Why an Oversized AC Unit Causes Problems

An oversized air conditioning unit causes problems. At first blush, you may think that purchasing an oversized HVAC unit will cool your home faster. However, a unit that is too large for your space will lead to inefficiencies, discomfort, and increased costs. Oversized units tend to turn on and off frequently, called “short cycling,” and can shorten the system’s lifespan.

In this article, you will learn about the issues that an oversized AC unit can cause.

Inefficient Cooling and Dehumidification

An oversized AC unit cools the air too quickly, and then the thermostat signals the unit to shut off even though the entire space hasn’t been properly cooled or dehumidified. This cycle is referred to as “short cycling.” Short cycling means the system is constantly starting and stopping, which is less efficient than running continuously at a lower capacity.

Lacking Dehumidification

Properly sized AC units dehumidify the air as they cool it. Oversized units don’t run long enough to remove sufficient moisture, leaving your home feeling damp and clammy even when the temperature is comfortable. This exacerbates humidity issues, leading to mold and mildew growth and water damage, and potentially causes health problems and damage to your home. 

Increased Energy Costs

Due to the constant on and off (short cycling), oversized systems don’t operate as efficiently as properly sized units. This leads to higher energy consumption and increased utility bills. In effect, you are paying for a larger system’s capacity that you aren’t fully utilizing.

Reduced Lifespan

The constant starting and stopping puts extra strain on the compressor and other components. Over time, the lifespan of the oversized unit is shortened. The overall strain can lead to premature wear and tear, resulting in more frequent repairs and a shorter overall lifespan.

Uneven Cooling

Oversized units may cool some areas of your home adequately while leaving others too hot or too cold. This creates uneven temperatures and uncomfortable hot spots throughout the house.

Higher Initial Cost

A larger HVAC unit costs more to purchase upfront. It is an unnecessary expense considering the problems associated with oversized units, especially when a properly sized system could provide better comfort and efficiency.

Unbalanced Air Pressure Due to Leaky Return Ducts

In oversized systems with high airflow, duct leaks become more problematic. These leaks can pull in unconditioned attic or crawlspace air and redistribute it into the living space, making cooling uneven and inefficient. 

If the house has duct leakage on the return side (especially in unconditioned spaces like attics), a higher-powered system can create negative pressure. This negative pressure pulls hot air from:

• Poorly sealed doors and windows
• Attic gaps
• Wall voids
• Crawlspaces

Overcooling Supply Plenum

An “overcooling supply plenum” refers to a situation in an HVAC system where the supply air plenum–the main box that distributes cooled air from the air handler to the ducts and vents–becomes excessively cold. This leads to the undesirable effect of overcooling the spaces it serves. Overcooling can lead to discomfort in certain areas, particularly in non-arid climates, where it can also reduce the system’s ability to properly dehumidify the air. Causes of overcooling in a supply plenum include:

• A cooling unit that’s too large for the space it’s attempting to cool can result in a higher supply airflow than is necessary, potentially leading to warmer supply air and overcooling in some areas.
• Uneven air distribution due to issues, causing an improper airflow balance.

How Home Energy Efficiency Solutions Can Help

To improve energy efficiency with an oversized air conditioning unit, consider strategies like improving airflow, adjusting thermostat settings, and potentially upgrading to a ductless mini-split system. Adding more ductwork to distribute air more effectively, using the “fan only” setting on your thermostat, and installing a smart thermostat can also help. Regular maintenance, including changing filters and cleaning coils, is essential for maintaining optimal performance.

These interventions are critical in determining the source of your issues:

• Attic insulation and attic ventilation to lower thermal load and reduce pressure gradients that suck in outside air
• Blower door testing can identify pressure imbalances
• Duct leakage testing to find return-side leaks pulling in hot air
• Air sealing attics, rim joists, and crawlspaces to reduce infiltration paths

Find the Right Fix for Your Cooling Issues

An oversized AC unit can do more harm than good, leaving your home with uneven temperatures, humidity problems, and a system that wears out long before it should. The good news? There are ways to correct these issues without replacing your entire system. From air sealing and duct improvements to smarter thermostat settings and maintenance, the right adjustments can restore comfort and efficiency. Fan Man Inc. is here to pinpoint the problem and guide you toward solutions that keep your home cool and your system running the way it should. Request a quote today by calling  704-759-9964 or filling out the form below.