run pretty much all day to keep it cool and it will not get any cooler than 80if it 96 or higher.i insulated ceiling between drywall and roof deck with r-21.i added an 8"duct and the other 2 ducts are 6"the room is still hard to efficentally keep cool.the return duct is an 12" which is about a '10 run to the unit the filter is about 2"off the floor and it is a 24"by24". the unit is in the back of house and centered. i have 12"running from a/c to a distribution box then 10"duct to 2 other boxes then to the rest of the rooms are 6" duct. what do you think i need to do with the duct work? the a/c is about 4yrs. old i clean it about every 6 months inside and out.also i read on another thread that you should have your return close to or in the ceiling in the south?i hope this is enough info for you.
thanks,ronny

no one has any suggestions? please help. thanks ,ronny

You said you need to to this. And by raiseing the ceiling you added square footage to the house.

So you have a large house with no insulation in the walls. And you think there is something wrong with your A/C?

I can why no one has posted to your question.

Fix the house, then worry about a/c

Chris

the house use to cool good before i raised the ceiling,and i upsized 1 a/c duct to an 8" how many 8" ducts do i need to run in the room to cool properly?the vents are 6 by 10. thanks

Like said You cant just add a another duct to a room off the same AC And hope to cool it If you made the room bigger. You need a bigger AC.

and insulation.......

Ronny, you've changed the dynamics, youve completly changed the space the system needs to cool.

I've sen in commercial applications the testing and balancing of systems be completely screwed by one open screw hole.

Same thing in your situation, you have added to your house, added square footage. Might seem small to you, but to the system you have it's a BIG difference.

Time for you to call a pro. And guess what, he/she will say to insulate the walls.

Chris

Man I have to learn to review before posting. My erlier replies make me look illiterate.