An over sized AC will run in short cycles while maintaing the temperature set point. The short cycles do not allow enough time for the air to circulate multiple times through the evaporator coils. The result is air which has a high amount moisture. The high humidity makes it feel uncomfortable. The user tends to lower the temperature of the thermostat to compensate for this uncomfortable condition. In addition the short cycle also wastes energy. It takes the AC condenser a period of time (10 - 15 minutes) to reach optimal operating conditions. The constant starting and stopping also create extra wear and tear on the blower and compressor.

An over sized furnace will also short cycle. The frequent cycling is a strain on the blower. If the duct work is not properly sized then the furnace could operate in a hot condition. This hot condition may cause the heat exchanger to fail prematurely.

Most over sized equipment comes with undersized duct work. The small duct work creates noise and blasts of air people associate with force air systems. An under sized duct creates high static pressure. This makes the system inefficient and forces to blower to work harder. In a variable speed system the blower will increase the speed as fast as necessary to maintain the correct air flow. This uses an additional amount of power to compensate for the poor set up.

Thanks! Oh the joys of building a new house....

Your HVAC team gracefully declined to help me due to the builder. I understand.

Upfront cost, operating cost, and comfort.

If in a location with high summer humidity, you will not get the inside dehumidification you need which could lead to mildew and mold issues.

There could be repair issues due to short cycling.

IMO

This post was edited by tigerdunes on Fri, Jun 7, 13 at 9:28

I would like to add we have all experienced oversized a/c. Enter a home and it feels really cool. In about 10 - 15 minutes you start to feel really warm and maybe even clammy.