Why does my pool heater keep shutting off?

A pool heater that shuts itself off is almost always protecting itself. The usual triggers are not enough water moving through it, water that is too hot inside the heat exchanger, a switch or time clock telling it to stop, or a failed ignition. Note what the pump, filter and water level look like, then have a licensed technician read the heater's fault display.

Start with the water flow

A gas heater needs water moving through it to stay lit. Raypak says its heater requires water flow and positive pressure to fire and operate properly.^[1] Raypak describes the water flow switch on the heater this way: it ensures that the heater operates only when the filter pump is in operation.^[1] When flow drops, the switch opens and the burners stop.

Raypak's troubleshooting chart lists the causes of a heater going on and off continuously: a dirty filter, a low pool water level, an external bypass setting out of adjustment, a pressure switch out of adjustment, and liming or scale forming in the exchanger.^[1] Three of those show up without touching the heater: a high filter pressure reading, a low water level, or a pump strainer basket packed with debris. A pool service can sort these out quickly, and they are the most common flow problems.

Two-speed and variable-speed pumps add one more cause. Raypak says that in some cases the flow on low speed is insufficient to operate the heater, and the pump must be run at high speed when heating the water.^[1] A heater that runs when the pump is on high and quits on low fits that description.

The heater overheating itself

Heaters carry two high limit switches that open when the water in the header gets too hot. On Raypak residential units both are automatic and set at 135°F (57°C).^[1] An open high limit shuts the burners down until the water cools.

Raypak notes that an erratic high limit is often characteristic of an internal heat exchanger problem, such as scale build-up or a defective bypass.^[1] That points to water chemistry (hard, scaling water) or a stuck bypass valve, both of which need a technician. A pump turned off too soon can cause the same shutdown. Raypak says the pump should shut off a minimum of 5 minutes after the heating cycle ends, and that failing to do so may cause residual heat buildup that triggers a high limit event.^[1]

Controls telling it to stop

Some shutoffs are not faults at all:

• Time clock. Raypak says a time clock should shut off the call for heat 15 to 20 minutes before the pool pump shuts down.^[1] On a digital heater the display shows "Clock/Fireman Sw" when the time clock has shut off the call-for-heat.^[1] A clock set wrong, or one that cuts the heater at the same moment as the pump, produces a heater that seems to quit on its own.
• Set point reached. The display reads "No Demand" when the heat demand is satisfied.^[1]
• Cold water. A Raypak digital control displays "Low Temp Lockout" when the water temperature is below 36°F (2°C).^[1]

Safety and ignition lockouts

Other shutdowns are real faults. Raypak's fault list includes an open water pressure switch, open high limits, an open rollout switch, and ignition lockouts where the pilot flame is not established or is lost during the trial to ignite the main burner.^[1] On its digital heaters, if the pilot flame is lost 4 times within the 8-second trial to ignite the main burner, the heater locks out for 60 minutes before retrying.^[1] A heater that cycles off and then restarts an hour later is showing exactly that pattern.

Raypak describes the flame roll-out switch as a "single-use" thermal fuse that must be replaced when it is disabled by an over-temperature condition caused by excessive restriction in the heat exchanger flue passage.^[1] A tripped rollout switch is a warning about the exhaust path, not a nuisance to reset.

Write down the message on the display before you call. It tells the technician which part of the chain opened. What a technician checks when a pool heater will not fire walks through the order of those checks.

What to note before you call

A technician diagnoses faster with a few observations:

• The message on the heater's display, written down word for word.
• When the shutoff happens: right away, after several minutes, or only at a certain pump speed.
• Whether the filter pressure gauge reads higher than usual, and whether the time clock and the heater's set point look right.

The heater itself is not a homeowner repair. Raypak cautions not to operate the heater without the function of a properly-adjusted water pressure switch or flow switch.^[1] It also says its instructions are intended only for qualified personnel, and that persons not qualified shall not attempt to fix the equipment nor attempt repairs.^[1]

When to call a licensed pro

A gas pool heater is gas-fired equipment, and Virginia regulates gas work by licence. No individual may engage in work as a liquefied petroleum gas fitter or natural gas fitter provider unless licensed under the same article of the Code.^[3] The definition of a liquefied petroleum gas fitter excludes hot water heaters, boilers and central heating systems, which require a heating, ventilation and air conditioning or plumbing certification.^[2] Ask the business which DPOR licence it holds for your heater, and check the licence before you pay a deposit. DPOR says that if the work is valued at $1,000 or more, a valid Virginia contractor's licence is required for the licence category in which the contractor is to work.^[4]

Ask for the diagnosis in writing, including which switch or sensor opened and why. If the fault is scale in the exchanger or repeated trips, read whether to fix a pool heater or buy a new one and what a pool heater repair visit costs in Virginia. More help is in the pool and spa equipment answers, and you can browse pool and spa businesses.

To get the shutoff diagnosed, describe the pool heater problem and licensed local businesses reply.