Hayward Pool Heater Not Heating: LO, HI, IF, and Bypass Valve Fixes

Hayward pool heaters display error codes on the front panel. LO = low pressure switch open (not enough water flow through the heater). This is the #1 Hayward complaint and is usually caused by a dirty pool filter (clean or backwash your filter), a closed or partially closed bypass valve, or a failing pump. The bypass valve on Hayward Universal H-Series heaters controls how much pool water flows through the heater vs bypasses it - if set too far toward bypass, not enough water flows through the heater and LO triggers. HI = high limit switch tripped (heater overheating from low water flow or thermostat failure). IF = ignition failure (check gas supply, pilot assembly, and flame sensor). Hayward heaters have a minimum flow rate requirement (printed on the nameplate) - if your pool pump provides less GPM than required, the heater cannot operate.