Heat Pump Not Cooling Due to Overflow Switch

Common Symptoms

• Heat pump outdoor unit not running in cooling mode
• Warm air from supply vents with thermostat set to cool
• Indoor fan/air handler running normally
• Water in condensate drain pan under indoor unit
• Overflow light or safety shutoff active on control board
• System works fine in heat mode but not cool
• Outdoor unit stops during hot, humid weather

Most Likely Causes

1. 1

   Clogged Condensate Drain — Float Switch Tripped (Most Common)

   Same root cause as conventional AC: algae and mold in the drain line back water up into the pan. The float switch opens the Y circuit, the contactor de-energizes, and the outdoor unit stops. Clearing the drain and resetting the float resolves it. Heat pumps produce roughly the same amount of condensate as standard ACs in cooling mode.

2. 2

   Overflow Switch in Secondary Pan Tripped

   Heat pump air handlers installed in attics or over finished ceilings use a secondary drain pan for overflow protection. If the secondary pan float trips, it means the primary drain is completely blocked and the primary pan is already overflowing — an urgent situation. Check the ceiling below for water damage before restarting the system.

3. 3

   Reversing Valve in Wrong Position — Not Actually Calling for Cool

   Before assuming a drain problem, confirm the heat pump is actually in cooling mode. On most heat pumps, the reversing valve energizes for cooling (O terminal energized). Set your multimeter to 24V AC and confirm voltage at the O terminal. No voltage at O when the thermostat is set to cool suggests a thermostat or O-wire wiring issue — not a drain problem.

4. 4

   Heat Pump Defrost Board Lockout

   Some heat pump control boards have a lockout delay when exiting defrost mode. If the system recently came out of defrost (common on a heat pump that also ran in heat mode), there may be a built-in delay before cooling is available. Wait 5–10 minutes and retry before diagnosing further.

Quick DIY Checks

1. 1Confirm the system is calling for cooling: set the thermostat to COOL and a setpoint at least 3°F below current room temperature. Set your multimeter to 24V AC and test at the air handler Y terminal. Confirm 24V is present — this means the thermostat is correctly issuing a cooling call.
2. 2Check the condensate pan: locate the air handler drain pan. If water is present, the float switch has tripped. Use a wet/dry vacuum to remove all standing water from the pan. This is step one before any electrical diagnosis.
3. 3Clear the drain line: use a wet/dry vacuum on the drain outlet (exterior discharge end) for 60 seconds. Then pour 1/4 cup of white vinegar through the cleanout port. Flush with clean water and confirm free flow at the discharge end.

1. 4Test the float switch after the pan drains: with power off, disconnect the float switch wires. Set multimeter to resistance. With float down, confirm 0 ohms (NC contacts closed). If OL with float down, the float switch has failed — replace it.
2. 5Restore power and confirm the outdoor unit starts: with the drain clear and the float switch reset, restore the thermostat cooling call. The outdoor unit should start within 60 seconds (accounting for any built-in delay). Confirm cold air from supply vents and verify the outdoor unit is running (compressor and condenser fan both active).

Repair vs Replace

Recommended Tools & Parts

• Mini Float Switch (NC, 24V, universal heat pump drain pan)

  Universal NC float switch for heat pump air handler condensate pans. Wires in series with Y terminal to shut off outdoor unit when pan fills. Fits most residential and light commercial air handler drain pans.

  $10–$20

  Buy on Amazon →

• Condensate Drain Pan Treatment Tablets (monthly)

  Slow-release algaecide tablets for heat pump condensate drain pans. Prevents algae buildup that causes drain clogs and overflow switch trips. One tablet per month during cooling season.

  $8–$15

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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