The heater calls all sound the same this time of year. It worked in July, nobody touched it, and now the nights are cold and the thing will not come on. Here is the part worth knowing before you pay anyone: a heater that misbehaves is usually reporting a problem somewhere else in the system, and occasionally the problem has whiskers.
Heaters fail in a handful of recognizable ways, and what the unit actually does narrows it down fast. Find your row, then read the section below it.
| What it does | What that usually means | Where to start |
|---|---|---|
| Completely dead — no display, no click | Power or a control signal, not the heater. A tripped breaker, a timer, or automation that never asked for heat. | Breaker, then the automation schedule |
| Display lights up, never fires | Gas supply, ignition, or a safety switch refusing to let it light. | Diagnostic |
| Fires, runs a few minutes, quits | Our most common call. Usually flow, sometimes a safety switch or poor heat transfer. | Filter first, then read below |
| Runs fine, pool never warms up | Frequently not a fault at all. Run time, heat loss overnight, or sizing. | Run hours and a cover |
| Quit suddenly mid-season, no warning | Often wiring damage rather than a failed part. Rodents are a real category here. | Open the cabinet and look |
Every row below the first one is worth understanding before you spend money.
A gas heater is not permitted to run without enough water moving through it, so when something is wrong it shuts the burner down deliberately. Short cycling is the heater protecting itself. It is a symptom, and several different things sit behind it.
Most often it comes down to flow:
But flow is not the only answer, and this is where a guess gets expensive:
Those last three cannot be sorted out from the driveway. They need a technician with gauges on the system, checking actual flow and actual temperature rise. That is what a real diagnostic buys you, and it costs a great deal less than a heater.
If a heater gets replaced and the new one short cycles the same way within a season, the old heater was almost certainly never the problem.
A heater cabinet is warm, dry, sheltered and nobody opens it for months at a time. From a rodent’s point of view that is a finished basement. We find nests inside heaters regularly, and where there is a nest there is usually chewed wiring.
It matters because a chewed sensor wire will stop a heater just as completely as a failed control board, and it costs a fraction to put right. What we typically find:
So if a heater quit suddenly with no lead-up, opening the cabinet and looking is part of a proper diagnostic. It is also why we would rather inspect a heater than take a phone description as gospel.
This one is usually physics rather than a fault. A pool loses most of its heat off the surface, overnight, to the air. A few degrees is normal, and several on a cold night or with wind moving across the water. If you are losing far more than that, something else is going on and it is worth having looked at.
Before you conclude the heater is weak, check three things:
If you are weighing running costs at the same time, our comparison of heat pump vs. gas pool heaters goes through which one actually suits a Southern California season.
Those five account for a surprising number of “dead” heaters, and none of them need a technician. Past that line it is gas, high voltage and control boards, and that is licensed work. Not because it is mysterious. Because it is gas.
One myth worth putting down: a tripped GFCI is rarely a heater problem. That is much more often a pool light. If a GFCI keeps tripping, look at the lighting circuit and have the heater diagnosed on its own.
A Pentair MasterTemp or a Jandy JXi will show a code or a light pattern instead of just sulking, and that code narrows the fault quickly. It tells you which direction to look. It does not tell you why, which is a different job.
We have written both up properly, code by code: Pentair MasterTemp error codes and Jandy JXi and LXi error codes. If yours is not on either list, photograph the display and the equipment pad and send both over.
Gas heaters in hard water do not last as long as the brochure implies. The question we ask on site is not how old the unit is but what failed, and how clean the rest of the system is.
| What failed | Usually | Why |
|---|---|---|
| Ignitor, flame sensor, pressure switch, thermostat | Repair | Wear parts. Replacing them on a sound heater is ordinary work. |
| Chewed or corroded wiring | Repair | Cheap to put right once you have found it, which is the hard part. |
| Control board, heater otherwise sound | Usually repair | Worth doing if the exchanger and burner tray are in good condition. |
| Heat exchanger scaled through or leaking | Replace | The expensive component. Once it is gone, the rest is not worth rebuilding around. |
| Corroded burner tray on an older unit | Replace | Combustion parts plus age together push it past economic repair. |
Which of those you are looking at is exactly what a diagnostic establishes, and it is worth paying for before a replacement quote. Real numbers for both paths are on our pool heater repair cost page, and the whole service is described on pool heater repair & installation.
Our water is hard, and a pool concentrates it. Every gallon that evaporates leaves its minerals behind. You can see the result on the tile line. You cannot see it inside the heat exchanger, which is where it does the expensive damage.
That is why a heater here can be short cycling at six years old while the same model runs longer in a soft-water state. And it is why replacing a heater without talking about water chemistry only resets the clock. Sometimes the honest finding is that the heater is fine and the water is not, which is a better conversation than the one most people are braced for.
Send a photo of the equipment pad and the display, tell us what it does, and we will tell you what it points to. If it needs a proper diagnostic we will say so, and what that involves.
Get a Free EstimateACE Pool Repair & Remodel is a licensed California pool contractor, CSLB #1075061, based at 251 E Imperial Hwy, Fullerton, CA 92835, serving Orange County since 2014. We do not run after-hours emergency calls, so if the heater is down, morning is the honest answer. Related reading: heat pump vs. gas heater, heater repair costs, all equipment services.
