Air-source heat pump outdoor unit beside a house in the snow
Generic photo: PeterEastern · CC BY-SA 4.0

Field pattern: air handler heat strips cycle off or some strips never heat (limit, sequencer, airflow)

Electric strips are switched by sequencers or contactors and protected by limit switches in series with each element. Low airflow trips the limits first, so the filter, blower and coil come before parts. The wrong turn is replacing a limit or element without measuring current per strip and proving airflow. All of this is 240 V work: CAUTION.

Cross-checked ~60 min advancedBrands: All

Before you start

  • 240 V and high current: CAUTION. Turn off and lock out every breaker feeding the air handler (strips often have their own breakers) before opening panels
  • Never run strips with the blower disabled
  • Never jumper a limit or fusible link to keep heat on
  • Refrigerant-side work is BLOCKED: refer to an EPA 608-certified technician

Diagnostic steps

Unlock all 8 steps with expected values

Full library + AI assistant + the OnCall Tech app for iPhone & Android. $34.90, one time.

Get lifetime access

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

More air conditioners guides

Guide · Air Conditioner

PTAC not cooling (fan runs, air not cold)

Most PTAC no-cool calls are airflow (filter, coils), a wrong control setting or a failed compressor start component. Rule those out before suspecting the sealed system.

6 steps ~60 minmoderate
Guide · Air Conditioner

PTAC not heating (electric heat or heat pump)

PTACs heat with electric strip heat, a heat pump, or both. Heat pump models switch to strip heat in cold weather. Check settings, then the heater element and its limits, then the reversing valve.

5 steps ~60 minmoderate
Guide · Air Conditioner

PTAC error codes: how to read and clear them

Most PTAC brands store fault codes that can be shown with a button combination on the control (for example, on GE Zoneline (2019 technical service guide) you hold FAN and the red AUX button together while the unit is OFF, and AUX clears stored faults; other models use other sequences, so use the model's tech sheet). Read the active code, fix the cause, then clear it.

4 steps ~20 mineasy
Field pattern: air handler heat strips cycle off or some strips never heat (limit, sequencer, airflow) · OnCall Tech