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Having the Wrong Size AC Unit

Did you know there are downfalls to having the wrong size AC unit?  With an outline that is reminiscent of Goldilocks and the Three Bears, people will have problems if their AC unit is too big or if it is too small.  The only solution when it comes to HVAC, is to have a system that is sized just right.

AC Unit Too Small

It’s easy for someone to understand the problems associated with an air conditioner that is too small because having an AC that isn’t big enough for your home will cause it to never quite get cold enough.  Having too small of a unit will cause it to have run cycles that are way too long.  When you combine these two issues, not only does it increase your electric bill beyond what it should be, but it puts unneeded stress on your AC.

Some people try to overcome the inadequate sizing by adding a window unit in an area that stays the warmest.  While this can be a valid solution in some cases, it does not come without its own issues.  This approach essentially creates “cool zones” and can throw off your AC system even more.  If one of the cool zones is anywhere near your thermostat it will affect the temperature readings.  Incorrect temperature readings can make your system think that it is doing what it needs to be, when in reality, it may not be.  Thereby, causing even less cooling to happen.

AC Unit Too big

The wrong size AC unit doesn’t just mean too small.  Having a unit that is too big is something that homeowners typically might not understand.  People tend to think that the age-old adage “bigger is better” applies here, but it does not.  In fact, if your HVAC system is oversized, it can cause just as many, or possibly even more problems than one that is undersized.  Some issues that stem from an oversized system would be short cycle times and high humidity.  To top it all off, larger units are also more expensive.

Cycle times will be short because the larger AC unit will push an increased amount of cool air at a much faster rate.  This makes the thermostat think that the house has reached the target temperature, when in fact, only a portion of the house has cooled down properly.  As the air temperature evens out, the thermostat senses the need to cool again, so the same thing happens repeatedly.

When an AC unit is too large, it never really gets to run for any length of time.  This means the evaporator coil is only able to remove minimum amounts of humidity.  The more humid the air is, the more heat it is capable of holding.  This is because excess moisture is considered latent heat.  If the humidity is allowed to remain, the HVAC system will continue to cycle even more (called short cycling) because of the incorrect size of the system.

But Wait! There’s More!

Continuous short-cycling will eventually cause premature failure of your HVAC system.  Whether this is in the form of a contactor going out or a compressor dying, something will break before it should.  This is due to the unit switching on and off more times than it is designed to.  Just like anything in life, if you abuse it, it will break.

There is also ductwork to consider.  If the ductwork in a house is not designed for such a large system, the heat will not be able to be removed from the air.  You can’t put 5 tons of cooling through a duct system designed for 3 tons.  This can cause repeated freeze-ups in which no cooling is taking place at all. As a resident of Florida, trust me, this is not something anyone wants.

AC Unit Sized Just right

The best way to avoid these problems, is to have the correct size HVAC system installed.  In order to get an AC unit that is the correct size, you need to hire professionals that know how to determine the correct size.  Proper heating and cooling requirements can only be determined by conducting what is called a Manual J load calculation.  There are many different factors that go into a Manual J load calculation.  Some of which include the direction a building faces, insulation ratings, quality and quantity of windows, square footage, and much more.  Some contractors have a rule of thumb where they assign a certain number of square feet per ton of cooling. This method can be inaccurate in many situations.


In the end, having the wrong size AC unit will do nothing but hurt your comfort and your wallet.  A unit that is too small will not cool properly and run endlessly.  Yet a system that is too large will short cycle until it eventually breaks down.  Whereas a properly sized air conditioner provides a whole house with comfortable air.  While simultaneously allowing the system to run in the manner it was designed to.  If you are experiencing any of these symptoms or think that your HVAC unit may be improperly sized, contact Beahan’s Heat and Air to schedule an appointment.

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