A/C system advice for 2 story residential

We have a 40 year old Florida home 2450 sq. ft. with large windows; also 1 acre lot with lots of big/tall oak shade trees. We currently have an outside older 3 ton Trane A/C unit (rated 10 seer) which is underpowered but with the help of a couple of upstairs window units we do ok. I think its time to upgrade/replace the Trane; what I am thinking is a 4 ton higher efficient (maybe 14 or 16) unit. My wife thinks that if we replace with another 3 ton higher efficient AND install a smaller unit for upstairs - programmable so that the bottom primarily runs during the day and the top primarily at night (upstairs is all bedrooms) we would possibly be more efficient and cooler. Obviously big up front install costs for upstairs unit but if we are willing to absorb those - is this usually the more efficient way to go? Thanks in advance - Bill
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I'm in a similar boat, 3 ton unit with one window unit upstairs, but in a cooler climate and about 3500 sq. ft. South facing upstairs rooms can reach 85+ with the AC running. The best solution would be a multi unit mini-split, but it comes in around 12K. A 4 ton unit might help some, but our ductwork sucks, so a mini split would be a better option. I bought a portable last year, but the aluminum wiring couldn't handle the load over the long haul. Plus it's a PITA to use with the casement windows.
Depending on your situation a separate unit for upstairs is probably the way to go. I might buy a single mini for the one room in the short term.
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Bill Stock wrote:

YES.
Window units cost a lot less than mini-splits, plus smaller BTUH ratings, and a small bedroom does not need a lot of BTUH to cool at night. Do a manual J on each room to get a number on heat-gain. http://www.udarrell.com/airconditioner_current_temperature_btuh_charting.html
Half-Ton Room A/C - Cools over 900 sq.ft. @ 104 HT Index - Daytime, entire first floor in older 1930's house - udarrell
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Thanks for the response - I guess what I'm really asking is if we are willing to "eat" the installation costs, would our power bill go up or down with 2nd unit upstairs? We have 4 bedrooms that each will have a window unit vs the central unit.
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Just cause your system has a 3 ton name plate, doesn't guarantee you're getting three tons of cooling. I'm remembering a call I had last year. the fellow had a 3.5 ton AC, and it wasn't cooling the house. He was thinking maybe needed 5 tons. I asked his permission to service the unit, and when I finished, it was cooling just fine.
You may replace the system if you like, but it's possible that a good servicing would get you by for several more years.
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