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Heaters

Gas Pool Heater Won't Heat or Keeps Cycling: What's Safe to Check

The owner-level checks Hayward and Pentair list (flow, filter, valves, settings), what the heater's safety switches do, and the point where you stop and call a pro.

As an Amazon Associate I earn from qualifying purchases. The "Search Amazon" buttons and the "Worth a look" links on this page are Amazon affiliate links; we show no prices or ratings. Facts recorded Oct 25, 2026 from the official sources linked on each section. Manufacturer pages and rules change, so check the current version and your model's manual.

Difficulty: owner-level checks only; everything gas-related is for a licensed pro

Safety first: gas and carbon monoxide

  • If you smell gas, Pentair's MasterTemp guide says: don't try to light any appliance, don't touch electrical switches or use a phone in your building, call your gas supplier from a neighbor's phone, and call the fire department if you can't reach the supplier.
  • Carbon monoxide symptoms (dizziness, headache, nausea and others the guide lists): turn off the heater, leave the area, get into fresh air immediately, and have a gas professional test the heater before it runs again.
  • Never bypass a safety switch. Pentair: "Do not bypass the Water Pressure Switch or render it inoperable," and do not operate the heater when the SERVICE HEATER light is on or if the blower or burner won't start.
  • Pro work: Pentair says the heater must be installed and serviced by a technician qualified in pool heater installation.

Source: Pentair MasterTemp Installation and User's Guide (Rev. M, 1-8-15)

The question

"Why won't my heater heat, or why does it cycle on and off, and what must a pro handle?"

Why heaters cycle on and off (Hayward)

  • Inadequate water flow from a dirty filter or closed valves
  • External bypass needs adjusting
  • Reversed water connections
  • Water pressure switch out of adjustment
  • Possibly the thermostat

Why a heater doesn't reach temperature (Hayward)

  • Thermostat set too low
  • Inadequate gas supply
  • Malfunctioning thermostat
  • Losing heat faster than it's added (the heater may be too small); see Pool heater sizing

For a leaking heat exchanger, Hayward lists chemical imbalance, sanitizer damage, winter freeze damage and failed joints, and says "a Hayward service company must be contacted."

Source: Hayward Pool Heaters FAQ

Owner-level checks, then stop

  1. Is the pump running and primed? Pentair's troubleshooting starts by making sure the pump is running correctly. If it isn't, see Pool pump won't prime.
  2. Clean the filter, skimmer and pump baskets. On a MasterTemp, the SERVICE SYSTEM light means not enough water flow, which Pentair says "usually indicates that the filter and/or skimmers should be cleaned (some filters may require back-washing)." If the light stays on after that, have a qualified service technician check the system.
  3. Valves and time clock. Pentair says to check that valves are correctly set and the time clock is set and running.
  4. Settings. Check the thermostat setting and pool vs. spa mode. A MasterTemp ships set to 78 °F pool and 100 °F spa.
  5. Pool cleaner and pump speed. Pentair says not to operate the heater while an automatic pool cleaner is running, and that a two-speed pump on low speed may not develop enough pressure to run the heater; run it on high, and if that doesn't solve it, don't run the heater and have the installation corrected.
  6. SERVICE HEATER light, or the blower or burner won't start: stop. Pentair says to shut the heater down (turn off gas per the manual) and call a qualified service technician.

Source: Pentair MasterTemp guide, Operation and Troubleshooting

What the safety switches do (so you know not to touch them)

Pentair's MasterTemp guide describes interlocks that must all be closed before the heater fires: a water pressure switch that senses the pump is running; a high limit switch that opens above 135 °F at the heat exchanger outlet; an air flow switch for the combustion blower; an automatic gas shut-off above 140 °F; and a stack flue sensor that shuts the heater down at a 480 °F flue temperature. Pentair warns that if suction is plugged (for example by leaves) there may not be adequate flow, and not to rely on the pressure switch in that case. Pressure-switch adjustments are for a qualified technician.

Source: Pentair MasterTemp guide, Safety Controls

Where this page stops

No instructions for gas valves, burners, igniters, venting, gas lines, pressure-switch adjustment or heat exchangers. Those belong to a licensed gas or pool-heater professional. Pentair's guide is a 2015 revision for MasterTemp heaters; other heaters have their own manuals.

Should you DIY? / When to call a pro

Fine to DIY: confirming the pump runs and is primed, cleaning the filter and baskets, checking valve positions, time clock and thermostat settings.

Call a licensed gas / pool-heater professional: no ignition, SERVICE HEATER faults, gas supply, burners, igniters, venting, pressure switch, bypass adjustment, leaks from the heat exchanger. Call 911 or your gas supplier if you smell gas, per your heater manual.

See our full When to Call a Pro page. Follow your model's manual; this site isn't a substitute for it, and it isn't legal advice.

If your model isn't listed

The numbers on this page come from specific manufacturer documents and models. If your equipment isn't one of them, don't assume the same numbers apply: find the model number on the rating plate, open that model's manual on the manufacturer's site, or contact the manufacturer. You can also email us with the brand and model number.

Facts from the manufacturer (and where else they came from)

Where to look

Heater links are in the “Worth a look” box above. These buttons cover safety and maintenance items. As an Amazon Associate I earn from qualifying purchases.

  • Carbon monoxide detector: Pentair recommends CO detectors near a gas pool heater; follow the detector's placement instructionsSearch Amazon
  • Cartridge filter cleaning wand: for cartridge filters onlySearch Amazon