Cannot keep up in a heat wave: it may be doing its job
Home air conditioners are sized to an outdoor design temperature, not the hottest day of the decade. A Georgia Tech mechanical engineering professor told NPR most new US systems are designed for a 95 degree day, and that they are built to hold the house about 20 degrees below outside.
What is probably happening
- The rule of thumb: a healthy AC holds roughly 20 degrees F below the outdoor temperature. At 105 degrees outside, 85 inside is not broken, it is the design limit.
- Systems are sized with ACCA Manual J to the local 1 percent cooling design temperature, meaning it is hotter than that only about 1 percent of the summer.
- A dirty filter, a dirty outdoor coil, or leaky ducts make it fall short of even that.
Do this in the next five minutes
- Leave the thermostat where it is. Setting it lower does not cool faster, it only runs longer.
- Close blinds and curtains on the sunny side. Run ceiling fans in occupied rooms.
- Replace the filter. Hose the outdoor coil gently from the outside, power off, and clear plants to 2 feet around it.
- Skip the oven and dryer until evening.
- If the vents are blowing cold and the house holds within about 20 degrees of outside, it is working. If the vents are not cold, go back and pick warm air.
Do not
- Do not let a heat wave stampede you into a replacement. Every contractor is booked and quotes are highest.
- Do not accept 'you need a bigger unit' without a load calculation. Oversized units cool fast and shut off before removing humidity.
- Do not add refrigerant on a hunch. A correctly charged system does not need more in hot weather.
Who to call
A licensed HVAC technician, after the heat wave
A tune-up with a coil cleaning and a temperature-split check tells you whether the unit is underperforming or simply at its design limit.
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What it should cost
$175 to $350
HVAC maintenance visit, national range, averaging $250
Source: HomeAdvisor, HVAC maintenance cost, updated June 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- What temperature split did you measure across the coil, and what does the manufacturer's data say it should be at today's conditions?
- Is the outdoor coil clean, and is the charge correct by subcooling or superheat?
- If you think it is undersized, will you run a Manual J and show me the number?
- Are my ducts leaking into the attic? Can you measure it?
End the call if
- A bigger unit recommended by square footage alone. ACCA's own data shows homes ranging from 624 to well over 1,400 square feet per ton, so square-foot rules of thumb mis-size systems.
- A verdict from the 15 to 20 degree split rule with no manufacturer data. ACCA calls that rule a notoriously lousy practice when used alone.
Where this comes from
- NPR, Your air conditioner is not designed for extreme heat, 2023
- ENERGY STAR, Design temperature limits (ACCA Manual J 1 percent design conditions)
- ACCA HVAC Blog, A split in diagnosis
- ACCA HVAC Blog, Manual J load calculations vs rules of thumb
- US Department of Energy, Energy Saver, Maintaining your air conditioner
- HomeAdvisor, HVAC maintenance cost, 2026