AC Help
Why will my house not cool below 78 degrees?
Use temperature trend, runtime, airflow, humidity, and outdoor conditions to separate a peak-load pattern from a system that needs service.
- Written by
- Andrew Carpenter
- Updated
The number 78°F does not tell you whether the AC is broken. The useful evidence is whether indoor temperature is falling, holding, or rising while the system runs, plus outdoor conditions, humidity, airflow, and whether the behavior changed. A correctly selected system may run for long periods near peak load, but a house that keeps gaining heat needs investigation.
Measure the direction, not only the setpoint
Record the thermostat temperature, setpoint, indoor humidity if available, and outdoor temperature. Repeat at the same thermostat after 30–60 minutes. Note whether the indoor and outdoor equipment ran continuously and whether supply airflow felt normal across several rooms.
Use the Should my AC be able to keep up? tool to organize the trend. The result is an observation, not a diagnosis.
Check the ordinary items
Confirm the thermostat is on COOL, fan is on AUTO, and no schedule or remote sensor is changing the command. Check the normal filter location and keep return grilles and supply registers open and clear. Do not close vents around the house to force air toward one room.
Look for visible water or ice without opening equipment. Listen for a new severe sound and note whether the outdoor unit stops while the thermostat still calls. Stop for electrical odor, a repeated breaker trip, water near wiring, or ice.
Reduce unusual loads while you observe: close exterior doors and windows, use exterior shade where available, and avoid large heat-producing activities. This does not fix a system fault; it makes the observation easier to interpret.
Why “it should maintain 20 degrees below outside” is not a rule
A fixed indoor-to-outdoor difference is not an equipment-sizing standard. Residential cooling load depends on the actual walls, roof, windows, air leakage, ducts, orientation, occupancy, ventilation, and moisture load. ACCA identifies Manual J as the residential load-calculation method (ACCA). Equipment selection and installation then have to match that load and the manufacturer’s performance data.
The supply-to-return temperature split is a different measurement. It can add context when measured correctly, but it does not establish whole-house capacity or identify a failed component. Use the temperature split guide if you can measure safely.
When the pattern points to a fault
Call for service when the temperature rises through sustained operation, the pattern is new, airflow is weak at many vents, one piece of equipment stops, or water, ice, severe noise, odor, or electrical symptoms appear.