Pool Error Codes

Hayward HS: Water Temperature Sensing Error

HS shows up when the sensor reading the water coming into a Hayward heater reports a temperature that's either genuinely too hot or too erratic to trust.

Medium severity

Applies to: Hayward Universal H-Series and H-Series residential gas pool/spa heaters

What it means

Hayward's own diagnostic documentation defines HS as triggering on "maximum return water temperature exceeded and/or rapid water temperature rise" — meaning the control board is watching for two different things at once: an absolute ceiling around 104°F at the inlet sensor, and a rate-of-change check that catches water heating up unusually fast even before it crosses that ceiling. That second half of the check exists because a rapid spike is often a sign of a real flow problem rather than just a hot spa on a summer afternoon — if water isn't moving through the heat exchanger properly, the small volume that is present heats up quickly, and HS trips as an early warning before a high-limit switch has to intervene.

This shows up more often on spa-only sessions than full-pool heating, and there's a straightforward reason why: a spa holds a comparatively small, contained body of water, so when the heater and pump are both running to hold a set temperature, that water can genuinely climb fast, especially if the spa is running on a lower jet/circulation speed that isn't pulling a full volume of water past the inlet sensor. A large in-ground pool rarely produces that kind of rapid rise because there's simply too much water for a working heater to overheat quickly, which is one reason HS turns up more on multi-body systems with a spa than on straightforward single-pool setups.

Likely causes

What you can check yourself

  1. Check actual water temperature at the equipment pad; let the water cool and circulate if it's unusually hot
  2. Verify normal flow (clean filter, open valves, pump running at adequate speed)
  3. Power-cycle the heater once flow and temperature look normal
Call a professional if…

If water temperature and flow are both normal but HS keeps returning, the sensor itself has likely failed and should be tested/replaced by a pool professional.

Sources

Related

This site is an independent reference, not a substitute for a licensed pool professional. Gas pool heaters can leak carbon monoxide and cause fire; pump and heater electrical circuits carry mains voltage. Anything involving the gas line, burner assembly, or house wiring should be handled by a licensed technician or electrician.