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HELP, MY HEATER ISN’T WORKING! (POSSIBLE CAUSES)

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Heater Error Messages: Water Flow Troubleshooting Guide

If your heater displays any of the following error messages, the issue is most likely related to water flow, not the heater itself:

  • “LO” – Hayward
  • “Check Flow” – Jandy
  • “Water SW Open” – Raypak/Rheem

These errors occur because the heater is designed to not turn on when water flow is insufficient, resulting in cold pool water. Before calling a technician, check the following:

1. Pump Status

A pump with poor flow may not deliver enough water to the heater, triggering these error messages. Common causes include:

  • De-primed pump: Water should reach the top of the pump lid.
    Solution: Turn off the pump, open the lid, fill with water, close the lid, and restart.
  • Low pool water level: Keep water between ½ – ¾ of the skimmer opening. Low levels can introduce air, causing the pump (and heater) to stop. Refill and re-prime the pump.
  • Clogged skimmer or pump basket: Check and empty skimmer and pump baskets regularly to ensure proper flow.

2. Filter Status

Dirty or clogged filters can also reduce flow. Signs include high filter pressure and weak return flow.

  • Sand filter: Backwash to flush out debris. Replace sand every 5 years.
  • Cartridge filter: Remove cartridges to clean or replace.
    • Cleaning methods: Power wash (keep distance to avoid damage), use a pleat comb, or chemical-based filter cleaner.

3. Additional Considerations

  • These steps assume all equipment is serviceable and the heater’s pressure switch is functional.
  • Corroded electrical connections inside the heater can cause intermittent operation.
  • Always eliminate non-heater causes first before scheduling a service call.

Note: These errors do not indicate ignition or gas-related problems. Ignition failures involve the igniter, flame sensor, or other gas/heater components. This guide only covers the water-flow related errors listed above.

Summary

Most “LO / Check Flow / Water SW Open” messages point to water flow issues. Check:

  1. Pump priming and operation
  2. Pool water level
  3. Skimmer & pump baskets
  4. Filter condition

If all of the above are in order and the issue persists, it may be time to consult a professional.

Need help? Visit us at Vaughan Pool Supplies:
545 N. Rivermede Rd, Unit 103, Concord, ON
905-597-SWIM

Written by: Mark Leaman President, Vaughan Pool Supplies York Pool Services Inc. Etobicoke Pool Services Inc. Richmark Pool Services Inc.

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