How much space am I trying to cool (and has it changed since the old AC was installed)?

If you have to replace a system the first thing most people think is, “give me the same thing I had.”  In most cases whatever size you had before will work for the replacement system, but there are circumstances where it will not.  For example, if you added windows or doors, added new rooms, zoned off unused rooms, or added insulation you may have affected the amount of cooling that is required.  Some people want to install a larger system “just to be safe,” but in reality this can cause problems down the road.  If your system is too big for your space your electric bill will be higher and you could negatively affect the system’s ability to control humidity, which can lead to several other problems. When it comes to replacing a system, if you’ve made any of the changes above it’s worth having a contractor perform a load analysis of your home to properly size the unit for your needs. If you don’t feel comfortable with the way your contractor is analyzing your space, call another and get a second opinion.  They should walk through the home and ask you about comfort and usage, not just ask about square footage.

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4 thoughts on “How much space am I trying to cool (and has it changed since the old AC was installed)?

  1. I have a rec room that essentially 19’x27′ 12′. It contains a 6×9′ bathroom.
    The air handling unit is in the attic. Both AH and compressor are 8 years old.

    I have had many repairs, its humid all the time (I use a dehumidifier) The last 2 repairmen have commented that I don’t need that much a/c and it is too complicated for what the room needs

    I will replace both units. Any advice? tonnage?

    • Hi Tom – Here are a few thoughts and suggestions. First of all what the contractors might be saying is that your system has too much capacity for the space you are trying to cool (the whole house). If you are in a humid area and your system is oversized, it will run until the temperature set point on your thermostat is reached. If this happens so fast that the cool air does not have a chance to cool all the rooms in the house then humidity will rise – especially in the rooms that are further away from the indoor cooling coil. If the air does not move across the coil then the moisture in the return air can’t condense on the coils and into the drip pan and to the drain. Some people who live in humid climates have installed systems with capacity modulation (either two-steps or variable speed) to address some of these issues. These systems provide longer run times to allow better humidity reduction. Many AC systems that are 16 SEER (efficiency) or greater have this variable capacity feature. Here is a link to a video that explains the benefits of variable capacity.

      http://www.ac-heatingconnect.com/comfort-of-home/

      One other suggestion would be to make sure the contractor does the proper measurements and calculations (ACCA Manual J is one) to insure that whatever system you install is sized properly for your home. It is important to consider more than just the size of your space to get it right and getting the capacity (tonnage) right is the key to having good comfort and low humidity throughout the cooling season. I hope this answers some of your questions.

  2. How do I measure the space to be air conditioned? Without utilizing a contractor.
    My home has two units. One for the upstairs/ bedroom area. The other for the first floor/ kitchen,dining, rm living rm and den and basement. Total sq footage is around 3250 living space
    I have two units. I have been told that I possibly should downsize the units ( 20 yrs old)

    • Hi Greg – Unfortunately, there are a lot of factors that go into calculating the amount of cooling you need for any given space. “Rules of thumb” based on square footage do not take into account the number of doors and windows, insulation, shade, roof type, etc. As to whether your current system is oversized or not you might have noticed more humidity problems if your system is too large but that is not always the case. Here is a link to a pretty good article on this topic which could provide some more background. It is somewhat promotional but it still provides a very good summary explanation of these issues.

      http://airconditioningsoutheast.com/tips/ac-installation/size-matters-the-importance-of-heating-and-cooling-load-calculations

      One other point we noticed about your post about your system being 20 years old. Our research shows that the average age of systems, when replaced in the US is about 16 years – maybe a little older for the North and newer for the South and also newer for heat pumps. So, at 20 years old, you might be ready to upgrade to newer, more efficient models. We encourage our readers to get 2-3 quotes from different contractors before deciding.

      Good luck with your HVAC project!

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