A gas pool heater that lights and then shuts off is usually a safety check doing its job. How long it runs is your best clue: a few seconds points at the flame, and a few minutes points at water flow or overheating. Here's how to read it, using Hayward's and Pentair's own manuals.
Redzone Pools repairs Hayward, Pentair and Jandy gas heaters across Cypress, Tomball, Katy, Spring and Greater Houston — licensed, insured, usually same day.
Pool heater short cycling is when a heater lights, runs briefly and shuts off, then tries again, instead of running until the water reaches the temperature you set.
Not every stop is a fault, so rule out the normal ones first:
If the water is below your set temperature, the pump is running and the heater still drops out, something is tripping it. On a MasterTemp, if any safety interlock opens while the burner is on, the burner shuts off immediately. A Hayward H-Series constantly monitors its blower vacuum switch, control loop, temperature sensor and flame sensor during a call for heat.
| What you see | Where to look first | Codes you may see |
|---|---|---|
| Lights, then goes out within seconds | The flame: gas supply, igniter or flame sensing | Hayward IF, Pentair ERR IGN, Jandy ignition lockout |
| Runs a few minutes, stops, then comes back | Water flow first, then overheating | Hayward LO or HS; Pentair ERR PS, HLS, AGS or SFS; Jandy CHECK FLOW or FAULT-HIGH LIMIT |
| Bumping or boiling sounds from the heater | Low flow, a blocked heat exchanger or a stuck bypass valve | Any high limit code |
| It started right after a new install or re-pipe | Reversed water connections | Hayward HS; Pentair says it will cycle continuously |
| Stops at about the same time each day | The pump schedule | Hayward LO |
| Stops at set temperature, fan runs on for under a minute | Nothing: that's normal | None |
Use this as a guide, not a rule. If the display shows a code, the code beats the timing: write it down before you cycle power, then look it up in our Hayward H-Series, Pentair MasterTemp or Jandy JXi and LXi guide.
A gas heater needs a minimum amount of water moving through it. Hayward warns that less than the minimum can cause dry firing, or water boiling, which trips the high limits and can damage the heat exchanger. These are the minimums the manufacturers list:
| Heater | Minimum flow |
|---|---|
| Hayward H150FD, H200FD | 20 gallons a minute |
| Hayward H250FD, H300FD | 25 gallons a minute |
| Hayward H350FD, H400FD | 30 gallons a minute |
| Pentair MasterTemp 175, 200 | 20 gallons a minute |
| Pentair MasterTemp 250 | 25 gallons a minute |
| Pentair MasterTemp 300 | 30 gallons a minute |
| Pentair MasterTemp 400 | 40 gallons a minute |
| Jandy JXi, LXi | 30 gallons a minute |
Hayward's figures are for heaters made after February 25, 2011, and Pentair's are from its 2015 guide, so check the manual for your own model.
You can check most of this yourself: make sure the pump is running, clean the filter, empty the skimmer and pump baskets, and confirm the valves are open. Pentair says that when the pump is running, ERR PS usually means the filter and/or skimmers need cleaning. If the filter gauge stays high after cleaning, see our guide to filter pressure that stays high after backwashing.
Variable-speed pumps are easy to miss. A low speed that's fine for filtering may not move enough water for the heater. Jandy lists a variable-speed pump set too low as a cause of CHECK FLOW, and on a Hayward the same thing shows up as LO with the pump visibly running. Raise the speed while the heater is on and see if it stays lit.
The pressure switch can be the problem too. It should be closed with the pump running and open with the pump off. Pentair's deck-level factory setting is 3.00 psi, and the switch has to be reset if it sits a foot above or below the pool's water level. Hayward says certain applications need it adjusted as well. That's a technician's job, because Pentair warns that a wrong setting can let the heater run with no water flow, which can severely damage it.
If flow looks fine and the heater still drops out after a few minutes, the next suspect is heat that isn't leaving the heat exchanger.
Liming is where Houston's water comes in. Pentair's remedy for a plugged heat exchanger includes correcting the water chemistry, and Hayward lists high pH, alkalinity or calcium hardness among its heat exchanger failure modes. Our guide to what hard water does to pool equipment explains why it builds up here.
Pentair gives one simple check: after start-up, the outlet pipe should feel slightly warmer than the inlet pipe. If it feels hot, or you can hear water boiling in the heater, there may not be enough flow. Make sure the filter isn't plugged, and if the water stays hot while the heater keeps running, turn it off and call a technician.
Inside the heater, a thermal regulator and an internal bypass control the flow through the heat exchanger. Pentair lists a stuck bypass valve as a cause of boiling, and if high limit trips persist, it says to check the internal bypass valve, the thermal governor and the heat exchanger for blockage. Hayward says its internal bypass should be inspected periodically, because it could be the cause of low or high flow through the exchanger. Those checks need a technician.
If this started right after a heater install or a re-pipe, look at the plumbing. Pentair's manual says a heater plumbed in backwards will cycle continuously, and to make sure the piping from the filter isn't reversed. Hayward's troubleshooting guide says reversed connections will often cause frequent HS codes. No amount of filter cleaning fixes this one: the plumbing has to be corrected.
A very short run usually means the flame isn't lighting properly, or the heater can't confirm it's there.
On a Hayward H-Series, the blower pre-purges for 20 seconds while the igniter heats up, then the gas valve opens for a 4-second trial. If it fails to light, the gas valve closes, the blower post-purges for 30 seconds and the sequence starts over. After 3 retries it locks out with IF, waits 60 minutes and retries 3 more times, then keeps retrying every 60 minutes until the call for heat stops. So a Hayward that tries, goes quiet and tries again an hour later hasn't fixed itself.
On a MasterTemp, the gas valve opens about 20 seconds after the call for heat, and the igniter then switches to a sensing mode and monitors the flame. Pentair's burner chart covers the case where combustion looks normal but the flame doesn't stay lit: the flame current isn't being sensed, so the checks are for a wet or damaged igniter and a properly grounded burner. It also says a flame that's too lean can look "fluttery", with an acrid exhaust smell, and the burner may fail to stay lit. Jandy's JXi and LXi stop after three failed attempts and try again by themselves after one hour.
You can confirm the gas is on at the main shutoff outside the heater. Gas pressures, igniters and flame sensors are technician work. After months of sitting idle, a dirty flame sensor is also easy to mistake for a dead igniter, and our guide to heaters that won't fire on the first cold morning explains why. If you ever smell gas, don't try to light anything: call your gas supplier from a neighbor's phone.
Pentair says the filter pump should run continuously while the heater is on and for at least 5 minutes after it turns off. If a time clock runs the pump, it calls for a low-voltage Fireman's Switch that shuts the heater off at least 15 minutes before the pump stops.
So if your heater stops at about the same time each day, check the pump's schedule first. On a Hayward, that shows up as LO, the code Hayward calls normal with the pump off. If it only happens when you switch to spa mode, the valves may be the cause: our guide to a spa that won't heat when the pool does covers spa settings, spillover and valve actuators.
The pump, filter, baskets, valve positions, pump schedule and the outside gas shutoff are all fair to check yourself. Stop there. Pentair's manual says only qualified, trained service technicians should service the heater, warns against jumpering switch terminals to get around a failed safety switch, and says not to bypass the water pressure switch.
Call if it still cycles with a clean filter, if a code keeps coming back after a reset, or if you hear boiling. Our heater repairs run from about $200 for a small component to $1,000 or more, and a replacement gas heater is roughly $4,500 to $5,000 installed, so a proper diagnosis comes first.
If it stays lit but the water won't warm, see our guide to a pool heater that runs but won't heat. If the repair looks big, see repair or replace. For everything else, see pool heater repair.
That usually points at the flame: gas isn't reaching the burner, or the heater can't confirm the flame is lit. A Hayward H-Series gives each attempt a 4-second trial and locks out with IF after 3 retries. Check that the gas is on at the main shutoff. If it is, igniters, flame sensors and gas pressure are work for a technician.
Usually low water flow or overheating. Make sure the pump is running, clean the filter, empty the baskets and check that the valves are open. If it still happens, a high limit may be tripping because of a scaled heat exchanger or a bypass problem, which needs a technician.
Yes, for a short time. Hayward lists a 30-second post purge on the H-Series, and Pentair says the MasterTemp blower runs for about 45 seconds. A fan that won't stop is different: on a Hayward with an HF code, it means heat or flame was sensed with the gas valve off, so call a technician.
Check the display. LO points at the pump, the flow, the pressure switch or the limit switches. HS means the water went over about 105°F (Hayward's owner's manual says 104°F) or rose too fast. After an over-temperature stop, the heater restarts 2 minutes after the water cools. IF means it failed to light after 3 retries, and it tries again every 60 minutes.
On a MasterTemp, ERR PS points at low water flow, usually a filter or skimmers that need cleaning. HLS and AGS mean the heat exchanger outlet went over 135°F or 140°F, and SFS means the flue reached 480°F. On a new install, check that it isn't plumbed backwards, because Pentair says that makes it cycle continuously.
No. Pentair says not to bypass the water pressure switch, or jumper a failed safety switch. The heater could then run with no water flow, which can severely damage it. Find out why the switch is opening instead, starting with the filter.
Tell us the make, any code on the display and how long it runs before it stops. If it's the filter or the pump, we'll tell you so; if it's a switch, the flame or the heat exchanger, we'll come out and fix it. Licensed, insured, serving Cypress and Greater Houston.