HLS
Pentair Pool Heater
Severity: CriticalWhat Does This Error Mean?
Pentair MasterTemp HLS means High Limit Switch.
Two switches sit on the heat exchanger — they open if water in the tubes exceeds about 135°F.
If they open, the heater stops firing and posts HLS.
This is a real protection event, not a nuisance code.
The most common causes are a clogged filter, a partially closed valve, or a heat exchanger fouled with scale.
Affected Models
- Pentair MasterTemp 200
- Pentair MasterTemp 250
- Pentair MasterTemp 300
- Pentair MasterTemp 400
Common Causes
- Pool filter dirty and restricting flow
- Valve at the heater inlet partially closed
- Pump cavitating or air-locked
- Heat exchanger scaled — a hard-water issue over years
- High limit switch itself failed open after age
- Thermostat set too high combined with low flow
How to Fix It
-
Stop the heater and check the filter.
Switch the heater off but leave the pump running.
Check filter pressure on the gauge.
If it reads more than 8-10 PSI above clean baseline, backwash a sand filter or hose off a cartridge filter.
Restricted filters are the leading cause of HLS. -
Verify all valves are open.
Walk the equipment pad.
Every valve in the loop — pump suction, filter inlet, heater inlet, heater outlet, return — should be in the fully open position.
A valve someone closed for service can absolutely cause HLS. -
Confirm flow at the return.
Look at a return jet in the pool.
Strong steady flow = good.
Weak or pulsing = poor flow, which is what triggers HLS.
Check skimmer baskets and pump basket for leaves choking the suction side. -
Let the heater cool fully.
Some HLS switches reset automatically once cool.
Others have a manual reset button (a small button on the switch body).
Press it firmly with the heater off.
You should feel a small click. -
Inspect the heat exchanger for scale.
Open the unit and look at the copper heat exchanger tubes.
White or grey crusty buildup = scale.
An acid wash by a pool tech (200-400 USD) restores flow through the exchanger and clears recurring HLS in older heaters. -
Replace the high limit switch.
If filter, valves, and flow are all good and the switch will not reset, the switch itself has failed.
Genuine Pentair HLS pairs are 50-120 USD.
Two screws and two wire terminals on each switch. -
Lower the thermostat.
Once back online, set the heater no higher than 88°F for typical pool use.
Setting at 95°F+ pushes the unit closer to the high limit threshold and increases the chance of recurring HLS during sunny afternoons.
Frequently Asked Questions
Is HLS dangerous?
The trip itself is the safety system working as designed.
The dangerous case is a heater that keeps overheating without a working high limit — that is how copper exchangers blow out.
Fix the cause, do not bypass the switch.