July 8, 2024

My thermostat is set to 72 degrees, but my HVAC system won’t keep my house cooler than 75 degrees…Is it broken?

On hot summer days, do you wonder why your home temperature doesn’t match your thermostat setting?

During summer heatwaves, our customer service team often hears many people wondering if their system is in need of repairs or even a full replacement.

We’ve devoted this blog to helping you understand your system’s design temperature limit. On extremely hot days, HVAC systems probably won’t be able to cool to your thermostat setting. System lag is normal, and in this blog, we are investigating it thoroughly.

First, did you know that HVAC systems are based  partially on your geographic area’s average temperatures?

The technician who installed your home’s HVAC system followed a certain set of standard practices and regulated your system to those standards. That means your system’s design considers the high and low temperatures and how long the area tends to stay at those levels.

You might be surprised to learn that Charlottesville’s temperature only rises above 92 degrees Fahrenheit one percent of the time (Source: Energy Star). To evaluate winter low temperatures, we look at the temperature the area stays above for 99% of the year, which is 16 degrees Fahrenheit in Charlottesville. 

Air conditioning systems are designed to maintain a comfortable (roughly 20-degree) temperature difference between indoor and outdoor temperatures. So, while you might have the AC set at 72 degrees, don’t be surprised if the system can’t cool your home to less than 75 degrees on the hottest summer days above 92 degrees. That’s because your system attempts to keep the 20-degree difference between the programmed one percent design temperature, which is 92 degrees. 

If HVAC systems were sized and designed according to the temperature of the hottest (or coldest) days of the year, they’d be far too large and frequently short-cycle throughout the year. 

Remember, Virginia tops these abnormally hot days only one percent of the time in a year. While they might be hot and uncomfortable, above 92-degree days are the exception. To make sure you’re getting the most out of your AC, make sure your HVAC filter is clean and your system can breathe.

Now, if your AC can’t keep up for an extended period of time…

If your system doesn’t seem to maintain a roughly 20-degree difference between the temperature in the home and outside, it might be time to call us. 

Poor cooling performance might be due to several factors:

  • An old unit. If your HVAC is over ten years old, Energy Star suggests replacing it.
  • Refrigeration leaks. Unlike a car, the refrigerant in your AC unit doesn’t run low. Unless there’s a leak, it will remain at the same level. If your system is blowing warm, there is no air, or it isn’t making sound, a refrigerant leak might be the issue.
  • Dirty filters. Believe it or not, dirty air filters can also affect your HVAC system’s efficiency. If it’s been over three months since you last changed your air filters, your allergies are acting up, and/or there’s visible dust accumulating around your air vents, change those filters ASAP! 
  • A suffocating condenser. Your condenser is the big box outside at the side or back of your home. If these outdoor units aren’t maintained properly, they can accumulate dirt, dust, and grass clippings, all of which affect their ability to do their job and make them extremely energy-inefficient, leading to higher electrical bills.
  • Inadequate sizing. It might sound counter-intuitive, but an AC unit can be too large for a space. 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 an appropriately sized unit. Due to short cycling, an oversized unit will not run long enough to effectively remove humidity from indoor air. The higher humidity will make you feel hotter and more uncomfortable, leading you to crank the AC—and your utility bill. It’s a vicious cycle!

If your HVAC tech concludes that your system is oversized or outdated and you’re due for a system replacement, don’t worry! Here at W.E. Brown, we have plenty of options for system upgrades that address your home’s unique needs–whether that’s mini splits for optimal cooling in bedrooms or home offices, or air purification systems for allergy-prone kids. 

Call us to set up your free in-home consultation with one of our team members. We’ll discuss your unique needs and goals and financing options to help ensure this expense is within your budget. 

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