What size AC do I need?


What size AC do I need?

Asked by Joey Kramer

I am getting suggestions all over the map and I want a rule of thumb besides my square footage. What are the 5 to 10 things that into the equation to determine the size for my home?

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Scott Grieves, CPM, CEA's picture


Okay partner, yes you will get more than one answer.

However my best advice is to do the math. You can't go wrong if its calculated correctly by a licensed A/C contractor or RESNET certified Energy Rater. Have both complete the task will help too...

  • You will need to complete/ confirm a 'Manual-J' report for code requirements and check it against Energy Gauge USA.
  • This will more than answer what is the proper 'btu' tons for your home.
  • GreenHomeGuide'senergy auditordirectory includes41 listings for the Phoenix area.

It's not just about the square footage!

Heat-transfer, conduction, convection and solar heat-gain also have to be considered.

  • For example: What if your home was facing west and had a lot of windows. This would cause a huge issue on 'btu' tons loads on the home.
  • For example: What if you have only R-13 (less than 4 inches) of insulation in the attic---this would also allow attic heat to transfer in the home faster; causing the A/C system to become over load.

So you see it's not just about the square footage these days or rule of thumb...You have to look at the home as a complete A/C system to achieve the highest SEER.

Get a blower door test

Another issue which many overlook is check the Duct-work, insulation R-value, orientation, and the windows.

A HERS-rating 'blower door' and duct test will help to determine if your home is very leaky and will need more or less 'btu' tons to supply the right amount of cooling or heating you desire.

To get the best A/C sizing estimate

In conclusion:

  1. Complete a Manual J, plus Energy Gauge USA report.
  2. Complete a 'Home Energy Rating' HERS-rating blower door and duct test.

This will give you the best solution or answer to your question and aid in selecting the most knowledgeable contractor to complete the work and stand behind the install/ warranty.

You could also contact your local power company and ask if they provide free a energy audit or energy survey of your home. This could help too and give you some non-biased information.



For more information:

Read "What size AC unit should I use for a 1,986 sq. ft. home in Kingwood, Texas?" a Q&A answered by Danny Kelly.