High CFM, high temp - or low CFM, low temp

I live in South Florida, have a single level wood home with very good insulation (R30 in attic - not sure about walls). I have a 3 year old 4 ton Trane XL14i with variable speed air handler. The company that installed it showed me the detailed load calculations and it was not just based on square footage. The even accounted for my 9 foot ceilings, and large bay windows so I am pretty sure I have the correct sized unit. All seems fine, but the unit is running about 10 hrs a day now (wife and kids are home during the day). Here are the details at about 2:00pm yesterday.
Outside temp = 95 Condenser exit air = 105 Temp at registers inside = 62-64 depending on which one.
I have the thermostat set at 78 and the house pretty much stays at 78 from 12:00 noon - 5:00pm and never gets down to 77 where it would shut off.

register and outside temp is OK.
I changed some dip switches on the air handler so that the max airflow went from 1600 CFM to 1800 CFM and I re-measured the register temps and they were the same 62-64. I then lowered the max CFM to 1400 and the temps went down to 60-62. My question is what is better? Or is it a wash? The factory setting is 1600.
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When was the last time your equipment was serviced by a HVAC professional?
Zyp

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1 yr ago. When I bought it, it came with a once a season service plan for 2 years. So they came, checked pressures, temps, etc. last year. That is when they told me a 25+ temp difference was a good indication the system was OK if I wanted a quick check. I also had my ducts leak tested 3 months ago through Florida Power & Light and all was OK.
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snipped-for-privacy@gate.net wrote:

If you are in a high humidity climate zone, a 16-F indoor split is can be acceptable. The outdoor split seems to be a little low, even for a 14-seer unit. On a 95 plus day, take all the filters out & then check the condenser split again to see if it goes up. You should have also checked the condenser split when you went to the 1400-cfm setting to see if it moved up or down. If it does not change or goes up then it has increased the latent condensation load on the E-Coil.
Get a hygrometer - a percent humidity gage, let us know the readings!
A lot depends on the amount of air infiltration. Is there a small duct bringing high humidity outdoor air into the system?
The 18-F split might work better to reduce the percent of humidity then if you had it ramp-up to the 1600-cfm setting this would again increase the ratio of the sensible load would & perhaps pull the temp down a degree or two more.
If you have the blower chart & know what the static pressure is the chart will show you the actual CFM the blower is delivering, which could be less, especially on a large 4-Ton unit.
Many times the Supply & Return ductwork is not properly sized. Make sure the Return ducting & the Filter Rack(s) Areas are plenty large! The lower resistance on the Return Air Suction side will increase the airflow on the Supply side & back through the blower - delivering more CFM. Here is a link for reference: http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
Think about all the things you can do to reduce the radiant heat load & the air infiltration latent load! udarrell
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I am on the water - very high humidity. Installer was wary of putting too large a unit (5 ton) because it would run shorter times and not get the humidity out.

I won't have a chance to take any more measurements during the heat of the day for a few weeks.

Nope. At least not on purpose. I can't imagine anyone around here would have that. Part of the FPL duct check was to make sure the intake ducts were sealed so that no hot humid attic air was being drawn in. I have also done checks myself and found no leaks.

The ramp up is automatic. I can only adjust the max setting and only by 400 CFM/ton per setting (3 settings).

My duct is well sized. The inlet is 1.5 times larger than the area going into the air handler. It is a short run - less than 8 feet with one right angle and tapers nicely down to the air handler.

I've thought about installing a radiant barrier up in the attic. Attic temps are 120 with ridge vents. Were 145 before I had those added. I'm also adding thermal windows and additional shade trees and bushes around the house.
Thanks for the help.

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