Why is “just right” Important to AC sizing?
An “oversized” air conditioner will cool your house quicker, but it will use more electricity and will not remove humidity adequately. Contrary to popular belief – and intuition – long AC system run cycles are far more desirable and energy efficient than short-run cycles.
Think of AC efficiency like this:
How efficient is your air conditioner at turning your hard-earned cash into cold air? The more efficient, the better! Just about any system will deliver cold air, but at what cost?
The greatest amount of energy usage and wear and tear on your equipment occurs at system startup (it takes tremendous torque and, hence, amperage to get your compressor turning). Once started, it takes 10 to 15 minutes before your system is operating at peak efficiency and performance. Because an oversized system will often bring the temperature down to the thermostat setting in just a few minutes, your oversized system will never get to operate at peak performance and will end up “short-cycling” — starting and stopping over and over again – a fact which will cost you plenty in terms of comfort, utility bills and added repairs.
Sizing your system with Manual J
Manual J is an industry standard that describes a method for determining, basically, how “big” your air conditioning system should be for the size and type of house you live in.
A If your contractor says that a Manual J isn’t needed for your system replacement — even if that contractor is a member of ACCA — you should find another contractor. Full system replacements always require Manual J as a fundamental step toward quality installation.
When so-called contractors say, “Oh, your home is X square feet, so you need a X-ton unit,” you should tread very carefully! This “rule of thumb” is always wildly inaccurate. It results in the installation of systems that are often too big for your actual needs.
It is against the law to install an oversized AC system.
Temperature & Humility
Another factor to consider with regard to system size is that comfort is a function of temperature and humidity. That is, you can have low humidity (relatively dry air) and be comfortable when the temperature is 77 degrees or be absolutely miserable when you have high humidity and a temperature of 72 degrees. One of the most important jobs your air conditioning systems performs is removing moisture (humidity) from the air in your home or office.
Removing moisture requires long run times so that enough moisture-laden air has a chance to blow across the cold indoor coil and condense into the water which then drains from your system. Today’s high-efficiency central air conditioners may remove as much as two gallons of water from the air in your home every hour, which makes you far more comfortable.
If your system is oversized, it will cool the temperature down to the thermostat setting too rapidly and the system will shut off before proper dehumidification of the air can occur. Taken to the extreme, a severely oversized system will create “rain forest” like conditions in your home: you’ll have water dripping from your cold supply grills and, very soon, mold and mildew covering your ducts and walls.
Duct system sizing must be considered too.
If your duct system is not big enough to handle the increased airflow required by an “oversized” air conditioner, the evaporator coil may freeze and the system won’t work at all. Even if the coil doesn’t freeze, reduced airflow will impact operating efficiency and effectiveness and cause premature failure of expense components such as the compressor.
Obviously the air conditioner must be sized big enough to cool your house, but too big is just as bad. We can measure the heating and cooling load on your particular home and then recommend a system that is properly sized to cool your home to 77 degrees when it is 100 degrees outside, then increase the size slightly, if needed, based on your temperature preferences.
To ensure that you're getting customized sizing, professional courtesy, and an expert installation, upgrade your system with McCullough. Call us to get started – (512) 280 0011.