Compressor Amps Climb Slowly Over 20 Minutes of Runtime: Decision Tree

Why this matters

A compressor that starts at nameplate amps and then creeps upward, 10 percent, then 20 percent, over the first 20 minutes is telling you that something is heat-soaking, restricting, or losing cooling as the system warms. It rarely throws a code until it finally trips on internal overload, so the customer reports intermittent cooling loss late in long cycles. The slow climb separates the dynamic faults (rising head pressure, recirculating condenser air, failing capacitor under thermal stress, overcharge that surfaces only at high condensing temperature) from the static faults that would show high amps from the first second. Reading the climb correctly avoids condemning a healthy compressor that is actually being strangled by airflow or charge.

Symptom presentation

The clean version: clamp-meter on the compressor common shows current within nameplate RLA at startup, then a steady rise across 15 to 25 minutes toward or past RLA, with discharge pressure rising in step. The unit may finally trip on the internal overload and restart after a cooldown. Indoor cooling fades as amps climb. No hard lockout code.

Variants that change the tree:

• Amps spike high immediately and stay flat: locked rotor, shorted winding, or stuck contactor, not a climb.
• Amps climb only on the hottest afternoons: condenser airflow or overcharge surfacing at high ambient.
• Amps climb and suction pressure also climbs: load or metering issue, not condenser side.
• Climb accompanied by hot motor and weak start: capacitor degrading under heat.

Quick checks before condemning the compressor

Capture a baseline at the moment of start: compressor amps versus nameplate RLA and LRA, discharge and suction pressure, outdoor ambient at the coil inlet, and condenser fan amps. Then log the same set every five minutes through the climb. The shape of the curve points to the branch.

Feel and read the condenser. Inlet air more than 5 F above shaded ambient means recirculation; a discharge pressure that climbs faster than ambient justifies means airflow loss. Check the condenser fan motor amps against nameplate; a fan motor slowly losing speed as its bearings or capacitor heat-soak raises head pressure and drags the compressor up with it.

Isolation tree

Branch 1: condenser heat rejection failing under runtime.

• Discharge pressure and saturated condensing temperature climb well above the 15 to 20 F over ambient target as the unit runs. Confirm coil cleanliness on all faces, fan blade pitch and direction, and fan motor amps holding steady.
• Fan motor amps that sag or the motor that cycles on its own overload mid-run: failing motor or run capacitor heating up. Replace and re-test the amp curve.
• Recirculation from a tight enclosure or top cover: the inlet temperature rise grows as the local air pocket warms. Relocate or clear the obstruction.

Branch 2: overcharge surfacing at high condensing temperature.

• Subcool that starts normal and climbs past 15 F as head pressure rises indicates liquid backing up in the condenser at the elevated condensing temperature. Recover to the correct weight and re-run the curve.
• Non-condensables (air in the system from a sloppy evacuation) raise head pressure abnormally as the gas heats; the dead-give-away is condensing temperature far above the pressure-temperature chart for the measured liquid line temperature.

Branch 3: electrical and mechanical compressor stress.

• Run capacitor measured under microfarads by 10 percent or more raises winding current; a capacitor that reads fine cold but the amps still climb suggests the cap or terminal connection heating up. Re-measure hot.
• Tight or seizing mechanical components inside the compressor draw rising current as oil thins with heat. This is a last-resort diagnosis after rejection and charge are cleared.

Branch 4: load and metering driving suction high.

• If suction pressure climbs along with amps, the coil is being overfed or overloaded: stuck-open metering device, flooding TXV bulb, or excessive return air heat gain. Check superheat trend, not just pressure.

Confirming the diagnosis

After the corrective action, repeat the 20-minute amp-and-pressure log under the same outdoor ambient. A healthy system holds compressor amps below RLA and discharge pressure stable at 15 to 20 F over ambient saturated condensing for the full cycle. A flat, in-spec curve confirms the fix. Photograph the start-versus-steady-state numbers; that curve is the proof and the baseline for the next service.

References

• ASHRAE Handbook HVAC Systems and Equipment, Chapter on Compressors - current, head pressure, and thermal-stress behavior.
• AHRI 540 - positive-displacement refrigerant compressor performance rating.
• Compressor manufacturer application bulletins (Copeland AE4-1301 and equivalents) - RLA, LRA, and overload behavior.
• EPA 40 CFR Part 82 Subpart F (Section 608) - recovery and recharge during charge correction.