Hayward Pool Heater LO Code - Low Water Flow Diagnosis and Fix

Error Code: LO Code - Low Water Flow

What it means: Your Hayward pool heater stopped firing because it detected low water flow. The heater uses a flow switch to confirm water is moving through the heat exchanger before it allows the burner to light. When that switch stays open, you get LO on the display. The heater is doing its job. It is protecting itself from overheating.

Common Causes

• Dirty filter creating backpressure and low flow
• Clogged pump basket or skimmer basket
• Pump lost prime from air leak on the suction side
• Flow switch paddle stuck or failed completely
• Internal bypass valve stuck open
• Heat exchanger scaling restricting water passage
• Pump impeller clogged with debris
• Closed or partially closed isolation valves

Step-by-Step Fix

Follow these steps in order. Each one rules out the most likely causes first.

1. Check the filter pressure gauge. A high reading above normal means your filter is dirty and needs backwashing or cleaning. A low reading means restricted suction somewhere before the pump. This single reading tells you which half of the system to look at.

2. Inspect the pump basket and skimmer baskets. Remove the pump lid and pull the basket. Remove debris from the skimmer baskets too. Reinstall the pump lid hand-tight with a clean O-ring. Any air leak here causes the pump to lose prime.

3. Verify the pump has prime. Look through the pump lid for a fully flooded volute with no air pocket. If you see air, check the pump lid O-ring and all suction-side fittings. A pump that runs dry cannot push water through the heater.

4. Check the filter pressure gauge again after cleaning. If the pressure drops back to normal range and the flow returns, you found the problem. The dirty filter was the bottleneck.

5. Examine the flow switch paddle. Remove the flow switch from the heater header. Look at the paddle. It should move freely through its full range of motion. If it sticks, replace the flow switch. For H-series heaters, the mechanical paddle must point upstream. For FD2 and ED2 heaters, the electronic flow switch has a smaller paddle that is more sensitive to scaling.

6. Test the flow switch with a multimeter. Set your meter to continuity mode. Disconnect the flow switch wires. With no water flow, you should read no continuity (open circuit). Manually lift the paddle and you should hear a click and read continuity. If the switch does not click or the meter shows no continuity when the paddle moves, replace the flow switch.

7. Check the internal bypass valve. The Hayward internal bypass diverts water around the heat exchanger during startup. If the bypass valve sticks open, water flows past the exchanger without carrying heat away. This keeps the switch cool and triggers LO. You need to remove the header to inspect the bypass spring and piston. A stuck bypass usually requires a rebuild kit.

8. Inspect the heat exchanger for scaling. Calcium buildup narrows the tubes and restricts flow. Remove the header and look inside the exchanger tubes. White crusty deposits mean scaling. A scaled exchanger needs chemical cleaning or replacement.

9. Clear the LO code. Once you restore proper flow, the LO code clears automatically. You do not need to press any buttons. The heater resumes normal operation once the flow switch closes.

Parts That May Need Replacement

Technician Tips

Spring startup causes most LO codes. Pool owners open the pool, turn on the pump, and fire up the heater without checking the filter. A filter that sat all winter with debris now restricts flow. Clean or backwash the filter first. You will save yourself the other eight steps more than half the time.

H-series heaters use a mechanical flow switch with a visible paddle. You can watch it through the clear PVC fitting. FD2 and ED2 heaters use an electronic flow switch with no visual indicator. You must test the FD2 switch with a multimeter, not with your eyes.

A dirty filter produces high pressure on the gauge. A clogged pump impeller produces low pressure. Low gauge pressure with a clean filter points to a suction-side problem, not the heater.

Need Replacement Parts?

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When to Call a Professional

Call a technician if you checked the filter, pump prime, and flow switch and still get LO. Persistent LO after clearing all blockages suggests a failed internal bypass or a scaled heat exchanger. Both require disassembling the heater header. A service call costs less than the damage from overheating the exchanger.

See Also

• Hayward Pool Heater IF Code — Ignition Failure Troubleshooting
• Pentair MasterTemp Heater Error Codes — Complete Guide
• How to Clean a Pool Heater Heat Exchanger
• Pool Heater Flow Switch Replacement Guide

Related Articles

• Hayward Pool Heater IF Code — Ignition Failure Troubleshooting
• Pentair MasterTemp Heater Error Codes — Complete Guide
• How to Clean a Pool Heater Heat Exchanger
• Pool Heater Flow Switch Replacement Guide