Low Flow Light On

Why Your System Shows “Low Flow” and How to Fix It

A Low Flow warning is one of the most common pool equipment alerts.

Your heater, salt system, or automation panel uses a flow sensor to ensure enough water is moving through the system. When flow drops too low, equipment will shut off to protect itself.

This guide explains the most common causes and what you can check before requesting service.

⭐ 1. Dirty or Clogged Filter (Most Common)

When the filter becomes clogged, water cannot pass through at the required rate.

Signs:

✔ Pressure gauge higher than normal

✔ Weak return jets

✔ Heater won’t ignite or shuts off

✔ Salt system displays “Low Flow”

Fix:

• Clean the cartridge
• Backwash sand/DE filters
• Confirm pressure returns to normal

A dirty filter accounts for over half of Low Flow alerts.

⭐ 2. Pump Running Too Slowly (Variable Speed Pumps)

Salt systems and heaters require a minimum RPM to operate properly.

Fix:

• Increase pump speed to 2,500–3,000+ RPM
• Use “Boost” or “Quick Clean” mode
• Ensure automation isn’t setting a lower speed

Spa heating generally requires 3,000–3,450 RPM.

⭐ 3. Pump Basket or Skimmer Basket Full

Debris restricts suction and triggers Low Flow.

Check:

✔ Skimmer baskets packed with leaves

✔ Pump basket full

✔ Skimmer socks clogged

Fix:

• Clean all baskets
• Rinse pump basket thoroughly
• Monitor baskets after storms or heavy leaf drop

⭐ 4. Air Leak on Suction Side

Air entering the system reduces water flow and can trip the Low Flow sensor.

Look For:

✔ Air in pump lid

✔ Pump struggling to fill

✔ Bubbles in return jets

✔ Water level low

Fix:

• Raise water level to mid-skimmer
• Clean & lubricate pump O-ring
• Tighten unions
• Check vacuum hose for leaks
• Ensure suction valves fully open

⭐ 5. Flow Switch or Flow Sensor Issue (Salt Systems)

Salt system flow switches sometimes fail or get stuck.

Signs:

✔ “Check Flow” even with strong jets

✔ Salt cell clean but still no production

✔ Heater running normally

Fix:

• Clean debris around the flow switch
• Ensure correct orientation of switch arrow
• Replace switch if faulty

Hayward flow switches are a common replacement item.

⭐ 6. Clogged Impeller (Internal Blockage)

Debris can pass through a damaged basket and lodge in the impeller.

Signs:

✔ Pump gets louder

✔ Flow fluctuates

✔ Baskets clean but flow still weak

Fix:

Requires opening the pump and cleaning the impeller.

⭐ 7. Valve Positioned Incorrectly

If a suction or return valve is partially closed, flow drops quickly.

Check:

✔ Vacuum line left open

✔ Suction valve between Pool/Spa not aligned

✔ Return valve partially closed

✔ Solar/heater bypass valve in wrong position

Fix:

Set valves to fully open or correct configuration.

⭐ 8. Heater Bypass or Check Valve Problem

If bypass valves fail, water may not flow through the heater efficiently.

Signs:

✔ Heater won’t ignite

✔ Heater cycles on and off

✔ Low Flow light only appears when heater is on

Fix:

Inspection needed. May require bypass or check valve repair.

⭐ 9. Air Leak at Salt Cell Union or Filter Tank O-Ring

An unnoticed air leak reduces flow and causes intermittent Low Flow warnings.

Fix:

• Tighten salt cell unions
• Inspect filter tank O-ring
• Ensure air relief valve is sealed

⭐ 10. Water Level Too Low

Low water level causes the skimmer to draw air, which reduces flow.

Fix:

• Maintain water level at halfway up the skimmer opening

⭐ When to Contact Professional Pool Company

Call us if:

✔ Low Flow persists after cleaning filter

✔ Pump speed increases don’t help

✔ Heater shuts off due to flow

✔ Flow returns temporarily then drops again

✔ Air constantly enters the pump

✔ You suspect an underground suction leak

✔ Impeller blockage is likely

We can diagnose:

• Suction leaks
• Damaged impellers
• Faulty flow switches
• Plumbing issues
• Internal heater bypass failures

⭐ Summary

A “Low Flow” alert is usually caused by:

• Dirty filter
• Low pump RPM
• Air leaks
• Clogged baskets
• Impeller blockage
• Valve misalignment
• Flow switch failure
• Low water level

Most issues can be corrected with a few basic checks.

⭐ Need Help?

If your Low Flow light is still on after trying these steps, contact Professional Pool Company. We can restore proper circulation and diagnose any equipment issues.

Updated on: 29/11/2025