Here's the thing about heater callouts: most of them aren't heater faults.
The heater has a set of safety interlocks, and when one isn't satisfied it refuses to fire. Which is exactly what it should do. What it can't do is tell you in plain English that your filter is filthy.
Key takeaways
- Most heater faults are flow faults. The heater is reporting a problem somewhere else.
- A variable-speed pump running too slow won't satisfy the heater's pressure switch.
- Check the filter before you touch anything on the heater itself.
- A heat pump that stops heating in air around 50°F is usually defrosting or in a low-temperature cutoff, which is normal operation.
- Gas train, burner and ignition work is licensed-professional territory. No exceptions.
Work through it in this order
1. Flow
The most common cause by a distance.
- Filter pressure high? Clean it. A clogged filter reduces flow below what the heater needs.
- Pump running slow? On a variable-speed pump, a low RPM setting that's fine for filtration is often below the heater's minimum flow. Schedule a higher-RPM block during heating hours.
- Valves. Anything partly closed on either side, including a bypass valve at the heater itself.
- Baskets. Skimmer and pump.
- Air in the system? Bubbles at the returns mean a suction leak, which means unstable flow.
If the pressure switch isn't closing, the heater won't try. That's the design.
2. The obvious things
- Thermostat set above current water temperature? More common than you'd think.
- Mode set to pool rather than spa, or vice versa.
- Power on at the breaker? Heaters have their own.
- Any automation blocking it? A controller schedule can be quietly overriding the panel.
- Error code on the display? Look it up in the manual before doing anything else. It'll save you an hour.
3. Gas supply, on a gas heater
- Is the gas valve at the heater open?
- Do other gas appliances in the house work? If the range and water heater are fine, supply is fine.
- On propane, is there gas in the tank? Check the gauge rather than assuming.
- Was a large appliance added recently? A new heater can outstrip an undersized meter or line, and the symptom is a heater that fires then drops out under load.
4. Air and ventilation
Gas heaters need combustion air and a clear flue. Leaves in the top, a bird's nest, a blocked intake screen, or an enclosure that got tightened up with new fencing — all of these cause lockouts.
And a lockout is the good outcome. A gas appliance starved of combustion air burns incompletely, and incomplete combustion of natural gas or propane produces carbon monoxide. Outdoors in open air that disperses. In a shed, a pump house, under a deck, or inside an enclosure somebody built around the equipment pad to hide it, it does not. The model code treats this as an air-quality requirement rather than an appliance one — equipment room ventilation has to address combustion air and air quality, not just keep the machine happy.
So: clear the blockage, never bypass the lockout. If the heater keeps shutting down and it lives in anything resembling an enclosed space, that is the point to stop resetting it and get a professional out — you are troubleshooting the symptom of a condition that has a second consequence.
Heat pumps need serious airflow across the coil. Leaves packed into the coil, shrubs grown in around it, or a tight equipment enclosure will kill performance.
5. Sensors and switches
Pressure switch. Closes when flow is adequate. Common failure part, cheap. But confirm you actually have good flow first — nine times out of ten it's working correctly and reporting a genuine problem.
High limit switch. Trips when water gets too hot, usually because flow is inadequate. Same story: it's the messenger.
Temperature sensors. Scale and fouling on a sensor gives false readings. Cleaning is often all that's needed.
Heat pump specifics
Cold air. Output falls as the air cools. Hayward and Pentair rate their current heat pumps at roughly 60 to 70 percent of full output in 50°F air, so a cool day alone does not stop one. What does is frost: around 50°F the coil has to defrost, and heating stops while it runs. Hayward's manual gives that as anything from 15 minutes to several hours, and Pentair's UltraTemp shuts down with a Low Ambient Temp alarm if a 15-minute defrost does not clear the coil. None of that is a fault. If it's April and you're getting nothing, check the air temperature before you call anyone.
Ice on the coil. Some frost during cool-weather operation is expected and the unit should defrost itself. Persistent heavy ice means a refrigeration problem, and that's a technician job.
Blocked airflow. Clear leaves and debris from the coil, cut back anything growing within a couple of feet, and make sure the discharge air isn't recirculating back into the intake.
The code depends on the model. Hayward's square HeatPro MCHX shows FLo for low flow. Its round 100K heat-and-cool model, the HeatPro HP31005T, shows E03 for the same thing, and E01 and E02 for its refrigerant pressure switches. Look the code up for your model number before you order a part.
When it fires and then stops
A heater that lights and then shuts down within a few minutes is a different problem from one that never lights at all, and it points at a much shorter list.
Flow that is adequate at ignition and not under load. A filter close to its limit passes enough water at first, then less as the heater's own restriction adds to it. The same thing happens on a pump that steps between speeds on a schedule: the heater fires during the high block and drops out when the schedule steps down.
Gas supply that cannot sustain the draw. A line or meter sized for the house before the pool heater was added can deliver enough for ignition and not enough for continuous firing. The tell is that it happens more often while other gas appliances are running — and on propane, more often when the tank is low, since a nearly empty tank cannot vaporize fast enough on a cold morning to feed a 400,000 BTU load.
High limit tripping. The exchanger reaches its temperature limit because water is not carrying heat away fast enough. That is flow again, arriving from a different direction.
A fireman's switch doing its job badly. The switch exists to shut the heater down a few minutes before the pump, so residual heat gets carried out of the exchanger. Wired or timed wrong, it kills both at the same instant, which cooks the exchanger a little every cycle and can trip the limit on the following start.
Work the flow items first. Nearly all of this is flow.
Scale in the heat exchanger
The slow killer, especially in hard water and on salt pools.
Symptoms build gradually: heating takes longer than it used to, the heater short-cycles, and filter pressure creeps up. Eventually it fails outright.
It's a chemistry problem. High pH plus high calcium deposits scale on the hottest surface in your system, which is the inside of the exchanger.
Worth knowing what you are protecting while you decide: what a heater actually costs to run is often the number that settles repair against replace, because an old heater fighting a scaled exchanger is burning fuel for heat it never delivers.
An acid flush by a technician can recover a scaled exchanger if caught in time. Left long enough it's a replacement — $900–$1,800 on a gas heater.
Prevention is keeping pH in range and watching calcium hardness. That's it. That's the whole prevention.
Maintenance that prevents most of this
- Keep the filter clean. Flow is the heater's whole world.
- Keep chemistry in range. Scale is what kills exchangers.
- Clear leaves from the top of a gas heater and the coil of a heat pump.
- Annual service on a gas heater in a heavy-use climate. Burner cleaning, flue check, gas pressure verification.
- Drain it properly at closing. A heat exchanger with water in it that freezes is a $900–$1,800 lesson.
Frequently asked questions
Why is my pool heater not turning on?
Most often insufficient flow. A dirty filter, a pump speed set too low, a partly closed valve or a clogged basket will all prevent the pressure switch closing, and the heater simply won't fire. Check flow before you assume anything is wrong with the heater.
Why does my pool heater keep shutting off?
Short cycling usually means the heater is reaching a limit and shutting down for protection. Common causes are low flow tripping the high limit, a scaled heat exchanger restricting water, poor ventilation on a gas unit, or a failing pressure switch. Persistent short cycling is worth a professional look.
How do I know if my pool heater pressure switch is bad?
Symptoms are a heater that won't fire despite good flow, or that cycles on and off rapidly. It's a common failure part and it's cheap. But confirm you genuinely have good flow first — the switch is usually doing its job correctly and reporting a real flow problem.
Why is my heat pump not heating the pool?
Check the air temperature first. Output falls as the air cools, and around 50°F the coil has to stop and defrost, which is normal operation, not a fault. Hayward's HeatPro manual tells owners to wait for the air to rise above 50°F and check again, and Pentair's UltraTemp shows a Low Ambient Temp alarm when a defrost does not clear. Also check that the air intake and coil aren't blocked by leaves or debris, since they need serious airflow to work.
Can I fix a pool heater myself?
You can fix the causes — clean the filter, correct the pump speed, open a valve, clear the airflow, clean a sensor. You should not open the gas train, adjust gas pressure, or work on burners and ignition controls. That's licensed work for good reason, and a mistake there is a fire or carbon monoxide risk.
