AC can't keep up in a heat wave
Equipment sized for an ordinary Boone County summer can lose three or four degrees on a 98°F afternoon and be perfectly healthy. Something is wrong if the registers blow air that isn't cold, the condenser fan has stopped, the copper is frosted, or the house is still warm at breakfast.
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What healthy looks like on the worst days
Contractors size for a typical summer afternoon, not the record. So when it is pushing 100 outside, a thermostat that climbs from 74 to 77 or 78 while the compressor never cycles off can be a system doing its full job. The airport recorded 12 days a year at 95°F or more over 2016-2025, and 16 of them in 2026 through October 24, 2026.
The test is the overnight. A sound system pulls the house back to the setpoint before morning. Warm nights make that harder, and Columbia has more of them than it used to: 37.3 a year now with a low of 70°F or higher at the airport, compared with 27.8 over 1991-2020. In 2026 there were 45.
Nights per year when the low stayed at 70°F or above
Signs you need a technician
- Supply air that feels about the same as the room. A common field check is a 15 to 20 degree difference between the air going into the return grille and the air coming out of a register.
- A condenser that hums with the fan blade still, or that isn't running at all.
- Frost or ice on the copper lines or on the indoor coil.
- Water collecting near the furnace, or the system shutting itself down.
- A house that is warmer at 7 a.m. than it was at midnight.
Giving the system a fighting chance
- Close the blinds on the south and west sides before noon.
- Swap a dirty filter.
- Leave the setpoint where it was. Dropping it to 65 doesn't make cold air come out faster; it only keeps the compressor running longer.
- Save the dryer, dishwasher and oven for after dark.
- With the power off, hose cottonwood fluff and clippings out of the condenser fins.
Keeping everyone safe
Heavy sweating and nausea are on the National Weather Service's list of heat exhaustion signs. Hot, red skin with confusion points to heat stroke, and that means calling 911. Put anyone at risk in the coolest room, a basement if you have one, or take them somewhere with working air conditioning. Per the CDC, a fan alone won't prevent heat illness once indoor temperatures climb high. The first hour after the AC quits.
What Boone County homeowners ask us
My AC hasn't shut off all day during this heat wave. Is that OK?
On the hottest afternoons a properly sized system may run without a break. It should still blow cold air and bring the house back to the setpoint overnight.
How cold should the air from my vents be?
Many technicians expect the supply air to be about 15 to 20 degrees cooler than the air entering the return. A much smaller gap suggests something needs a look.