Why won't my AC cool the house when it's extremely hot outside?
Older air conditioners and heat pumps struggle to overcome extreme outdoor temperatures above 95 degrees, causing them to run continuously without reaching the thermostat setpoint. This constant runtime leaves indoor temperatures several degrees too high, preventing the home from staying comfortable during severe heatwaves.
What the numbers show
Counted from the threads quoted below. How we count
- People have been saying this in most months since July 2026.
- This comes up 11.7× more often in r/hvacadvice than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1adding window AC units to specific roomsDid this help you?
- 2using thick blackout curtainsDid this help you?
- 3sealing drafty doors and windowsDid this help you?
- 4shading windows from the outsideDid this help you?
- 5hosing off the outdoor condenser unitDid this help you?
- 6Replacing or cleaning air filtersDid this help you?
- 7Adding insulation, radiant barriers, and air sealingDid this help you?
- 8Using solar mesh screens, window film tinting, or foam boards in windowsDid this help you?
- 9Running a dehumidifier to make higher temperatures feel coolerDid this help you?
- 10Keeping doors and windows closed to minimize heat gainDid this help you?
- 11renting an insulation machine to blow in cellulose insulation oneselfDid this help you?
- 12sealing penetrations, vents, and HVAC ducts with HVAC tapeDid this help you?
- 13replacing seals and weatherstripping, caulking, and fixing windowsDid this help you?
- 14adding powered attic fans to existing ventilation systemsDid this help you?
- 15Trying window fans switched to exhaust modeDid this help you?
- 16Consulting with HVAC professionals for custom solutionsDid this help you?
- 17closing window blindsDid this help you?
- 18using circulating fansDid this help you?
- 19putting tinfoil or outdoor shades on windows facing direct sunlightDid this help you?
- 20installing an attic fanDid this help you?
- 21adding more attic insulationDid this help you?
- 22running an additional dehumidifierDid this help you?
- 23spraying a light mist of water on the roofDid this help you?
- 24Managing the thermostat by bumping up the temperature a degree or two during peak afternoon heatDid this help you?
- 25Upgrading attic insulation, air sealing, and ductworkDid this help you?
- 26Using supplemental window unitsDid this help you?
- 27Opening windows briefly only during the coolest parts of the day (early morning or late evening)Did this help you?
- 28Running ceiling fans, oscillating fans, and Vornado fans to keep air circulatingDid this help you?
- 29Using standalone HEPA air purifiers with activated carbon filtersDid this help you?
- 30Building and using a DIY Corsi-Rosenthal box filterDid this help you?
- 31Turning on bathroom and kitchen ventilation exhaust fansDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“It kept running at 76, so I see it to 77 and it’s been running constantly like before”source ↗
“It was 100 degrees today”source ↗
“Oklahoma has been hellish this summer with record consecutive days of triple digit temps, yet I've heard numerous acquaintances claim that they just set their thermostat to 68° and it keeps up even on the hottest days. So if you're in that club, how do you do it?”source ↗
“it was 97°F today with a heat index 105 where i live and i kept my 20 year old goodman heat pump set to 72. it got as high as 78 inside with the heat pump running constantly”source ↗
“Here🖐️ Dealing with this today!”source ↗
“I’ve found that with the AC temp set to around 74, throughout the day and especially into the early evening, our indoor temp creeps up to 77-78.”source ↗
“Our HVAC system had this problem during heatwaves and the house would get well above 80 during the worst of it.”source ↗
“My house can smell a bit stale and odors tend to linger. The first recommendation from folks online is always to open a window, but I would love to know what y’all’s AC bills look like because I can’t exactly do that when it’s 85 degrees and 90% humidity outside even in October!”source ↗
“Spot on—anyone telling you to 'just crack a window' in 90% humidity has clearly never felt their indoor air instantly turn into soup.”source ↗
“We couldn't afford one more mini split when we got rid of our central AC. We decided to keep my adult daughter's bedroom shut, and her one window closed to curtains... Naturally, it was the room we didn't throw a mini split in.”source ↗
“I just had a friend in HVAC come by to give us a quote. The bad news is that, because we have an open, shared attic space with all our 2nd floor neighbors, we can’t install my precious fan without impacting them.”source ↗
“We’ve tried the window fans that can be switched to exhaust and they just don’t do anything. Nowhere near powerful enough.”source ↗
Where this came up
People with this problem also raised
- 6Why is just one upstairs room so hot in the summer?
- 3How to cool down a house when the AC breaks during a heatwave
- 6Why is my attic apartment so hot in summer and cold in winter?
- 6Why does my HVAC fan run continuously and blow hot air?
- 3Is Japan still enjoyable in summer or is it too hot?
- 2How to choose a mini-split for an upstairs room