Sizing a New Air Conditioner

Different sized ACs
Ensuring you have the proper size of AC is extremely important. You can simply look at your old unit to determine the tonnage (see below) and use that as a marker for the new unit you'll be purchasing. However, let's first discuss if your old unit was properly sized.

Undersized Units

Undersized units simply won't get the job done. They're more of a drain on your electric bill as they don't run efficiently and will run constantly.

  • Did your old unit run non-stop? Was it commonly running for more than 2-3 hours at a time?
  • Did your old unit ever get the temperature to the desired level?*

*Barring any of the common issues discussed earlier

Oversized Units

Oversizing a unit is a common problem. People often want bigger and better. But with an air conditioner oversizing it can actually make the unit have more problems. As a unit starts and stops frequently it's harder on the motor which can lead to higher maintenance costs. Since an AC helps dehumidify your house if it's only running for very short durations it will not assist in this process very well ultimately leaving you less comfortable.

  • Did the unit start and stop frequently? In 20 minute or less bursts?
  • If you live in a humid area was the humidity in your home often comparable to outside?


ACs are rated by tonnage. This measurement is derived from how many BTUs it takes to lower the temperature 1 degree. 12,000 BTUs = 1 ton. There are many factors that go into determining the size of unit you'll need and it is often recommended to seek the guidance of contractor when determining this. We'll cover things you should discuss with your contractor but here are some items you can look into ahead of time:

One obvious trick is to just find out what size your neighbor has assuming they have similar square footage and the house lies on a similar piece of property.

This link is a good guide to consult for determining the tonnage you should look at. Keep in mind though that your specific location may require more or less based on some of the below factors:

Factors that make it cooler and necessitate a lower tonnage
  • Do you know what rating your home insulation is? R22? R38?
  • Is your property pretty well shaded?
  • Do you live near a park or open space and have medium to strong breezes?
  • Do you live at a higher elevation?
Factors that make it warmer and necessitate a higher tonnage
  • How many south facing (equator facing) windows do you have?
  • Do you live in a new development with smaller trees, less shade?
  • Is there a lot of asphalt around?
  • Do you have a home office/theatre with lots of computers and electronics running?
  • Do you suffer from allergies and often keep all the windows closed?

Need an energy audit? Do it yourself and save money! Check this DIY Energy Auditor out!

Furnace/Air Handler Fan Capacity

Your furnace/air handler fan motor is rated for a specific tonnage output of an air conditioner. You may be able to find this on the sticker on your fan motor and it should certainly be in the owner's manual of your furnace/air handler, many of which are easy to find online.

If you're looking at a unit that does exceed the capacity of your furnace/air handler fan motor then you'll need to upgrade the fan motor at the same time.

One of the most important things you'll look at when purchasing a new unit is how efficient it is. Next >>>