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Don’t Install an Over-sized Air Conditioner

Air ConditionerBefore I knew much about building science, I thought the bigger the air conditioner the better, for any size house.  My ignorance got the best of me on a home that I was remodeling years back.  I was replacing the air conditioner and the HVAC contractor told me the size of the unit that would be needed.  The day he came to install it, he said to me that he had good news and that the distributor gave him a larger unit on accident and now I would be getting the larger AC for the same price.  Not knowing much about HVAC at the time, I thought this was great news!  In hindsight, this could have been problematic.  Now keep in mind, I have not done a Manual J calculation on this house, so to this day I don’t know for sure if the AC was over-sized.  The point I am trying to make is that just because the AC unit is bigger, doesn’t mean it’s better suited for a particular situation.  Here is why.

Comfort for the occupants

I think we can all agree that indoor comfort is important.  An air conditioner’s purpose is to lower the home’s temperature and remove moisture from the air.  When air passes over the evaporator coil (which has a very cold surface) the water vapor condenses.  On a properly sized system you will get enough water vapor on the coil for it to drip down into the pan below and drain outside.  Many times on an over-sized system you won’t get enough condensation on the coil for the water to drip down and drain out.  This means the water can evaporate back into the air that will be distributed through the home, meaning the air was not dehumidified.  For this process to work properly, the unit has to run long enough, and over-sized systems will tend to short cycle.

Short cycling

Over-sized air conditioners will also short cycle.  A larger AC will hit the thermostat set-point much quicker and turn on and off more times during the day.  This will cause more wear and tear on the unit, which can shorten the life and require more maintenance.  This can also have an effect on the comfort of the home, since a larger unit will be blasting cold air then shutting off quickly.  A unit that runs under 10 minutes at a time is a good indication of an over-sized unit.  A properly sized AC should have at least 30 minute run times.  There is also the flip side of an under-sized air conditioner, but will be discussed on a later BLOG post.

There has been debate in the industry on whether or not an over-sized system will cost you more or less on your utility bill.  Chances are it will be incremental savings and really depends on the behavior of the occupants.  A larger system will cost you more money upfront and ongoing.

The bottom line is, make sure your HVAC contractor does a full HVAC design process to size the equipment and not just a rule of thumb calculation.  Ask if they use a Manual J calculation and that you expect to see it.  If they don’t, I suggest you find a new contractor.  HVAC is one of the more important aspects of a home’s efficiency, so make sure you take this seriously.

A HERS Index Rating is very important to utilize during the designing phase of any new construction and existing home renovation project.  Contact us today to find out more about his process or any questions you may have.

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