Oversizing: The #1 Mistake in Air Conditioning Design

In HVAC, bigger is not necessarily better.  It is the most common mistake we see in air conditioning design.  Compared to a correctly sized system, an over-sized system cools poorly, costs more to operate, is noisy, and has a shorter lifespan.

Design occurs in the field

HVAC systems are not designed in the factory.  They are designed by HVAC contractors.  Each system is unique to the home it will be servicing.  We account for your home's design, layout, the type of construction, its orientation tot he sun, shade, our local weather, and (most importantly) your lifestyle.  How you plan on using your air conditioner has a big impact on the design.

After we collect the necessary information, we will perform a load calculation (a highly specialized set of engineering calculations).  Sometimes, we make the calculations on the spot.  Other times, we return to the office to use special design software.  When we perform the calculations in the field, it is not unusual for us to double check them in the office.

You will be unhappy with a HVAC system, no matter how reliable and well-made, if other components of the design are wrong (i.e. poor field fabrications, sloppy installation, etc).

The impact on comfort

When an air conditioner is over-sized, it powers on, runs for a few minutes, lowers the air temperature at the thermostat to the setting, and stops.  A few minutes later, the air temperature rises, and the system powers back on.  This is called short-cycling and is indicative of an over-sized system.

The impact on accoustics

The HVAC system and duct system should be designed together.  If the system is sized larger than the duct system, the volume of air pushed through the ducts raises air velocity, creating what feels like a windstorm in your home.  Noisy grilles, registers, and diffusers indicate over-sized equipment or under-sized ducting.

The impact on efficiency

Short-cycling is not efficient.  It takes a lot more energy to start a motor than to keep one operating.  With your air conditioner, you start three motors: the compressor, the condenser fan motor, and the indoor fan motor.  In addition, you are operating larger, more expensive motors than you need.

Think of a properly sized HVAC system as a fuel-efficient car rolling down the highway.  The over-sized system, by contract, is like a 1960's muscle car in stop-and-go traffic.

The impact on equipment longevity

Short-cycling causes your equipment to wear out sooner.  Just like an old car with lots of highway miles has a longer life than one used for city driving, a correctly sizes air conditioning unit will outlast an over-sized unit.

A continually running air conditioner does not guarantee that it is sized correctly.  Over-sizing covers up other problems.  For example, if your system is over-sized and has a refrigerant leak, it may not be readily apparent until the compressor fails.

If your air conditioner starts and stops regularly on very hot days, it's probably over-sized.  Call Elite and have us perform a load calculation for your home.  Call us today!  918.610.7300