Raypak HL1: High Limit Switch 1 Tripped

HL1 means the primary high-limit switch on your Raypak heater opened because water at the heat exchanger got hotter than it should.

Why does a Raypak heater shut itself down and flash HL1 right after it seemed to be running fine? The high-limit switch isn't a random glitch — it's a physical safety thermostat clamped to the heat exchanger, and it only opens when the water passing through actually reaches an unsafe temperature, typically somewhere around 135°F. That almost always traces back to water sitting in the exchanger too long rather than moving through it, which is why flow is the first thing worth checking, not the switch itself.

Applies to: Raypak Digital ASME and low-profile residential gas pool/spa heaters

What it means

High limit 1 tripped — the primary high-limit switch opened because water temperature at the heat exchanger exceeded its safe threshold (roughly 135°F). Raypak heaters run two separate high-limit switches for redundancy, and HL1 is the first line of defense; it's designed to trip well before things get dangerous, so a single trip on its own isn't an emergency, but it is the heater telling you something upstream of the burner needs attention.

Likely causes

• Restricted or inadequate water flow through the heater
• Scale buildup inside the heat exchanger
• A failed or miscalibrated high-limit switch

What you can check yourself

1. Verify and restore normal flow (clean filter, open valves, adequate pump speed)
2. Let the heater cool, then clear the fault and retry

Sources

• Raypak 206A Installation & Operating Instructions Manual, Diagnostics section (ManualsLib)

Related

• All Raypak error codes
• Raypak HL2 — the backup high-limit switch, which often follows an unresolved HL1
• Raypak PRS — the low-flow fault that frequently sets HL1 off in the first place
• Pentair ERR HLS — Pentair's equivalent high-limit safety trip
• Flow / Low-Flow Error — the shared symptom behind most high-limit trips