Lennox Heat Pump Runs Constantly: Causes, Fixes & Costs

Last updated Oct 19, 2026 · By fixme.vip Editorial

Lennox heat pump runs constantly: what it means

“Runs constantly” is not a fault code that a Lennox board stores. It is a symptom. It falls into two different situations, and it helps to know which one you have:

1. It runs nonstop but the house does reach setpoint, or gets close. This usually points to lost capacity: a dirty coil, low refrigerant, or poor airflow. It can also be normal behavior in extreme weather or on variable-speed and two-stage Lennox models, which are designed to run long, quiet, low-capacity cycles.
2. The outdoor unit keeps running after the thermostat is satisfied or switched to Off. This is a control fault, usually a stuck contactor or a shorted call wire. Shut the unit off at the outdoor disconnect and call a pro. Unlike the first situation, this one will not fix itself.

There is no reset for this symptom. If you have a communicating iComfort, S30, S40, or E30 thermostat, check its alert screen. Logged alerts such as low-pressure trips or a low-capacity notice give your technician a head start.

This page focuses on nonstop running in general. If your situation is narrower, see these related pages:

• Backup heat-strip operation: Lennox Heat Pump Aux Heat On Constantly
• Cold air in heat mode: Lennox Heat Pump Blowing Cold Air in Heat Mode
• The indoor fan running alone: first confirm the thermostat fan is set to AUTO, not ON. Most heat pumps use an air handler indoors; if your system uses a furnace as the indoor unit instead, see Lennox Furnace Blower Won’t Shut Off.

Lennox heat pump runs constantly: common causes, ranked by probability

1. Normal operation for the conditions or model. Heat pumps lose capacity as outdoor temperature drops. Near and below the balance point (often 25–35°F), continuous running is expected. Variable-speed Lennox units (EL18XPV, SL25XPV, XP25) and two-stage units also run long by design.
2. Restricted airflow. A clogged filter, closed or blocked registers, or a dirty indoor coil or blower wheel all reduce airflow, so the system has to run longer to deliver the same heat or cooling.
3. Dirty or blocked outdoor coil. Cottonwood, grass clippings, leaves, or snow and ice packed around the outdoor unit reduce heat transfer. In heating mode, this ties into defrost issues.
4. Low refrigerant charge from a leak. Capacity drops gradually, and run times stretch out over weeks or months.
5. Stuck or welded outdoor contactor. The compressor and fan run even with no thermostat call. This is common on single-stage units after years of cycling.
6. Thermostat problems. Possible issues include:
   • A thermostat mounted near a draft or heat source
   • A wrong setting, such as fan ON or an incorrect system type
   • Shorted Y or W wiring
   • Bad calibration or a failing thermostat
7. Reversing valve leaking internally or stuck partway. This bleeds capacity in both modes, and the unit runs without ever catching up.
8. Duct leakage or an undersized system. If long run times have existed since installation, ducts or sizing are likely. Leaky ducts in an attic or crawlspace are especially common.

How to fix Lennox heat pump runs constantly: safe checks first

• Check the thermostat.
  • Confirm the mode (Heat/Cool) and setpoint are what you expect.
  • Set the fan to AUTO, not ON.
  • Replace the batteries if your model uses them.
  • Make sure no sun, lamp, or supply register is hitting the thermostat directly.
• Replace the air filter. If you can’t see light through it, replace it. Use the size printed on the old filter’s frame.
• Open supply and return registers. Clear away furniture, rugs, and drapes blocking them.
• Look at the outdoor unit from outside.
  • Clear leaves, grass, and snow at least 2 ft around it.
  • Don’t open the panels or spray inside the cabinet.
  • A light rinse of the exterior fins with a garden hose (no pressure washer) is fine with power off at the disconnect.
• If the outdoor unit runs with the thermostat off, switch off the outdoor disconnect or its breaker and leave it off until a tech checks it.
• Check the condensate line. In cooling mode, confirm the line is draining. A clogged drain with a float switch can cause odd cycling.
• Test against a mild day. If the unit still can’t keep up when it’s 45–60°F outside, something is wrong. Long runs at 15°F may be normal.

How a technician will diagnose it

• Confirm the complaint by comparing indoor temperature to setpoint, checking the outdoor temperature, and checking the model’s staging.
• Check for 24 V across the contactor coil with the thermostat satisfied:
  • No voltage but the compressor still runs: the contactor is welded.
  • Voltage is present: look for a shorted Y wire or a faulty thermostat.
• Measure supply and return temperature split, and total static pressure, to evaluate airflow.
• Inspect the indoor coil and blower wheel, and clean the outdoor coil.
• Connect gauges or probes and check superheat and subcooling against the Lennox charging chart on the unit panel. If the charge is low, they will leak-search before recharging.
• Test reversing valve operation through temperature checks across its lines, and confirm the solenoid coil gets voltage.
• Read any stored alerts from iComfort or S30-class thermostats.
• For long-standing complaints, inspect the ducts, check for leakage, and review sizing (Manual J).

A good quote names a measured finding, such as “subcooling 3°F vs 10°F target” or “contactor welded.” Be wary of a quote that says “needs refrigerant” with no leak search.

Symptom, cause and what to do

Lennox heat pump runs constantly repair cost

These are typical US ranges, including labor:

A typical leak search, repair, and recharge together runs roughly $550–$1,900. Costs reach $3,000 only when the evaporator coil has to be replaced.

R-454B pricing applies only to newer-production units. Older systems use R-410A, so ask your technician which refrigerant your unit takes.

Lennox parts warranty (often 5–10 years with registration) may cover the part on coils and valves, but not the labor. Warranty terms vary by series and install date, so check your own registration and warranty documents before approving a repair.

Related codes

• Lennox Heat Pump Aux Heat On Constantly
• Lennox Heat Pump Short Cycling
• Lennox Heat Pump Not Heating
• Lennox Heat Pump Not Cooling
• Lennox Heat Pump Blowing Cold Air in Heat Mode
• Lennox Heat Pump Frozen and Not Defrosting
• Lennox Heat Pump Ice on Outdoor Unit
• Lennox Furnace Blower Won’t Shut Off (for systems that use a furnace, not an air handler, as the indoor unit)
• Lennox Furnace Runs Constantly But Not Enough Heat