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Apparently, we have yet to write an article about oversizing HVAC equipment. Shame on us! In case you haven’t heard, oversizing is a really bad idea. Because many homeowners don’t understand how oversizing affects them, we will give you a brief rundown before getting into some of the most frequently asked questions.

The Oversizing Rundown

Oversizing occurs when a piece of HVAC equipment is installed that is too big for your home. Sadly, oversizing occurs more often than you’d think. Some HVAC contractors use an outdated rule of thumb method to calculate what size equipment will work best in your home. Even though they know the importance of right-sizing, they can’t seem to break free of the “bigger is better” bias.

Most people understand that bigger equipment means bigger initial cost. What most may not know is bigger equipment has a negative effect on efficiency, costing you more over the lifespan of your air conditioner, gas furnace, or heat pump. Oversizing your HVAC equipment is detrimental to your wallet, comfort, and the environment.

Frequently Asked Questions

How do I tell if my air conditioner is oversized?

The easiest way to tell if your air conditioner is oversized is to time how long it runs on a hot day. If your unit runs most of the day and keeps the place cool, you’re in good shape. If your unit runs for a few minutes at a time and spends a better part of the time turned off, your AC is oversized. This same test can be done on your gas furnace by measuring run times on a very cold day.

Wouldn’t a unit that runs less often be more energy efficient?

You would think so, but it’s quite the opposite, my friend. Think of it like stop-go traffic compared to highway driving. Stop-go traffic is harder on your car. Having your air conditioner or gas furnace constantly turn on and off is harder on the compressor. System efficiency goes down as cycles become shorter. Also, an oversized compressor will have a much shorter life than a right sized compressor.

Will an oversized unit affect my comfort?

Absolutely. If your furnace only operates for 20 minutes per hour on a really cold day, your home will start to cool from the outside in once your furnace cycles off. Your thermostat is most likely located in a hallway, away from windows and outer walls that could affect the temperature reading. By the time the thermostat senses the temperature change, the perimeter areas may several degrees cooler. This same scenario plays out in cooling mode as well.

When an air conditioner is oversized, what’s the biggest concern?

The biggest consequence of oversizing an air conditioner is the lack of moisture removal. When an AC is sized correctly, the indoor coil is able to get cold enough to condense water vapor. This process takes about 10-15 minutes every time your AC kicks on. When oversized, shorter run times cause your AC to stay in this “dry coil” phase a good portion of the time, thus removing less moisture.

Problems caused by an oversized AC:

  • Elevated moisture levels can lead to mildew, dust mites, and mold.
  • Fungi thrives in areas with high humidity levels.
  • Mildew will eventually damage host materials.
  • Wood flooring or wainscoting can bow or cup if the humidity gets too high.

How do I prevent oversizing?

The easiest way to prevent oversizing is to rely on one of your trusted HVAC technicians at Suter to accurately size your equipment. Our Suterheroes don’t use an outdated rule of thumb method. Instead, we use the most cutting edge software to perform our HVAC load evaluation calculation. The size and age of your home, U-value of the windows, directional orientation, and tightness of the envelope are just of few of the variables that go into calculating your home’s HVAC load.