What PoolBoss Says
Most often the heater is not getting enough water flow, usually from a dirty filter or clogged baskets rather than a gas fault. The other common causes are gas supply problems, a failed igniter or gas valve, a dirty flame sensor, and blocked airflow or venting. Controllers typically lock out after 30-60 seconds of failed ignition attempts.
A heater lockout is the equipment call most likely to end with somebody hurt or a company sued, which is why it is worth knowing exactly where your scope stops. The homeowner troubleshooting guides answer one error code on one brand. They do not tell a service company what it may legally touch.
That boundary is most of the job. What follows puts the causes in the order worth checking, starting with the flow problem that sits squarely inside a route tech's scope, then draws the line at the gas train and covers how to hand the rest to a licensed tech without losing the account.
At a glance
Key takeaways
- Check water flow first: a dirty filter or clogged baskets cause more heater lockouts than gas faults do.
- Read filter pressure against that pool's own clean baseline, and treat 8-10 psi over it as both the cleaning and the lockout threshold.
- The flow switch is why a perfectly good gas train still reports ignition failure, so confirm flow before you suspect gas.
- A unit that lights and then drops out after a few seconds points at the flame sensor, not at the igniter.
- Gas train, burner, venting, and line-voltage wiring need a licensed gas contractor in most states, whatever your pool license covers.
- A slow-heating or overheating unit is usually a scaled heat exchanger, which is a water-balance failure from months earlier.
- Own the referral - name the specialist and hand over your readings, or the gas company that also does pool service quotes your account.
Why is a pool heater not firing?
Check water flow before anything else, because it is both the most common cause and the only one you are unambiguously allowed to fix. A heater will not light without enough water moving through it, and it does not know why the flow is low. A filter at 24 psi against an 11 psi clean baseline, a skimmer basket packed with oak leaves, or a pump basket nobody emptied all produce the same lockout as a genuine gas fault, and all three are inside a normal stop.
Once flow is confirmed good, the remaining causes are gas-side and airflow-side. Gas supply problems come first: a closed or partly closed valve, an empty propane tank, or pressure dropping when another appliance calls at the same time. Then the ignition components, meaning a failed igniter or a gas valve that is not opening. Then a dirty flame sensor, which lets the burner light and then shuts it down seconds later because the controller cannot confirm a flame. Last, blocked airflow or venting, which is usually a bird's nest, a wasp nest, or debris in the burner tray after a windy spell.
The controller gives you a clock to work against. Most units make two or three ignition attempts and then lock out after roughly 30-60 seconds, displaying an ignition-failure or ignition-lockout code. That timing is a useful diagnostic on its own: a unit that locks out without the burner ever lighting points at gas, ignition, or flow, while a unit that lights and then drops out after a few seconds points hard at the flame sensor.
The most common cause of ignition failure is a dirty filter, not a gas fault
Every gas heater has a flow switch, and it is a safety device rather than a suggestion. It is a simple pressure-actuated switch that stays open until water is moving fast enough through the heat exchanger to carry away the heat the burner is about to make. If the switch does not close, the controller never energizes the gas valve, and the unit reports an ignition failure even though the gas train is in perfect condition. The heater is not diagnosing the pool. It is only reporting that it did not get permission to light.
This is why the first move on a lockout call is a pressure gauge, not a screwdriver. Read filter pressure against that pool's own clean baseline rather than against a generic number, because a cartridge system that runs at 11 psi clean is in trouble at 21 psi while a sand filter that runs at 18 psi clean is not. A rise of 8-10 psi over baseline is the standard clean-the-filter threshold, and on a heated pool it is also the standard heater-lockout threshold.
The practical consequence is that a large share of heater calls are a $0 parts fix performed inside the normal stop. Pull and rinse the baskets, read the pressure, and if it is high, cleaning the filter that is starving it of flow restores flow and the heater fires on the next call. That is worth knowing before you tell a customer they need a heater technician, because the alternative is a service company that dispatched a specialist to clean a cartridge.
What you may check, and what needs a license
The line is not about skill, it is about gas. In most states, work on the gas train, the burner assembly, venting, or the heater's line-voltage wiring requires a licensed gas or plumbing contractor, and a pool service or pool contractor license does not extend to it. Water-side work, cleaning, and reading what the controller already displays are a different matter and sit inside normal pool service.
The rules vary by state and sometimes by county, and they change, so treat the table below as the shape of the boundary rather than as legal advice for your jurisdiction. Confirm your own position against your state board, and see which licenses a pool service company actually needs for how the base licensing question works before you start adding trades to it.
| What you see | Likely cause | Who does the work |
|---|---|---|
| Lockout code, filter high over baseline | Restricted flow from a dirty filter or full baskets | Route tech - clean it at the stop |
| Lockout code, flow and pressure normal | Gas supply, igniter, or gas valve | Licensed gas tech - stop and refer |
| Lights, then drops out after a few seconds | Dirty flame sensor or a flame-rectification fault | Licensed gas tech - stop and refer |
| No display, no response at all | Tripped breaker, blown fuse, or failed transformer | Route tech may reset a breaker once; the rest is electrical |
| Rust streaks or water under the unit | Corroded or leaking heat exchanger | Licensed gas tech - and usually a replacement quote |
| Runs, but the pool never reaches temperature | Undersized unit, scaled exchanger, or heat loss with no cover | Route tech diagnoses; replacement is a quote |
Scale on the heat exchanger is a chemistry problem showing up as an equipment problem
A heater that runs but heats slowly, short-cycles, or overheats is often reporting a water-balance failure from months earlier. The heat exchanger is the hottest, fastest-moving water in the whole system, and that is precisely where calcium carbonate comes out of solution first. Scale builds on the tube walls, insulates them, and forces the burner to run longer for less temperature rise until the unit starts tripping its own high-limit.
The chemistry driving it is not one number. Scale forms when calcium hardness, pH, alkalinity, and water temperature push the balance saturated, and heated water pushes it further than the same chemistry would on an unheated pool. The reverse failure costs just as much: water running below about 7.2 pH turns aggressive and pulls copper straight out of the exchanger. A heater replacement runs roughly $2,500-$4,500 on a typical residential pool, which makes the exchanger the single most expensive thing your dosing decisions touch.
None of this is visible at the moment it matters, which is the argument for a consistent pad routine rather than a heater inspection. The walk-around that catches this a season earlier costs about ninety seconds a stop, and on a heated pool the readings it captures are what let you say the exchanger was scaling from March rather than arguing about it in October.
The referral is what keeps the customer yours
The risk in a heater call is not the repair, it is the phone call the customer makes next. Told only that they need a heater technician, most homeowners search for one, and a large share of gas and HVAC companies in Sunbelt markets also do pool service. You have just handed a competitor a warm introduction to your account and a reason to quote the weekly service while they are standing there.
The fix is to own the handoff rather than announce it. Name the specialist, make the call yourself or text the customer the contact, tell them what you already ruled out, and say when you will be back to check the pool. The message is that the referral is part of your service, not the end of it. Operators who run heated routes generally keep one gas contractor on standby and agree the terms in advance, either a flat referral or the sub-billed through your invoice at a small margin.
What makes that credible is the record you hand over. If the equipment history for that pool already shows basket condition, filter pressure before and after, the action taken, and a photo of the controller display, the gas tech arrives knowing flow was verified and can go straight to the gas side. And when a unit is genuinely finished, the replacement is the moment to reopen the equipment conversation, because choosing between solar, a heat pump, and gas is a decision the customer will otherwise make with whoever happens to be standing at the pad.
Two Gulf Coast pools, the same lockout, two different verdicts
An operator running 140 pools across Sarasota and Bradenton takes the same complaint twice in one week: the heater locks out every morning. The two stops end nothing alike.
The Sarasota pool is a cartridge system with a clean baseline of 11 psi. The gauge reads 24. The tech pulls the cartridges, finds them loaded after a windy stretch, cleans them, and reassembles. Pressure comes back to 12, the pump pulls full flow, and the heater fires on the next call and holds. Parts cost is $0, the stop runs about twenty minutes long, and the customer is told what happened and why it will recur if the filter goes unserviced through leaf season.
The Bradenton pool shows the identical code with baskets clear and pressure at 13 against a 12 psi baseline. Flow is not the problem, so the tech stops. He photographs the controller display, notes the filter readings that rule flow out, records what was checked, and calls the gas contractor he already has an arrangement with. The customer gets a name and a same-week appointment from him rather than from a search result.
The second stop took eleven minutes and produced no revenue that day. It also kept a $210-a-month service account that a gas company doing pool work on the side would have been happy to quote. The difference between the two outcomes was not diagnostic skill, because both techs read the same code. It was having a stopping rule and somewhere to put what they found.
FAQ
Frequently asked questions
Can I reset a pool heater error code for a customer?
Clearing a lockout is generally fine, but clearing it repeatedly is not. Most controllers reset by cycling the unit's power switch or the breaker, and a single reset after you have fixed the actual cause - cleaned the filter, emptied the baskets, restored flow - is reasonable and part of finishing the job. What you should not do is reset a heater you have not diagnosed. A lockout is a safety response to a failed ignition sequence, and repeatedly clearing it without knowing why it locked out can mean sending unburned gas into a combustion chamber every attempt. The practical rule most operators use is one reset after a fix you can explain, and no reset at all on a unit heading to a gas tech. Note the reset and the reason on the visit either way, because the next person to touch the unit needs to know it has already been cycled.
Do I need a gas license to service a pool heater?
For anything on the gas side, in most states yes, and a pool contractor or pool service license does not cover it. The trades are licensed separately, so the gas train, burner assembly, venting, and line-voltage wiring typically sit with a licensed gas fitter, plumber, or HVAC contractor, while water-side work stays with you. The specifics vary by state and sometimes by county, and some states carve out narrow exceptions for like-for-like appliance work, so verify your own position with your state board rather than assuming. Two things matter beyond the license itself. Your general liability policy very likely excludes gas work you were not licensed to perform, so an uninsured mistake is uninsured twice over. And a customer who is injured after unlicensed gas work has a straightforward claim. This is the one area on a pool pad where the cautious answer is also the commercially correct one.
How do I know if the heat exchanger is scaled?
The clearest sign is a heater that runs normally but produces less temperature rise than it used to, on the same pool with the same run time. A healthy gas heater lifts water roughly 1-2 degrees per hour on a typical residential pool depending on size and unit output, so a pool that used to reach temperature overnight and now needs a day and a half is telling you something is insulating the tubes. Short-cycling is the other tell: a scaled exchanger cannot shed heat into the water fast enough, so the unit trips its own high-limit, cools, and relights in a repeating pattern. Neither symptom identifies itself as scale from the outside, which is why the water history is the real diagnostic. A pool that has run high calcium hardness and high pH through a heated season has a scaled exchanger until proven otherwise, and confirming it visually means opening the unit, which is licensed work.
Why does the heater work in the afternoon but not the morning?
Nine times out of ten this is a timing problem rather than a heater problem. The heater is calling for heat at the start of the pump cycle, before flow has fully established and the flow switch has closed, so it makes its ignition attempts and locks out within the first minute of the day. By afternoon somebody has reset it, or the second pump cycle starts with the plumbing already primed, and it lights normally. The fix is scheduling rather than parts: start the pump 10-15 minutes before the heater is allowed to call, so flow is stable before the first ignition attempt. The other common morning-only version is debris. Overnight leaf fall loads the skimmer basket, flow is marginal at the 7am start and fine once the basket is emptied later in the day. Both causes are inside a route tech's scope, which is worth checking before anyone calls a gas contractor.
Should my company offer heater service at all?
Most route companies should not, and the ones that should treat it as a separate licensed trade rather than an add-on. The economics are poor at the margin: heater work is low-frequency, needs parts you will not stock, carries the highest liability on the pad, and pulls a tech off a route where the stops are already scheduled. Referring it costs you the repair revenue and keeps the recurring service revenue, which is the larger number over any reasonable horizon. The case for bringing it in-house arrives when heated pools are a large share of your route and you are referring out often enough to fund a licensed hire, which in practice means a commercial-heavy or resort-adjacent book rather than a residential one. The middle path most operators land on is a standing arrangement with one gas contractor: you keep the relationship and the account, they do the licensed work.
What do I record before handing a heater job to a licensed tech?
Record enough that the specialist does not repeat your work. The useful minimum is a photo of the controller display showing the actual code, the filter pressure reading with that pool's clean baseline next to it, the condition of both baskets, whether you reset the unit and how many times, and the water temperature. Add whether the burner lit at all before lockout, since lit-then-dropped and never-lit send a gas tech to different components. Note the time of day, because a morning-only lockout is a real diagnostic clue. Photograph the data plate too if it is legible, so the model is known before anyone drives out. This takes about two minutes on the stop and does two things: the gas tech skips the water side entirely because you have already ruled it out, and you keep a dated record of what the equipment was doing, which is what makes the eventual replacement quote a documented pattern rather than an opinion.
