Why Your A/C Runs Constantly But Never Quite Cools

First, decide whether it is actually a problem

On a 115-degree Phoenix afternoon, a correctly sized air conditioner is supposed to run nearly continuously. That is the design intent. Equipment is sized to hold setpoint at a design temperature — for most of the Valley that is somewhere around 108 to 110 degrees — and on days above that, continuous operation is normal and healthy.

The question is whether it is holding temperature while it runs. A system that runs constantly and keeps the house at 78 is working. A system that runs constantly while the house drifts from 78 to 84 over the afternoon has a real fault.

The ten-minute diagnostic you can do yourself

Two measurements tell you most of what you need to know. You need a cheap probe thermometer or even an instant-read kitchen thermometer.

1. Pull the filter and hold it up to a light. If you cannot see light through it, that is your answer — replace it and re-check in a few hours.
2. With the system running fifteen minutes, measure the air temperature at the return grille.
3. Measure the air temperature at a supply register close to the air handler.
4. Subtract. That difference is your temperature split, and it should land between 16 and 22 degrees.

A split below about 14 degrees points toward low refrigerant charge or a compressor that is not pumping properly. A split above 24 degrees points toward restricted airflow — a filter, a dirty evaporator coil, a failing blower, or crushed ductwork.

Cause one: airflow restriction

This is the most common and the cheapest to fix. Filters loaded with Arizona dust, evaporator coils with a film of dirt that insulates them, closed or blocked supply registers, collapsed flex duct in the attic, and undersized returns all produce the same symptom.

Restricted airflow also causes the coil to run colder than designed, which is how you end up with an iced evaporator and a system that blows nothing at all by evening.

Cause two: low refrigerant charge

Refrigerant is not consumed. A system low on charge has a leak, full stop. Anyone who tops you off annually without finding the leak is selling you an annual subscription to the same problem.

Low charge shows up as a reduced temperature split, longer run times, ice on the copper suction line at the outdoor unit, and rising electric bills. On an R-22 system, it is frequently the point at which replacement becomes the better decision.

Cause three: a condenser coil packed with dust

The outdoor unit rejects heat to the air around it. When its fins are packed with cottonwood fluff, dryer lint, grass clippings, or fine desert dust, it cannot reject heat efficiently. Head pressure rises, capacity falls, and compressor amp draw climbs — which you pay for twice, in comfort and on the bill.

This is the single highest-value thing a homeowner can address between professional visits. Power off at the disconnect, rinse gently from the inside out with a garden hose, and never use a pressure washer.

Causes four and five: the expensive ones

A compressor that is running but not pumping efficiently — worn valves, internal leakage — produces exactly this symptom, and it is diagnosed by comparing measured pressures and amp draw against the compressor nameplate. On a system past twelve years, a failing compressor usually means replacement.

Genuine undersizing is less common but does happen, particularly after additions, garage conversions, or window replacements that changed the load. It also happens when a previous contractor sized by square footage rather than running a load calculation. The tell is that the system has never held temperature on the hottest days, even when new.