A pool heater is supposed to turn off. When the water reaches the temperature you set, it stops, and when the water cools, it starts again. That's normal. What isn't normal is a heater that lights, runs a few minutes, shuts off and lights again, long before the pool is warm. That's short cycling, and it is almost always the heater protecting itself from something.

This guide covers gas heaters, using Pentair's MasterTemp and Hayward's Universal H-Series manuals. For a heat pump, or a heater that won't come on at all, start with why your pool heater isn't heating.

Safety first

If you smell gas, leave the area and call your gas utility from a safe place, before anything else. Pentair's instructions are specific: don't try to light any appliance, don't touch any electrical switch, don't use a phone inside the building, and call the gas supplier from a neighbor's phone. If you can't reach them, call the fire department (MasterTemp manual, p. 42).

Everything in this guide is done from outside the heater cabinet. Inside are gas controls, line voltage and hot surfaces. Pentair says never to jumper a safety switch to get around a failed one (MasterTemp manual, p. 49), and some codes mean the heater should be shut down and checked by a technician, not reset again and again (MasterTemp manual, p. 45).

Why a heater shuts itself off

A gas heater only fires while a chain of safety switches agrees it's safe. On the MasterTemp, those include a pressure switch that confirms the pump is running, a high-limit switch that opens if water leaving the heat exchanger goes above 135°F, and an automatic gas shut-off that opens above 140°F (MasterTemp manual, p. 10). If any one opens, the heater stops. When it closes again, the heater tries again.

That's why the pattern matters. The MasterTemp's flow error (ERR PS) and high-limit error (ERR HLS) both clear themselves once the condition goes away (MasterTemp manual, p. 48), so a heater sitting right on the edge of one of them turns on and off all day. On a Hayward H-Series, the same kind of problem shows as LO (limit open), which Hayward says "may relate to water flow", and which is normal whenever the pump is off (Hayward H-Series guide, p. 48).

1. Not enough water flow (the most common cause)

A heater needs a minimum flow to keep its pressure switch closed and its heat exchanger cool.

Pentair MasterTemp: 20 gallons per minute for the 175 and 200, 25 for the 250, 30 for the 300 and 40 for the 400 (MasterTemp manual, p. 14).
Hayward H-Series: 20 to 40 gallons per minute depending on the model (Hayward H-Series guide, p. 61).

Hayward explains what happens below the minimum: the heater can dry fire, or the water can boil, which trips the high limits and can damage the heat exchanger (Hayward H-Series guide, p. 61). So low flow causes both kinds of shut-off, the flow switch and the high limit.

What you can check:

The baskets. Empty the skimmer and pump baskets.
The filter. If the pressure gauge is reading higher than usual, clean it. Pentair's first step for a flow error is to service the pump and filter (MasterTemp manual, p. 53). See filter pressure too high.
The pump speed. This one is easy to miss on a variable-speed pump. A slow, quiet program can run all day without moving enough water to keep the heater's switch closed. When the heater is meant to be heating, the pump needs to be on a speed that meets the heater's minimum. If your pump shows its own codes, see the Pentair IntelliFlo and Hayward TriStar VS guides.
The valves. Make sure water is actually going through the heater, not around it.

If the pump itself isn't moving much water, see pump running but no water flow.

2. The high limit is tripping

If flow is good and the heater still lights and then stops, the high limit or the gas shut-off may be opening because the water inside the heater is getting too hot. Pentair's diagnostic chart for ERR HLS and ERR AGS starts with flow, then asks you to confirm the water coming in is below 104°F. If the problem persists after that, it points at the heater's internal bypass valve and thermal governor, and at a blockage in the heat exchanger (MasterTemp manual, p. 53). On a Hayward, the LO troubleshooting goes on from the pressure switch to testing the high-limit switches themselves (Hayward H-Series guide, p. 49).

Pentair says not to operate a MasterTemp showing ERR HLS or ERR AGS, but to shut it down and have it checked (MasterTemp manual, p. 45). This is not a code to keep resetting.

3. The internal bypass or thermal regulator

Gas heaters like these don't send all the pool water through the heat exchanger. On the MasterTemp, a thermal regulator and an internal bypass control how much goes through, to keep the outlet at the right temperature (MasterTemp manual, p. 10). When those parts stick, the heater misbehaves:

Pentair lists a bypass valve stuck open and a thermal governor stuck closed as causes of boiling in the heat exchanger, which can come with "bumping" sounds (MasterTemp manual, p. 55).
Hayward says the internal bypass should be inspected periodically, because it can cause either low or high flow through the heat exchanger (Hayward H-Series guide, p. 61).

These parts are inside the heater. A heater that bumps, boils or short-cycles with good flow needs a diagnosis, not more resets.

4. Too much flow

It sounds backwards, but too much flow is a problem too. The MasterTemp's maximum is 120 gallons per minute, and Pentair notes that pumps of 2 horsepower or more can exceed it, so a manual bypass should be installed and set to bring the flow into range (MasterTemp manual, p. 14). Hayward gives a maximum of 125 gallons per minute and says flow above it can damage the heat exchanger by thinning the tube walls (Hayward H-Series guide, p. 61). If a big pump was installed, or a variable-speed pump is running flat out while the heater fires, ask whether the plumbing has a bypass.

5. It isn't the heater at all

A few on-and-off patterns come from outside the heater:

The pump or automation turns off. If the pump's schedule ends, or an automation program changes its speed, the heater loses flow and stops. Check what the pump is doing at the moment the heater stops.
A temperature lock-out. Hayward H-Series heaters have a maximum temperature lock-out, set by default to 90°F for the pool and 104°F for the spa (Hayward H-Series guide, p. 14). If your setpoint is above the lock-out, the heater stops at the lock-out.
Failed lights. A heater that can't light keeps trying. A Hayward H-Series tries three times, shows IF, waits an hour and tries again (Hayward H-Series guide, p. 8). That looks like on and off, but it's a different fault. See the Hayward heater code guide and the Pentair MasterTemp code guide.

Why it matters

Short cycling isn't just annoying. The conditions that cause it, low flow, overheating and too much flow, are the same ones Hayward lists among the causes of heat exchanger damage (Hayward H-Series guide, pp. 61 and 63), and the heat exchanger is the most expensive part of the heater. Finding the cause early is the cheap way out.

When to call

Call when the heater keeps cycling after you've cleaned the filter and baskets and confirmed the pump is on a speed that meets the heater's minimum, when it shows a code that Pentair or Hayward says to have checked, or when it bumps or boils. Our heater diagnostic checks the flow, the pressure switch and limits, and the bypass and regulator, reads the code against the manufacturer's manual, and tells you what has failed and what putting it right involves before anything is quoted.

If you don't know what's wrong, a diagnostic is $120, and it's credited toward your repair if you go ahead. Start from pool heater repair, or answer a few questions in the Pool Problem Finder. A note of the code and how long the heater runs before it stops helps.

Sources

Every manufacturer statement above links to this list. Page numbers are the printed page numbers in each document.

Pentair, MasterTemp Pool and Spa Heater Installation and User's Guide, P/N 472592 Rev. Q (7/30/23).
Hayward Industries, Universal H Series Heaters Troubleshooting Guide, TSG-UHS16b (2017).

Pentair and MasterTemp are trademarks of Pentair. Hayward and Universal H-Series are trademarks of Hayward Industries. They are named here only to identify equipment.

Why your pool heater keeps turning on and off