Too Big, Too Small, or Just Right?
Customers depend on the expertise of contractors in selecting air conditioners. Yet contractors generally size air conditioners at least a half-ton larger than necessary and often oversize by a ton or more. Even the most conscientious contractor is driven to avoid call-backs (or even lawsuits). An oversized air conditioner can mask problems from duct leaks, improper flow across the coils, and improper charge.
At the same time the energy savings from reducing the size of air conditioners has been substantially overestimated. The main reason to reduce the size of an air conditioner is to be able to design and install a duct system that will give adequate flow without too much duct surface area.
There are even times that an oversized air conditioner may be more efficient:
- If the air conditioner is also a heat pump – bigger is better because the electric resistance back up heat is very inefficient
- If the air conditioner is a two speed or variable speed machine a large unit will run in its most efficient low speed more of the time.
CAVEATS: When a larger air conditioner is installed, the duct system needs to be designed with short runs and adequate airflow. Heavily insulating the ducts or putting them in conditioned space is also necessary.
While the vast majority of contractors do not use ACCA Manual J, experts know that even if you do, you