There are several reasons your home could be staying hot, even if your air conditioner is running. Common issues range from simple things like a clogged HVAC filter to more complex problems like refrigerant leaks to an improperly sized unit.
You can diagnose most of these issues yourself, but you’ll likely only be able to fix a few without the help of an experienced HVAC contractor
1. Ductwork is Leaking
Leaks in heating, ventilation, and air conditioning ductwork allow air from your furnace or air conditioner to escape. This wastes energy and raises your cooling costs.
You can often find leaks where lines branch off or separate. When your furnace fan is running, you can feel air escaping the duct.
Once you’ve found the leak, turn off the fan and apply metallic foil tape over the hole or gap. Then apply duct mastic over the tape and joints to improve the seal.
Watch How to Repair Leaks in HVAC Ductwork to learn more.
2. Your Air Filter Needs Replacement
How many times have you heard how important it is to change your heating and cooling system’s air filter? And how often have you wondered why a $10 or $15 filter is crucial to your system’s operation?
Here’s the fact: All your home’s air passes through the A/C return grill and the filter before it gets to the air handler. When an air filter gets dirty, that dirt passes through the air handler and the buildup makes it work much harder.
This buildup doesn’t just cause an increase in your cooling bill; it also could lead to a broken air handler unit and expensive repairs.
Watch How to Change the Air Filter in Your Home to learn more.
3. Your Windows are Leaking
If you feel radiant heat in your home when blinds are closed and curtains are drawn, your windows could be leaking.
This happens one of two ways:
Your single-pane windows’ glazing is failing. Single-pane windows need glazing putty to prevent leaks. Over time, glazing putty can crack and even fall off.
If this happens, use a putty knife or paint scraper to remove old glazing around the window opening. Then apply window glazing (available in a can or caulking tube) around the window frame.
Next, hold a putty knife at an angle to the frame, and use it to smooth the glazing compound.
Finally, paint over the glazing with exterior house paint after the glazing has set.
The window transmits solar heat gain. When solar radiation penetrates window glass, a room’s thermal energy increases.
To reduce harmful UV rays and solar heat gain, add window film that can reflect up to 72% of the sun’s heat and block up to 99% of UV rays.
4. Heat Coming from Your Attic
During the summer, heat seeps through your roof, into the attic and causes your air conditioner to work overtime.
There are two things you can do to prevent heat in the attic from entering your living space:
Install an attic vent fan. Add an attic exhaust fan near the peak of your roof to keep the attic cooler, lower cooling bills, and make your roof last longer.
Add more insulation. Insulation is important to reduce heat gain and save energy. How well a particular type of insulation works is determined by its R-value, which measures the insulation’s resistance to the transfer of heat.
5. Heat Coming Through Your Walls
If you have a brick home, you know how fast solar heat cooks those bricks — and how soon you start to feel the effects inside.
That’s why wall insulation is so important.
Again, before you purchase insulation, consider R-values. Fiberglass batts for standard 2-by-4 walls are available in low, medium, and high-density products that range from R-11 to R-15. Spray foam insulation in the same wall cavity can range from an R-14 to an R-28, depending on the product used.
6. Refrigerant Leak
Suppose your AC unit has a leak. In that case, it can have low refrigerant and won’t be able to cool your home effectively. Refrigerant leaks can occur due to damage to the refrigerant lines or improper installation. Signs of a refrigerant leak include warm air coming from the vents, hissing sounds from the AC unit, and a spike in your energy bill.
To fix a refrigerant leak, you’ll need to call an EPA-certified HVAC technician. They can find and repair the leak and then recharge the system with the proper amount of refrigerant.
7. Clogged Evaporator Coils
The evaporator coils in your AC unit absorb heat from the air inside your home and expel the warm air outside. Over time, these coils can get clogged with dust, dirt, and debris — especially if you don’t keep up with filter changes. If it gets caked in dust and grime, it reduces its ability to absorb heat effectively. This will eventually lead to your air conditioner pushing warm air throughout your home.
To fix clogged or dirty evaporator coils, turn off your AC unit and clean the coils with a soft brush, vacuum, or compressed air. If the coils are heavily soiled, you may need to use a coil cleaner or call an HVAC contractor for help.
8. Blocked Ductwork
If your ductwork is blocked or damaged, it can block cool air from circulating properly throughout your home. You’ll likely notice uneven cooling and hot spots in certain rooms if this is the case. Debris, such as dust, dirt, and insulation, or damage by pests or improper installation can block your air ducts.
To fix blocked ductwork, you’ll need to locate and remove the blockage. However, you’ll likely need an HVAC cleaning company to scope your ductwork with specialized tools to find the clogs. So, it’s best to get their help if you suspect this issue.
9. Aging or Inefficient AC Unit
If your air conditioner is old or inefficient, it may struggle to cool your home effectively. As AC units age, their components can wear out and become less efficient, leading to reduced cooling performance and higher energy bills.
If your air conditioning unit is 15–20 years old (or older), you should consider a replacement. Newer, high-efficiency models can provide better cooling performance and energy savings in the long run. If you’re not ready for a full replacement, regular maintenance and tune-ups can help improve the efficiency of your existing unit.
10. Improperly Sized AC Unit
An air conditioner that is too small or too large for your home can lead to ineffective cooling and a hot home. If your AC unit is too small, it will struggle to cool your home adequately. Conversely, an oversized unit will turn on and off frequently, causing hot and cold spots throughout your home — along with higher energy bills.
To fix an improperly sized AC unit, you’ll have to hire an HVAC technician to perform a Manual J load calculation to determine the correct size for your home. They can then recommend a properly sized replacement unit that will provide optimal cooling performance and energy efficiency.
Why Is My Upstairs So Hot?
If your upstairs is significantly hotter than your downstairs, it’s likely due to a combination of factors. Hot air naturally rises, leading to warmer temperatures on the upper floors. Poor insulation, air leaks in the attic, insufficient or improperly sized ductwork, blocked air vents, and direct sunlight through windows can also contribute to a hot upstairs.
Any of the 10 common issues above can be a reason your second floor is staying warm. If the indoor unit (air handler) of your air conditioner is in your basement, you’ll likely notice the symptoms on the top floor first, since it’s furthest away from the air handler.
To fix the problem, focus on improving insulation, sealing air leaks, ensuring proper ductwork configuration, and keeping air vents open and unblocked. If these measures don’t resolve the issue, you should consult with an HVAC technician to assess your home’s cooling system.
Why Is One Side of House Hot?
One side of your home may stay warmer than the other side for most of the same reasons the first and second floors vary in temperature. Poor insulation, uneven sun exposure, and any of the 10 common problems I reviewed above.
If you can’t get your home to cool evenly on both sides of your house after troubleshooting it yourself, calling an HVAC pro is what you need to do next.
Final Thoughts
If your home is staying hot in the summer, you likely have one (or more) of these 10 common issues with your AC unit or other items in your home. If none of your DIY troubleshooting and repairs/improvements help your home get cooler, contact a local HVAC company for a full assessment. They can diagnose the problem and recommend a solution to get your home cool and comfortable all season long.
Frequently Asked Questions
How can I tell if my air conditioner is not working efficiently?
You can turn on your AC unit by lowering the setpoint on your thermostat. Then, hold your hand over a duct register — you should feel cold air moving somewhat fast. If it doesn’t feel cold or you don’t feel any airflow, you have an AC system issue. Additionally, your thermostat never getting to the temperature you set is another sign your AC unit is inefficient.
When should I call an HVAC professional to help with my home's cooling issues?
You should call an HVAC company after ruling out common issues you can fix yourself — check the thermostat, replace the air filter, and verify the breaker for your AC unit is on. You can troubleshoot things further, but you’ll likely need an HVAC technician to help you fix it anyway.
What are the most common reasons for uneven cooling in a home?
The most common reason for uneven cooling in a home is sun exposure. In the U.S., rooms facing toward the south get much more sun exposure. This heats them more than the north side of the house. Other common reasons include an oversized AC unit and poor ductwork design.
