5 ton unit on 4 ton ductwork?

[Jax]

I agree with your contractor on one point, 4 tons is not enough for your 2700 sf home in Oklahoma. I live in Oklahoma.

But increasing to a 5 ton?,, the answer is NO, It will not work. The answer is in simple physics. You can not pump more air through the same sized ducts without increasing static pressure. If it is properly sized for a 4 ton, the static pressure is maxed. If you are not getting equal air flow in all rooms, the ducts are improperly sized, which will translate into static that is too high already.
Have your contractor run a static pressure test and you will see what your are up against.
There is no easy solution to your problem. Pumping air 70 feet is a log way.
I would opt for a 4 ton with the VS blower.
Jax

[tinknocker service tech]

variable speed blower will for the most part overcome the static presure and yes your air noise will go up to. every action has an equal reation. 1600cfms to 1800cfms is not enough diferance to realy matter and sorry to say wont change anything as far as cooling the. my opinion only
5 ton is 2000 cfms 1600 is 4 tons why split the differance
get other prices and opinions from other contractors first.

[dash]

Humid climate,set it up for 350 cfm per ton,for better dehumidification.

[pstu]

You *might* get a real increase in duct noise when blowing more air thru your system. I am a S.Texas homeowner who has experienced upon upgrading an air handler. ACCA Manual D (I know a little not a lot here) urges you to keep air speed below 700 fpm. Your contractor has given some meaty info so far, it could well be he has checked this out.

In my opinion, I would lean toward getting a variable speed air handler. That could help maintain air flow "no matter what" with a marginal supply system -- within limits of course, as Dash has pointed out. The choice of 350 cfm/ton sounds like a good choice to me, so you will be asking the ducts to carry just 1750 cfm or so, vs. the 2000 cfm at the classic 400 cfm/ton.

You should get radiant barrier in your attic if you don't already have it, that could be relied on for about a 10% load reduction. And probable comfort improvement. If you can get your house leakage tested, and find some ways to decrease air infiltration, that will be a material help also. For both comfort and A/C adequacy. Are you sure your duct leakage is as low as it can go?

Best of luck -- P.Student