How Long It Should Take — By House Size
These are real-world estimates for a properly sized, well-maintained central AC cooling a house that's been sitting warm, on a hot Chicago day (~90°F outside):
| Home Size | Drop 5°F | Drop 10°F | Drop 15°F |
|---|---|---|---|
| Under 1,000 sq ft | 30–45 min | 1–1.5 hrs | 1.5–2.5 hrs |
| 1,000–1,500 sq ft | 45–75 min | 1.5–2.5 hrs | 2.5–3.5 hrs |
| 1,500–2,500 sq ft | 1–1.5 hrs | 2–3 hrs | 3–5 hrs |
| 2,500–4,000 sq ft | 1.5–2 hrs | 3–4 hrs | 5–8 hrs |
| Over 4,000 sq ft | 2+ hrs | 4–6 hrs | 6–10+ hrs |
Based on a properly sized system, well-insulated home, outdoor temp ~90°F. Chicago humidity and direct sun exposure both increase these times.
A healthy central AC cools 1–3°F per hour. If your AC has been running for 3+ hours and the temperature hasn't moved at all, that's not a slow day — that's a problem that needs a technician.
What Makes Cooling Take Longer
Your AC cools air and removes moisture. On a 90°F day at 70% humidity — a typical Chicago July — the system spends significant capacity dehumidifying. The same 90°F feels harder to cool in Chicago than in dry Phoenix because of this.
A house sitting at 85°F all day takes much longer to cool than one that drifted up to 78°F. The larger the gap between current temp and your thermostat setting, the longer the recovery.
Chicago homes built before 1980 often have R-11 attic insulation or less. The DOE recommends R-38 to R-60 for Chicago's climate zone. Poor insulation means heat bleeds back in from the attic constantly while the AC tries to cool — it can never fully catch up.
A west-facing room with large uncovered windows in afternoon sun adds the equivalent of a small heater's worth of heat gain. Blackout shades on south- and west-facing windows from 2–6 PM make a real, measurable difference.
An undersized system runs constantly and never catches up on hot days. An oversized system short-cycles — cools the thermostat quickly but leaves the air clammy because it didn't run long enough to remove humidity.
The Tech Check: Temperature Split
When SAVA technicians check an AC's performance, one of the first things measured is the temperature split — the difference between air entering the return vent and air coming out of the supply vents.
💡 You can do a rough version yourself with any cheap thermometer. Hold it at the return vent for 60 seconds, then at a nearby supply vent. Under 10°F difference = something is wrong.
When It Means Something Is Wrong
- AC has been running 3+ hours with no temperature change
- House cools down but warms back up within 30 minutes of AC shutting off
- AC runs continuously without ever reaching your thermostat setting
- Some rooms cool fine but others never cool down regardless of runtime
- Air from vents is barely cooler than room temperature
- You hear grinding, squealing, or hissing while the system runs
AC Not Cooling Right? We'll Find Out Why.
SAVA provides same-day AC diagnostics across Chicago and suburbs. We measure refrigerant levels, temperature split, airflow, and electrical components — and tell you exactly what's wrong.