July 15, 2019

My A/C won’t keep my house cool enough on a 95-degree day.

It’s 95 degrees outside, and you can’t get your indoor temperature to go below 75 degrees—even with your thermostat set on 72 degrees. It seems as if the AC can’t keep up. First thing you might think; okay, let’s get a larger AC to keep perfectly cool when summer temperatures hit above 93 degrees. However, when the temperature outside is above 93 degrees, an oversized AC is not the solution to keep you more comfortable. First of all, an oversized unit is more expensive! It also has a shorter lifespan, doesn’t regulate humidity effectively, and will generate higher utility bills.

Here is why over-sized—is not right-sized.

  • An oversized unit will short-cycle; that is, cool your home fast then shut off—over and over again—and decline from overuse faster than a properly sized unit.
  • Due to short-cycling, an oversized unit will not run long enough to remove humidity from indoor air effectively. The higher humidity will make you feel hotter and more uncomfortable.
  • You’ll feel hot and uncomfortable because of the increased humidity and keep cranking the AC—and your utility bill. It’s an uncomfortable cycle!

Redefining comfort on a HOT day.

Comfort on an atypically hot summer day is not going to be an indoor temperature of 72 degrees; comfort is the difference (say 20 degrees) you experience indoors versus outside. AND HERE IS THE REASON WHY.

HVAC systems are based on your geographic area’s design temperature.

In Virginia, our one-percent design temperature during the summer is 93 degrees which means temperatures in our area will go above 93 degrees only one percent of the hours in a year—based on a 30-year average.

During the winter, our 99-percent design temperature is the temperature in Virginia that stays above 17 degrees for 99 percent of all the hours in the year—based on a 30-year average. So the outdoor air in Virginia is going to be colder than 17 degrees for just one percent of the hours during the year! (More on the winter design temperature and load calculation the Fall!)

When an HVAC consultant designs a system for your home, they review the 99-percent and one-percent design temperatures for your area. HVAC units are not sized based solely on the one-percent, 93-plus-degree days we get during the year. If your system were sized based on the hottest or coldest days of the year—your system would be oversized.

During the hottest one-percent degree days, your system will maintain about a 20-degree difference because your AC was designed to provide optimal comfort when the outdoor temperature is under 93 degrees—which is 99 percent of the time. Remember, Virginia tops these abnormally hot days around one percent of all the hours in a given year; while they might be hot and uncomfortable, those above 93-degree days are the minority. Make sure your HVAC filter is clean and turn on those ceiling fans!

What if your HVAC can’t keep up for an extended period of time?

If your system cannot keep up with a 20-degree difference for an extended period of time, your system may need professional servicing to check for inadequate sizing, ductwork or refrigeration leaks, dirty filters, or a suffocating (dirty) condenser. And if your unit is over ten-years-old, Energy Star suggests thinking about replacing it.

If you just can’t stand the heat, Mitsubishi mini-split heat pumps are an excellent solution to supplement your existing HVAC—or they can stand alone!

  • Mini-splits are extremely efficient (up to 26 SEER).
  • Mini-splits can more efficiently match the heating and cooling loads of a home.

Talk with an HVAC consultant to design a comfort system that you understand—and one that works best for you every day of the year and in every room of your home. You want to be comfortable; that is so worth having a conversation with a comfort specialist!

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