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ErrBase

Hayward Pool Heater LO Error Code: Low Water Flow

Quick Answer

Hayward LO means the flow switch did not close because water flow through the heater is below the 20 GPM minimum. A pump not running at heating speed is the most common cause for variable-speed pump owners. A clogged filter is the next most common.

Hayward LO means the flow switch did not close because water flow through the heater is below the 20 GPM minimum. A pump not running at heating speed is the most common cause for variable-speed pump owners. A clogged filter is the next most common. Before replacing the flow switch, always confirm water flow is actually adequate - the flow switch is the messenger, not the cause.

HaywardPoolheaterSeverity: high85% DIY Success
Time to Fix
15–60 min
Difficulty
beginner
Parts Cost
$20 – $50
Tools Needed
Multimeter, Pool pressure gauge
Wrong code?

Not seeing LO on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

low water flow pressure switch fault
PentairE05

What Does the LO Code Mean?

LO means low water flow through the Hayward heater - check pump speed, filter pressure, and bypass valve position before replacing any parts.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Variable-speed pump running at too low an RPM for heating mode40%
Clogged pool filter creating high resistance24%
Heater bypass valve open or partially open14%
Flow switch failure12%
Pump basket or strainer full of debris10%

Symptoms You May Notice

  • LO displayed on panel
  • Heater will not fire at all - shuts down immediately
  • LO appears within seconds of a heat attempt
  • Heater worked yesterday but shows LO after filter cleaning

Can you reset a Hayward poolheater to clear the LO code?

After restoring adequate water flow, press Mode to enter standby, wait 30 seconds, and restart the heater. The LO code clears automatically once the flow switch closes during the heat attempt. If LO returns, check that the pump is maintaining speed throughout the heat cycle, not just at startup.

Tools Required for Diagnosis

MultimeterPool pressure gauge

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

1

Confirm the pool pump is running and visually check that water

Confirm the pool pump is running and visually check that water is flowing at the returns in the pool. If using a variable-speed pump, verify the current RPM on the pump display.

2

Read the filter pressure gauge

3

Locate the heater bypass valve and confirm it is fully in the

4

Inspect the pump basket and any inline strainer baskets for debris

5

With confirmed adequate flow

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name
Flow switchCHXFLO002 · $20–$50

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs
Labor Approach
Our Verdict
REPAIR
Estimated Cost: $35 (Parts) vs $3000 (New Machine)

Frequently Asked Questions

What does LO mean on a Hayward pool heater?
LO means the heater detected low or no water flow through the heat exchanger. The flow switch remained open because flow was below the 20 GPM minimum threshold required for safe heater operation.
What is the minimum flow rate for a Hayward H-Series pool heater?
Hayward H-Series pool heaters require a minimum of 20 GPM flowing through the heater. The H400FDN at 400,000 BTU may need slightly more at peak load. Variable-speed pump owners must ensure the pump schedule includes a high-speed heating program.
Why does my Hayward heater show LO only when the filter is dirty?
A dirty filter significantly increases system resistance, which reduces the flow rate the pump can deliver. Even with the pump at full speed, a severely clogged filter can drop flow below 20 GPM at the heater, triggering LO. Clean the filter when pressure rises 10 PSI above the clean reading.
Can a bad flow switch cause LO on a Hayward heater?
Yes, but it is the least common cause. Always confirm actual water flow is adequate before replacing the switch. A flow switch that has failed open will cause LO regardless of actual flow. Test with a multimeter - if the switch stays open with confirmed adequate pump flow, it has failed.
Will a LO code damage my Hayward pool heater?
The LO lockout prevents damage - that is its purpose. If the heater were to fire without adequate water flow, the heat exchanger would overheat and potentially crack. As long as the heater stays locked out during the LO condition, no damage is occurring.

Related Hayward Poolheater Error Codes

Code: HUBHayward Pool Heater Error Codes GuideCode: RESETHow to Reset a Hayward Pool Heater

Same Fix on Other Brands

Pentair poolheaterE05: Pentair Pool Heater E05 Error Code

Models Known to Experience LO Errors

This repair applies to most Hayward poolheaters with this error code. Common model numbers include:

H400FDN, H250FDN, H150FDN, H400FDP

Last verified for technical accuracy on March 14, 2026