We are used to doing home inspections in the heat in central Texas. In Austin the temperatures in August are very hot. Bigger is not always better! That is very true in relation to sizing your air conditioning unit. The demand on the a/c is at peak performance. Clients quite frequently ask if they should have a larger a/c installed when it is time to replace the system. As an experienced home inspector my answer is always the same, contact your licensed a/c contractor. There are many factors that are required to size an a/c system correctly that the home inspector does not access to or the training required. As this article states, oversizing a unit will cause more humidity and the system will short cycle more frequently causing unnecessary repairs & higher utility cost. You should be knowledgable before your make the huge purchase by contacting your trusted a/c contractor & reading this article.

One of our recommendations was “Be sure that the cooling load is calculated and that the air conditioner is sized for that load.”  That advice proved easier to give than to follow. Only one of the four hvac contractors Bill talked to would submit a sizing calculation (two others just wanted to know the home’s square footage). So Bill hired the contractor who did the calculation, and installed a high-efficiency four-ton unit. The local utility gave Bill a rebate for buying efficient equipment.   Unfortunately, this is not really a success story. Bill’s new air conditioner is much too large for his house.
Why did Bill get an oversized unit? Four things went wrong:
1. The design temperature for the area is 97°F. The contractor increased the outdoor design temperature by 8°F.
2. The recommended design indoor temperature is 75°F. The contractor lowered that figure by 5°F. (Added up, the temperature “fudges” increased the estimated inside-to-outside temperature difference by 59%.)
3. Next, the contractor added 20% to the calculated load as a safety factor.
4. Finally, while the cooling load he calculated was already too high and could have been met by a three-and-a-half-ton air conditioner, the contractor convinced Bill to buy a four-ton unit “because then you will always have plenty of cooling.”

A two-and-a-half-ton air conditioner would have been perfect for Bill’s house; instead, he paid extra for an additional one-and-a-half tons of cooling. Not only did it cost more to buy, Bill’s air conditioner costs more to operate because it “short-cycles” — it turns on and off frequently, even during the hottest weather. And Bill’s house is uncomfortable
because the air conditioner removes very little moisture from the air. The utility also loses out, since the oversized unit helps to increase summer peak-load requirements.