Hayward H-Series LO Error Code: Water Pressure Switch Fault Troubleshooting

Understanding the LO Code

The LO error code on your Hayward Universal H-Series heater indicates the water pressure switch is open, telling the control board there's insufficient water flow through the heat exchanger. This is a critical safety feature that prevents the heater from firing without adequate water flow, which could cause severe damage.

The pressure switch is a simple device that closes when it detects adequate water pressure (typically 1-3 PSI) in the heater header. When water flow drops below the minimum threshold, the switch opens and triggers the LO code.

How the Pressure Switch System Works

The water pressure switch in the H-Series heater consists of:

• Diaphragm assembly: Responds to water pressure in the header
• Microswitch: Sends signal to control board
• Spring mechanism: Sets the activation pressure
• Adjustment screw: Some models allow minor adjustment (not recommended)

When the pump runs, water pressure pushes against the diaphragm, closing the switch contacts. The control board continuously monitors this switch - if it opens during operation, the heater immediately shuts down and displays LO.

Common Causes of LO Codes

Flow-Related Issues (80% of cases)

• Pump not running: Timer off, breaker tripped, or pump failure
• Dirty filter: Restricted flow through clogged filter element
• Closed valves: Bypass or isolation valves not fully open
• Air in system: Lost prime or air pockets reducing flow
• Pump running on low speed: Variable speed pumps set too low

System Configuration Problems

• External bypass open: Too much water bypassing the heater
• Multiple heaters: Insufficient flow when split between units
• Solar system interference: Diverter valves routing flow away
• Undersized pump: Pump can't deliver minimum required flow

Pressure Switch Issues

• Stuck switch contacts: Corrosion preventing closure
• Failed diaphragm: Internal damage to pressure sensing mechanism
• Loose wiring: Poor connection to control board
• Adjustment drift: Switch out of calibration (rare)

Step-by-Step Troubleshooting

Basic Checks (Homeowner Level)

1. Verify Pump Operation

• Confirm pump is running and water is moving
• Check pump basket for debris or blockage
• Look for signs of cavitation (air bubbles, unusual noise)
• For variable speed pumps, ensure adequate RPM setting

2. Check Filter System

• Note filter pressure gauge reading
• Clean or backwash filter if pressure is high
• For cartridge filters, remove and inspect elements
• Look for damaged filter grids or cartridges

3. Inspect Valve Positions

• Ensure all valves in heater plumbing are fully open
• Check heater bypass valve is in correct position
• Verify multi-port valve is not in closed or winterize position

4. Test Flow at Returns

• Place hand over return jets to feel flow strength
• Compare current flow to normal operation
• Look for weak or no flow from certain returns

Intermediate Troubleshooting

5. Check for Air in System

• Look for air bubbles in pump basket
• Check for suction side leaks (pump lid, valves, fittings)
• Verify water level is adequate (midway on skimmer)
• Bleed air from filter if equipped with air relief

6. Measure Actual Flow Rate

• Use flow meter if installed in system
• Calculate flow using filter pressure rise method
• Minimum flow requirements:
  • H150: 20 GPM minimum
  • H200: 20 GPM minimum
  • H250: 25 GPM minimum
  • H300: 30 GPM minimum
  • H350: 30 GPM minimum
  • H400: 40 GPM minimum

7. Bypass Test

• If equipped with manual bypass, adjust to increase flow through heater
• Close bypass completely to route all flow through heater
• If LO clears, flow rate is borderline

Advanced Troubleshooting (Technician Level)

8. Test Pressure Switch Electrically

• Turn off power and access control board
• Locate pressure switch wires (usually violet/gray)
• With pump running, test continuity across switch
• Switch should show continuity (closed) with good flow
• No continuity with pump running indicates switch or flow problem

9. Manually Test Switch Operation

• With power off, disconnect switch wires
• Use jumper wire to bypass switch temporarily
• Power on heater - if LO clears, switch is suspect
• Remove jumper immediately after test - never operate heater with switch bypassed

10. Inspect Switch Mechanism

• Remove pressure switch from header
• Check for debris in switch port
• Test switch manually by pressing diaphragm
• Listen for clear clicking sound when activated
• Check for corrosion on electrical contacts

11. Measure Header Pressure

• Install pressure gauge in drain port
• Run pump and note pressure reading
• Should see 5-30 PSI depending on system
• Less than 5 PSI indicates flow problem

Flow Rate Calculations

Using Filter Pressure Method

1. Note clean filter starting pressure
2. Run system for exactly 60 seconds
3. Note pressure rise
4. Use manufacturer's flow chart to determine GPM

Using Return Flow Estimation

• Count return fittings and estimate flow per fitting
• Standard 1.5" eyeball: 10-15 GPM
• Standard 2" return: 20-30 GPM
• This gives rough estimate only

Solutions by Cause

For Flow Problems

• Dirty filter: Clean or replace filter media
• Closed valves: Open all valves fully
• Low pump speed: Increase variable speed pump RPM
• Air in system: Find and seal suction leaks
• Undersized pump: Upgrade to larger pump

For Switch Problems

• Failed switch: Replace with OEM part (see parts list below)
• Loose wiring: Secure connections at switch and board
• Corroded contacts: Clean or replace switch
• Clogged switch port: Clean debris from header opening

For System Configuration

• Bypass issues: Adjust bypass valve for more flow through heater
• Multiple heaters: Install separate pump or increase pump size
• Long pipe runs: Increase pipe diameter or pump size

Testing After Repairs

1. Start pump and verify strong flow at returns
2. Clear error code from heater display
3. Set heater to call for heat
4. Monitor for at least 10 minutes of continuous operation
5. Check that heater cycles normally without LO code
6. Test with filter at various stages of cleanliness

Replacement Parts

Common parts for LO code repairs:

• Water Pressure Switch: CZXPRS1105 (most models)
• Pressure Switch Cable: IDXLWPS1930
• Header with Tapped Port: Model-specific part number
• Control Board: IDXL2CB1931 (if switch tests good)

Prevention Strategies

• Maintain proper filter cleaning schedule
• Keep pump basket clear of debris
• Program variable speed pumps for adequate heater flow
• Service pump regularly (seals, impeller, motor)
• Install flow meter to monitor system performance
• Check and adjust valves seasonally
• Maintain proper water chemistry to prevent scale buildup

Special Situations

Variable Speed Pump Integration

When using variable speed pumps:

• Program minimum speed of 2400 RPM for heater operation
• Use pump's heater mode if available
• Consider relay control to boost speed when heater calls
• Verify flow at lowest operational speed

Salt Cell Installations

With salt chlorinators:

• Install heater before salt cell in flow path
• Account for flow restriction from cell
• Check for scale buildup in cell affecting flow

Solar Heating Systems

Combined solar/gas heating:

• Verify adequate flow when solar is active
• Check diverter valve operation
• May need larger pump for combined system

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