Heater calls come in waves. In the first cool week of autumn, owners who have not touched the heater since last season switch it on, wait an hour, put a hand in the water and call you. Usually the heater is not broken. A pool heater fires only when a chain of conditions is met: something is asking for heat, it has power, water is moving through it fast enough, and every safety switch agrees. Break any link and the heater does nothing, which from the patio looks exactly like a dead heater.
So the order of checks matters more than any single fix. Causes outside the heater, such as a low setpoint, a valve sending water to the spa or a loaded filter, cost nothing to correct. Causes inside the cabinet involve gas, combustion, refrigerant or mains voltage, and most belong to a qualified heater technician. The three heater types also fail differently, and a heat pump that seems not to be heating in 48°F air is often doing exactly what its manual says.
This is the diagnostic order, what each type's failures look like, where a service technician should stop, and how to run a heater call so every trip is paid for.
Owners use the same words for both, so ask what they saw. "It won't turn on" should mean no fan, no click, no flame and no compressor. "It's not heating" usually means the heater runs but the water warms more slowly than expected. The first points at the conditions that let the heater start; the second at how much heat it delivers against how much the pool loses, and whether the heated water reaches the pool.
| What you find | What it usually means | Check first |
|---|---|---|
| Display blank | No power reaching the heater's controls | Breaker, disconnect and any GFCI on the circuit |
| Display on, heater never starts | No call for heat, or a safety interlock is open | Setpoint, pool or spa mode, automation, then flow |
| Starts, then shuts down or locks out | Ignition, or a safety switch opening during the run | The code shown, then flow, then the gas supply |
| Runs, but the water stays cold | Output below the pool's heat loss, or water bypassing the heater | Run time, air temperature, cover, valve positions |
| Heated well, then slowed over weeks | Something loading up gradually | Filter pressure, the heat pump coil, signs of soot |
Display blank
Display on, heater never starts
Starts, then shuts down or locks out
Runs, but the water stays cold
Heated well, then slowed over weeks
A general guide drawn from the troubleshooting charts in the Pentair MasterTemp, Hayward Universal H-Series, Hayward heat pump and Raypak E3T manuals. The manual for the model in front of you takes precedence.
These apply to every heater, in this order, because each is quicker than the next and each can make the later ones meaningless. Pentair's own troubleshooting section starts the same way: pump running, nothing blocked, valves set, time clock correct.
The first three steps settle most heater calls without a panel coming off.
Confirm there is a call for heat
Pentair's manual is plain that the burner will not fire unless the setpoint is above the actual water temperature. Check the mode, because after a spa session the heater and valves are often still set to spa. Check the maximum-temperature setting too: MasterTemp heaters have POOL MAX and SPA MAX limits and Hayward's Universal H-Series has a temperature lock-out, and either can cap the setpoint below what the owner asked for.
When automation runs the heater, its own panel is often in a remote or bypass mode, waiting for the automation to call; Raypak's electric heater even has a code for it. A time clock's fireman's switch is the older form of the same interlock. Pentair specifies one that turns the heater off at least 15 minutes before the pump stops, so a stopped or mis-set clock can keep the heater from ever firing.
Confirm power
A blank display means no power to the controls. Hayward notes that some breakers move only slightly when they trip, so switch the breaker fully off and back on rather than trusting the handle. A breaker or GFCI that trips again after one reset is reporting a fault, and that is an electrician's call.
Prove the flow
Heaters carry a water pressure switch or flow sensor that stops them when flow is low, and the minimum flows in the manuals cited here run from 15 gpm for a small electric spa heater to 40 gpm for a 400,000 BTU gas heater. Check the pump is running and primed, the baskets are clear and the filter is near its clean starting pressure, then follow the valves to make sure the water passes through the heater to the right body of water.
Variable-speed pumps are the modern version of the problem. A pump turned down to save power can deliver less than the heater needs, and Raypak and AquaCal both say to run a multi-speed pump faster when the heater reports low flow. A heater well above or below the pool needs its pressure switch set for that, which Pentair reserves for a qualified technician.
Read the display, and look the code up
Write down exactly what the display shows before any reset, because a reset can clear the evidence, then look it up in that model's manual. Codes are not shared between brands: Hayward's Universal H-Series uses one code, LO, for an open water pressure switch, vent pressure switch or temperature limit, while Pentair's MasterTemp reports each separately and keeps the last five errors in a history menu.
Give it time to start
Pentair describes the blower running for about 20 seconds before the gas valve opens, Hayward's heat pumps wait five minutes after most interruptions, and Raypak's electric heater waits a minute after sensing flow. Watch a full start before deciding nothing is happening.
A dirty filter is the classic dead heater
Pentair's manual says a low-water-pressure fault with the pump running usually means the filter or skimmers need cleaning, and that a qualified technician should check the system if the fault survives that. On a route, the heater call and the filter clean are often the same visit.
If the gauge is well above the pool's clean start, clean the filter properly, restart, purge the air and try the heater again before anything else.
Gas pool heaters light in a fixed order, and where the order stops tells you which link failed. In Pentair's description of the MasterTemp, the safety interlocks must be closed first: the water pressure switch, the high limits and an air flow switch that proves the combustion blower is running. On a call for heat the blower and hot surface igniter start, after about 20 seconds the gas valve opens and the burner lights, and the igniter switches to sensing the flame.
Hayward's Universal H-Series runs a four-second ignition trial; with no flame sensed it closes the gas valve, retries three times, then locks out with an ignition failure code and retries hourly while the call for heat remains.
| Where it stops | Likely link | Who takes it from here |
|---|---|---|
| Nothing at all, not even the blower | No call for heat, no power, or an interlock open | Service tech for setpoint, power and flow; heater tech if an interlock stays open with good flow |
| Blower runs, no ignition attempt | Air flow or vacuum switch, igniter, control board | Heater technician |
| Ignition attempt, no flame | Gas supply, gas valve or low gas pressure | Service tech confirms the valve is open and there is gas; the rest is for a gas-qualified technician |
| Lights, then drops out within seconds | Flame sensing, or gas pressure falling under load | Heater technician |
| Runs, then shuts down on a temperature code | Low flow through the exchanger, or an internal bypass fault | Service tech restores flow; heater tech if it recurs |
Nothing at all, not even the blower
Blower runs, no ignition attempt
Ignition attempt, no flame
Lights, then drops out within seconds
Runs, then shuts down on a temperature code
Based on the sequence of operation and troubleshooting charts in the Pentair MasterTemp and Hayward Universal H-Series manuals. Other brands differ in detail.
Confirm the manual gas shut-off outside the heater is fully open and, following the lighting instructions on the heater, that its own gas control is on; Pentair says to turn that control by hand only and to call a qualified technician if it will not move. On propane, check the tank gauge. A tank that ran dry means calling the propane supplier and having the system leak-checked before the heater is relit, because the National Fuel Gas Code calls for a leak check when gas is restored after an interruption of service.
On natural gas, ask whether the other gas appliances work. Pentair notes the meter must supply the heater and everything else on it at once, and lists a larger gas line, regulator or meter among the fixes for a heater starved of gas, so an ignition fault that comes and goes is worth a gas pressure test under load by a qualified technician.
On the MasterTemp, one high limit opens if water leaving the heat exchanger passes 135°F and an automatic gas shut-off opens above 140°F; Hayward's equivalent code reports maximum return water temperature exceeded or a rapid rise. The usual reason is too little water moving through the heater, so restoring flow is the service fix and replacing the switch rarely is. A switch that trips with good flow, or will not reset, goes to a heater technician.
Never bypass a safety switch
Jumpering a pressure switch, high limit or flow switch to get a heater running removes the thing that stops it overheating or firing with no water to carry the heat away. Pentair warns of explosion or fire if the safety interlocks are disabled. If you smell gas, do not try to light the heater or touch electrical switches; leave the area and call the gas supplier from somewhere else.
An idle gas heater is a home for things that block air and gas. Pool & Spa News has reported nesting spiders blocking the openings at the bottom of heaters and stopping them starting, and rodents chewing ignition and control wiring. Look from outside for webs, nests and debris around the base and vent, and clear what you can reach without opening the cabinet. The burner and orifices are for a qualified technician; Pentair says the burner needs no maintenance or adjustment by the user.
Soot is the sign to stop. It is the visible product of incomplete combustion, the condition that also produces carbon monoxide, and one industry writer calls it often the first sign of a problem and often ignored. Black deposits at the burner area or exhaust mean the heater is switched off and handed to a heater technician the same day.
Carbon monoxide
Pool heater exhaust contains carbon monoxide, which you cannot see or smell. Pentair strongly recommends carbon monoxide detectors near the heater and in any adjacent occupied space, and annual inspection of the heater and venting by a trained technician. A damaged, disconnected or blocked vent is a reason to switch the heater off and tell the owner in writing.
A heat pump moves heat from the air into the water, so its output falls as the air cools. AquaCal's troubleshooting advice starts there: in cooler air a heat pump heats less efficiently and has to run longer. Below a certain air temperature it stops. Hayward's owner's manual puts that low-ambient shutdown anywhere from about 45 to 55°F, and the unit shows LP and restarts on its own when the air warms.
As the air approaches 50°F, frost can begin to form on the evaporator coil, which AquaCal treats as normal. Heat-only units defrost by shutting off until the ice melts; models with active hot-gas defrost, such as AquaCal's IceBreaker, can defrost as often as every 50 minutes in very cold weather, for up to ten minutes an hour. No heat goes into the water meanwhile, so a heat pump pausing on a cold morning is behaving as designed, and AquaCal advises running heat pumps in the warmest part of the day.
Airflow and water flow are what a service technician can fix. Hayward asks for two feet of clearance around a heat pump and six feet above, and its coil cleaning is done with the power off, spraying from inside the unit outward with the nozzle 12 to 18 inches from the fins. Its codes show why the manual matters: PS means improper water flow, but HP, a high refrigerant pressure trip, can also mean low water flow, because in heating mode the water carries heat away from the refrigerant. LP covers cold air, lost refrigerant, a failed fan, a frozen coil or blocked airflow.
To tell whether a heat pump is working, stand at the fan. AquaCal says the air leaving should be 9 to 12°F colder than the air entering, and that a working unit makes plenty of condensation. Hayward's test for condensation against a leak is to switch the heat pump off for at least two hours with the filter pump running: if the water dries up, it was condensation. Hayward also says water is warmed only three to five degrees on its way through, so do not expect a hot return.
If the air is mild, the coil and fan are clear, flow is good and the unit still trips on a pressure fault or blows air no colder than it takes in, the problem is probably in the sealed system or its controls. Under the EPA's Section 608 rules, attaching refrigerant gauges or adding or removing refrigerant requires a certified technician, so book an HVAC or refrigeration technician.
Electric resistance heaters are most common on spas, and with no combustion their failures are simpler. Raypak's E3T manual is typical: the elements will not energise below 15 gpm, and its troubleshooting table for a heater that does nothing lists low flow, a tripped high limit with a manual reset button, a thermostat not calling for heat, and low supply voltage, which it hands to an electrician.
The failure most often caused by the service visit itself is a dry fire. Raypak explains that elements are damaged when powered with air around them, that a cartridge filter's casing holds air after cleaning, and that circulation should run for at least two minutes, with the air purged, before the heater gets power again. Build that into filter cleaning on any spa with electric heat. The same manual links repeated element burnout to poor water chemistry and a split element to an over-acid condition.
Anything past the reset button is electrical work. Turn the power off at the breaker before opening any panel, and leave element testing, a breaker that keeps tripping and all wiring to a licensed electrician.
Many "not heating" calls are heaters working as rated on a pool that needs more heat than the owner imagines. One BTU raises one pound of water by 1°F and a gallon weighs about 8.34 pounds, so a 20,000-gallon pool needs roughly 167,000 BTU per degree before any losses. On a Pentair MasterTemp the model size is the input rating, so a 400,000 BTU heater could manage about 2.4°F an hour only if every BTU of input reached the water, which it does not. A heat pump with an output of 110,000 BTU an hour manages under 0.7°F an hour on paper, and less in cool air.
The losses are mostly evaporation: Hayward attributes about 75% of an uncovered pool's heat loss to it and estimates a cover roughly halves total losses. The heater also works only while the pump runs, which is why Hayward tells owners to run both continuously until the pool reaches temperature and to allow at least 48 hours before it feels noticeably warmer. Pentair says a properly sized gas heater should recover a 10 to 15°F weekday setback within about a day.
For spas, Raypak's estimate for its electric heaters is kilowatts times 410, divided by the spa's gallons, in degrees per hour before losses, so an 11 kW heater on a 400-gallon spa gains about 11°F an hour.
The busiest week for heater calls is the first cold snap, when heaters that have stood idle for months are asked to work at once. Pentair's pre-start checklist makes a good field routine: clear debris from in and around the heater and its vent, check the ventilation openings and water connections, and run the pump for several minutes to clear air before calling for heat.
A winterised heater has to be returned to service first. Pentair's draining procedure removes the water pressure switch and plugs its port, and bags the air inlet against insects; both must be reversed, the drain cock closed and water circulated until the air noises stop. Heat pumps disconnected at their unions need reconnecting. If someone else closed the pool for winter, assume nothing about how the heater was left.
Undo the winterising before you diagnose
A heater with its pressure switch still out, a bag over its air inlet, a drain cock open or its unions disconnected will not run properly, and none of that is a fault. Walk around the heater and check each one before reading anything into a code.
Expect some water from a gas heater fired into cold water; brief condensation at start-up is normal for most gas pool heaters. Pentair warns that running continuously below 68°F causes harmful condensation, and for a spring start below 50°F says to run the heater continuously until the water reaches 68°F. Dripping that continues once the pool is warm needs a technician, and Pentair says a heater any part of which has been under water must be inspected by a qualified technician before use.
The manuals draw the line clearly: Pentair says only qualified, trained technicians should service its heater, and Hayward that only qualified technicians should remove the panel. The service side is what the owner's instructions cover: setpoints and modes, a breaker reset, flow, the filter, valves, reading codes, a single reset where the manual allows it, clearing debris and cleaning a heat pump coil with the power off.
The other side is gas components, gas pressure testing, burner and orifice cleaning, igniters, flame sensors and controls, pressure switch adjustment, refrigerant, and any electrical work beyond the breaker. Who may legally do that work varies by state and city, and gas and electrical work commonly require a specific licence; our licensing guide covers how that is usually split. One rule does not vary: a safety switch is never bypassed to get a heater running.
Price the diagnosis as a job in its own right. A heater call is one visit to find the cause and often a second, by someone else, to fix it, and the first costs you the same whether the answer is a failed igniter or a dirty filter. A flat diagnostic charge, quoted before you go and stating that repairs are quoted separately, keeps the conversation short. Build it like any other service rate.
The repair needs authorisation. A good service agreement sets a limit below which you can proceed, a rule above it, and whether heater repairs are done by you or referred. If yours says none of that, start with what a service agreement should include.
Set expectations on the first call in hours and days: a heat pump in cool weather needs days, a gas heater most of a day for a decent rise, and both need the pump running and the cover on. Owners told that in advance do not call at the end of the first afternoon.
Chemistry matters as well: Hayward lists low pH, high sanitiser levels and scale-forming water among the causes of heat exchanger failure, and Pentair excludes chemical damage to the exchanger from its warranty and warns against tablets in the skimmer, which send concentrated chlorine into the heater when the pump stops. If an exchanger fails on a pool you maintain, recorded water tests are the answer you want to have.
Heater calls on the same route, run two ways
Reactive
Systematic
What to record on every heater call
0 / 7
The habit that makes heater calls manageable is checking the cheap things in order and writing them down; a log sheet will do. Put a test firing on the last visits before the weather turns, alongside the rest of the seasonal maintenance schedule, and the first cold weekend becomes a list of known heaters rather than a week of phone calls.
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