Signs Your AC Is Too Small

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Updated May 17, 2023

There are many things to contemplate when buying an air conditioner, the most important of which is power output. Even the best air conditioners will be ineffective if they don’t have the power to match the room’s square feet. A too-small AC may run much longer than required if it doesn’t have the power to cool the room, wasting energy and putting unneeded stress on the unit. So, to save you trouble further down the road, stick around as we explore the question, “What if my air conditioner tonnage is too small?”


  • Undersized air conditioning units result in higher energy bills and lower comfort levels.
  • Tonnage measures an air conditioner’s hourly capacity to remove heat from the air.
  • To guarantee the right AC unit size, owners must measure a room’s square feet, ceiling height, and other factors affecting heat levels.

What Happens if Your Air Conditioner Tonnage Is Too Small?

Air conditioners run on cycles, meaning they are programmed to condition air to a specific temperature. Once it reaches that temperature, the unit shuts off. However, when an AC is too small, the room never reaches the desired temperature, so the unit never shuts off. That’s why it’s important to take into account the size of the room when selecting an AC. During summer months, a small window unit may not process enough air to keep circulating in larger rooms. Speaking of temperature, if you want to know more about AC cooling, check out our article discussing the coldest temperature for an air conditioner. And if you have a larger system, check out our page on sizing a mini-split.

Discovering your air conditioning unit is too small for the space is something homeowners often find out too late. It begins with low cooling efficiency, leaving you wondering if something’s broken, followed by high energy bills. Then, after spending hours or days hunting down the problem, you find out you’ve got an undersized air conditioner.

Insider Tip

Frequently changing or washing the air filter will help ensure that your AC works at peak cooling capacity and energy efficiency.

What Does This Mean for Cooling Efficiency?

The cooling power of air conditioners is rated in British Thermal Units (BTUs). BTUs measure how much energy it requires to heat or cool one pound of water by one degree Fahrenheit. An ACs amount of BTUs is often translated into what’s known as tonnage. Tonnage calculates the amount of heat an air conditioner can remove per hour. If you are curious, we have a page to help you figure out the AC tonnage you need.

To figure out a unit’s tonnage, a handy formula to remember is that a one-ton air conditioner is the equivalent of 12,000 BTUs. For a deeper look into AC cooling measurements, read our piece explaining what BTUs mean for air conditioners.

Too low of a BTU can have your AC running constantly. That result can be disastrous. You’ll notice your energy bills soar because the unit runs continuously. It also puts enormous stress on the motor, resulting in shorter longevity and subsequent repairs or replacement. Additionally, the indoor climate will never reach a comfortable level because the unit simply can’t lower the ambient temperature enough. If you’re still not sure about what BTU is suitable for your space and you want a complex model, you can consult with a professional for all types of ACs, like a mini split installation.

STAT: The EPA’s rule of thumb is that every square foot of room should have 20 BTUs of cooling capacity. (source)

Regardless of AC size, a great way to reduce a monthly energy bill is to understand what the air conditioner’s energy saver mode does. For those looking for a new dynamic cooling system that adjusts to the season, check out our article explaining what a SACC air conditioner is.

What if My Air Conditioner Tonnage Is Too Small FAQs

Is it ok if my air conditioner is constantly running?

During inordinately hot or humid spells, this can be normal. But if the weather isn’t extreme, it shouldn’t be running all the time.

Is it bad to have an oversized AC unit?

Like an undersized unit, oversized air conditioners can create excessive energy costs and uncomfortable temperatures. For ACs, it’s wise to operate under the goldilocks method.

Should I turn off the AC when I leave the house?

Central AC owners shouldn’t turn their units entirely off because it takes much more energy to start them up than it does to keep them running.
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