Gas Pool Heater Won't Stay Lit: Short-Cycling Causes Explained

Your gas pool heater lights, runs a minute or two, then shuts back down. This guide walks through the causes of short-cycling in order of likelihood — from a dirty filter to low gas pressure — with the safe checks for each. Perfect Catch is the Pinellas County company licensed for the gas and electrical side of these repairs.

Short-cycling problems fall into two families. Water-side faults — low flow and high-limit trips — are where homeowners can actually help themselves, and they're the most common. Flame-side faults — ignition, flame sensing, gas pressure, venting — are licensed-gas territory, full stop.

1. Low Water Flow — the #1 Cause of Short Cycling

A dirty filter is the single most common reason a gas heater won't stay lit. Most heaters need roughly 25–30+ GPM moving through the exchanger; when a clogged filter, full baskets, or a low pump speed starves that flow, the heater's safeties shut the burner down to protect the exchanger — then it retries, lights, and trips again.

• Clean or backwash the filter, and empty the skimmer and pump baskets.
• Run the pump on high speed while the heater is heating (variable-speed schedules often drop below the heater's flow requirement).
• Verify the valves are actually routing water through the heater.

2. Dirty or Corroded Flame Sensor

The flame sensor's only job is to prove to the control board that a flame exists. When it's fouled or corroded — which happens fast in Pinellas County's salt air — the board loses proof of flame seconds after ignition and closes the gas valve. The heater lights, then drops out, over and over.

• Nothing to clean or adjust here — a power-cycle at the breaker, once, is the only safe move.

3. Low Gas Pressure

A heater on an undersized gas line, a low propane tank, or a weak utility supply gets enough gas to light but not enough to sustain the burner at full fire — it lights briefly and can't hold on. This is one of the most commonly missed causes because everything looks fine until the burner is under load.

• Confirm the gas supply valve to the heater is fully open.
• On propane: check the tank level — low tanks lose pressure well before they're empty.

4. Wind, Downdraft, or Rain Intrusion

Florida afternoon storms and gusty coastal wind can physically blow the flame off the burner tray or drive rain and sprinkler spray into the combustion area. If your heater only drops out on windy or wet days, this is a strong suspect.

• Redirect any sprinkler heads that spray toward the heater.
• Note whether shutdowns line up with wind or rain — that pattern is gold for the service visit.

5. High-Limit Trips from a Scaled Exchanger

When the heat exchanger scales up — common in hard Florida water — water moves through it too slowly and overheats, tripping the high-limit switch even though the filter is clean. The heater shuts down, cools, relights, and trips again in a slow cycle.

• Do the same flow checks as cause #1 first; if flow is good and it still trips hot, note it for the service call.

6. Failing Thermocouple (Millivolt / Pilot Models)

Older millivolt heaters with a standing pilot rely on a thermocouple generating roughly 600 mV to hold the gas valve open. As it ages, output sags — the pilot lights, the burner may even fire, and then the valve drops out. Classic symptom: the heater stays lit less and less time before quitting.

• No safe homeowner steps beyond one power-cycle — thermocouple testing and replacement live inside the gas train.

7. Venting or Air-Intake Restriction

A burner needs air as much as it needs gas. Leaves, nests, or debris around the top of the heater — or an obstructed vent on indoor installs — starve the flame until a safety takes the unit down. Bad combustion air is also how carbon monoxide events start.

• From the ground, look for obvious debris on or around the top of the heater; keep plantings trimmed back from the cabinet.

Why Homeowners Across Pinellas Call Us for Gas Heaters

• Gas ignition faults are licensed-gas work, and we hold the license. Flame sensing, gas pressure, burners, and venting carry explosion and carbon-monoxide risk — we service all of it under LI4527, plus the electrical side under EC13011994.
• We fix the cause, not the symptom. Half of "heater problems" in Pinellas are really flow problems — dirty filters, scaled exchangers, VS pump schedules — and we're pool-licensed (CPC1459998) to correct that side too.
• We never bypass a safety. High-limits, rollout switches, and pressure switches are the only things standing between a fussy heater and a fire or CO event — a heater that trips them repeatedly gets diagnosed, not jumpered.