Oversized AC

The Problem of Short Cycling

An oversized AC unit is like a race car engine in a city. It gets to its destination (the thermostat's set temperature) far too quickly. This leads to a phenomenon called short cycling, where the AC unit turns on and off in rapid succession.

• Poor Dehumidification: The primary issue with short cycling is that it fails to address the home's latent heat load (moisture). An AC system's ability to dehumidify is a function of its run time. It needs to run for a sustained period to allow the evaporator coil to get cold enough to condense water vapor out of the air. When an oversized unit short cycles, it cools the air quickly, satisfies the thermostat, and shuts off before it has had a chance to remove a significant amount of moisture. This leaves the home feeling cool but uncomfortably clammy, or "wet-cold."

• Reduced Energy Efficiency: Every time an AC unit starts up, there's a surge of energy consumption. Short cycling means the unit is constantly in this high-energy-demand start-up phase, leading to higher electricity bills. Furthermore, the compressor, which is the heart of the AC system, is constantly subjected to the stress of starting and stopping.

• Blast Cooling: Due to the oversized nature of the AC unit, the air coming out of the registers is much colder than normal, leading to condensation on supply registers.

Wear and Tear on Equipment

Short cycling puts immense strain on the AC system's components, particularly the compressor.

• Increased Wear: The compressor is designed for sustained operation. The frequent start-up and shut-down cycles put unnecessary stress on its internal parts. This can lead to premature failure of the compressor, the most expensive component of the AC system to replace.

• Reduced Lifespan: An oversized unit that short cycles can have a significantly shorter lifespan than a properly sized system. A system designed to last 15-20 years might fail in just 5-10 years due to the constant stress.

Why Correct Sizing is Critical

A building scientist determines the correct size of an AC system by performing a detailed Manual J load calculation. This analysis takes into account all sources of sensible heat (temperature) and latent heat (moisture) gain in a home, including:

• Home size and layout

• Window and door efficiency

• Insulation levels

• Air leakage (building envelope tightness)

• Local climate and sun exposure

• Internal heat from occupants and appliances

This calculation provides the precise BTU/h (British Thermal Units per hour) required to keep the home comfortable in the hottest months. A system that is sized correctly will run in longer, more efficient cycles, allowing it to both cool the air and remove moisture effectively, creating a truly comfortable and healthy indoor environment.