On the hottest days of the summer does your AC run a lot? Almost continuously? Your unit may be oversized but your ductwork also could be lacking. What size is your current unit?

I think the current unit is 3.5 tons. Its probably a 15 year old unit and was serviced when we bought the house two years ago.

Our HVAC guy said the ducts look to be appropriately sized - he actually owns the same model home we do.

Trying to think back - in the summer when in use it - I can't recall if it runs continuously, but I don't think it does.

The following paper may help:

http://www.buildingscience.com/resources/mechanical/hvac/conditioning_air.pdf

If the zoning is installed correctly, the unit is sized correctly, and duct is correct your unit should be able to hold a 20 degree difference between the summer outdoor and indoor temperatures plus an indoor humidity of between 45 and 50 percent. You should not hold a humidity level of over 50% indoors in the summer.

In order to check it correctly use professional testing equipment. The portable humidistats and thermostats you can purchase at the local stores can be off by as much as 5 percent.

We were not actually using a humidistat to check the humidity level in the house.

We ran the dehumidifier because the house felt uncomfortably humid, and using it definitely helped increase the comfort level.

Residential AC is 'temperature comfort' conditioning. Humidity control is more of an afterthought.
The system has a design temperature that is the maximum it will handle and maintain the interior set point. Once you pass the design point the interior rises degree for degree with the outside temperature.
Humidity removal will be acceptable in most cases except in the fall and spring. At these times the humidity may be higher than average, but the temperature is lower than average. Under these conditions the system is oversized and short cycles resulting in less moisture removal. Cold and clammy is the result.
Some newer systems try and solve the problem by installing two compressors (and the associated valves to separate them) to produce a system that actually has two different design points. The smaller compressor can run on the lower temp/higher humidity days and since it is smaller it ill run long enough to control humidity on these days.
Commercial systems often run both heating and cooling year round. The air is first cooled to remove any humidity, then heated (and humidified) to the desired point. These systems are much more complicated than a simple home system, and contain dampers, humidifiers, sensors to measure both humidity and temperature (often at multiple points in the building and in the system), and may even contain volume to allow refrigerant to be Âstored and metered dynamically while the compressor simply runs based on the stored refrigerant level. You do not want to pay for the maintenance on these systems.
An older system with a large capacity will be very oversized in the spring and fall, and there is no simple way around this problem.

Any air conditioner that leaves the temperature at 85 when the outside temp is 105 is sorely undersized. Maybe thats fine where it only gets to 88, but in North Texas it won't cut it. Any time I hear this 20-degree rule from an HVAC guy, I find someone else.

I'm uncomfortable even at 75 when I am trying to relax indoors.

For these reasons we are putting Trane XL16i units in our new house. They can run on low for humidity removal and kick into high if it gets really hot.

I may be a little too hard on the new high SEER units. Maybe they do better than my non-experience with them, due to my retirement in 1993, assumes.

I would like to hear from more consumers & techs in different climate zones concerning their performance at real high load conditions. udarrell _ 2007

Here is a link that might be useful: udarrell.com