How Long Should It Take to Cool a House? Time Estimate Calculator

How the estimate works

Cooling a house is not just cooling the air. The air in a 2,000 sq ft home holds only a few hundred BTU per degree. Drywall, framing, furniture, flooring and contents hold far more, and they release that heat slowly as the room cools. Meanwhile, heat keeps leaking in through walls, windows and the roof. The air conditioner can only lower the temperature with the capacity left over after it keeps up with that heat gain.

Heat gain = Area × Weather factor × Insulation factor
Usable capacity = Tons × 12,000 × 0.95
Spare capacity = Usable capacity − Heat gain
Thermal mass = Area × 3 + Area × Ceiling height × 0.018 (BTU per °F)
Time (h) = Degrees to drop × Thermal mass ÷ Spare capacity

The weather factor is an approximate heat gain in BTU/h per square foot for an average home. The 0.95 allows for typical duct losses. The thermal mass assumes about 3 BTU per °F per square foot of floor for the building and contents that respond within a few hours, plus the air itself (0.018 BTU per cubic foot per °F). These are simplifying assumptions, so treat the result as a ballpark.

Worked example

A 2,000 sq ft house with 8 ft ceilings, a 3-ton AC, a warm day and average insulation, cooling by 5 °F:

• Heat gain = 2,000 × 10 × 1.0 = 20,000 BTU/h
• Usable capacity = 3 × 12,000 × 0.95 = 34,200 BTU/h
• Spare capacity = 34,200 − 20,000 = 14,200 BTU/h
• Thermal mass = 2,000 × 3 + 2,000 × 8 × 0.018 = 6,288 BTU/°F
• Time = 5 × 6,288 ÷ 14,200 = 2.2 hours, or about 27 minutes per degree

Those are the calculator's defaults. On a mild day the same house cools in about 1.6 hours (19 minutes per degree); on an extreme day with poor insulation the gain can match or exceed the unit's capacity and the house may not reach the setting until evening.

Typical cool-down rates

A system that cools very fast is not necessarily a good sign. Oversized units hit the setting before removing much moisture; see does AC remove humidity.

Why the AC may never catch up on the hottest days

Residential systems are sized to hold a set indoor temperature (usually 75 °F) at the local design temperature, not to pull a hot house down quickly. On a day hotter than design, a correctly sized system may run nonstop and still let the house drift up a degree or two. That is expected. If the house cannot hold 75 to 78 °F on an ordinary summer day, though, something is wrong: work through thermostat not reaching temperature and AC not blowing cold.

Large setbacks on hot days can backfire. Letting the house climb to 88 °F while you are out leaves a lot of heat stored in the structure, and the AC may still be catching up at bedtime. A setback of about 4 to 7 °F is usually easier to recover from. See ideal AC temperature.

Ways to speed up cooling

1. Replace a dirty filter and open all supply and return grilles.
2. Close blinds on sunny windows, especially west-facing ones, in the afternoon.
3. Avoid ovens, dryers and dishwashers during pull-down.
4. Use ceiling fans in occupied rooms so you feel cooler sooner (see ceiling fan direction).
5. Keep the outdoor unit's coil clean and shaded from obstructions but not boxed in.
6. Longer term: attic insulation and air sealing reduce heat gain, which directly increases the spare capacity in the formula. See the insulation R-value chart.

Limits of this estimate

The calculator ignores humidity (in humid climates part of the capacity goes to removing moisture, which slows temperature drop), assumes heat gain stays constant as the house cools (it actually rises slightly), and uses rated capacity rather than the real output at your outdoor temperature. Two-stage and variable-speed systems may run at reduced capacity at first, which is slower but more comfortable. Use the result to judge whether your system is roughly normal, not as a performance test.

Frequently asked questions