Pool Heater Not Heating? The 12 Most Common Causes, in Order

Pool heaters fail in a more predictable order than almost any other piece of pool equipment, and a large share of "my heater is broken" calls in the Redding area turn out to be a heater doing exactly what its safety circuit told it to do: refuse to fire because not enough water was moving through it. This guide takes the twelve causes we see in the order a technician would rule them out, explains what each one looks like from the outside, and is clear about which checks are yours and which are gas-line work. What each repair costs, and when a heater is worth replacing instead, is on the pool heater and filter repair page — this page's job is to get you there knowing what's wrong.

A note on what's installed here: most Redding residential pool heaters are natural-gas or propane units, which is why they get the most space below. Heat pumps and solar systems fail differently and get their own sections toward the end.

The eight-step diagnostic

1. Confirm the pump is running at full flow and the filter is clean. Run the pump at high speed (or full speed on a single-speed pump). Read the filter pressure gauge: if it’s 8–10 psi or more above its clean baseline, clean the filter first. Empty the skimmer and pump baskets. Most “dead heater” calls end here — the heater was waiting on water flow it never got.
2. Check that the heater is being told to heat. Set the heater to Pool (not Spa) mode, set the thermostat well above the current water temperature, and make sure any automation system isn’t holding it off with a schedule, a “heater cooldown” timer, or a mode setting. On a heater with a display, read the current status line — most modern units tell you exactly what they’re waiting on.
3. Confirm gas supply. Gas heater only. Is the gas valve at the heater open (handle in line with the pipe)? Is the shutoff at the meter or tank open? On propane, is there fuel in the tank? Did another gas appliance stop working at the same time? Do not do anything beyond looking at valve handles and gauges — gas-line work is licensed work.
4. Watch an ignition attempt. Stand by the heater and call for heat. Listen and watch through the sight glass if there is one. No click and no sound: power, a control board, or a safety switch is holding it off. Clicking with no flame: igniter or gas valve. Flame that lights and dies within seconds: flame sensor or a flow/pressure problem. Note the exact sequence and any error code — it points the tech at the right part.
5. Look at the burner tray and the exhaust. Power and gas off. Look in at the burners: rust flakes, a mud-dauber nest, leaves, or spiderwebs in the burner ports are common after a heater sits idle through a Redding summer. Look at the top of the heater for soot or heat discoloration — a sign of poor combustion or a blocked exchanger that needs a professional.
6. Check the bypass and the temperature rise. If the heater lights and stays lit but the pool isn’t warming, measure the water temperature going in and coming out (a pool thermometer at the pad works). A gas heater in good order should raise water temperature noticeably across the exchanger. Little or no rise with a steady flame points to a scaled or bypassing exchanger, or a bypass valve stuck open.
7. Do the run-time and cover math. Note how many hours a day the heater actually runs (it can’t heat when the pump is off) and whether the pool is covered overnight. An uncovered pool in a dry, windy Redding spring can lose heat to evaporation faster than a residential heater adds it. If the heater is only on four hours a day and the pool is open all night, the heater may be fine.
8. Stop and call at the first sign of gas, soot, or a scaled exchanger. Anything involving the gas valve, the igniter, the control board, sensors, or the heat exchanger is CSLB D-35-licensed contractor work in California, and gas-line connections are not a homeowner job under any circumstances. Bring the tech what you observed in steps 1–7.

Cause 1: Not enough water flow (the one to check first)

Every gas pool heater has a pressure switch (some newer units use a flow switch) wired into its safety chain. Its only job is to keep the burners off unless water is moving through the heat exchanger fast enough to carry the heat away — a heater firing into still water would boil it and crack the exchanger in minutes. So when flow drops, the heater doesn't light, and it's not broken; it's protecting itself. Flow drops for boring reasons: a filter that's loaded up after a windy week, a variable-speed pump idling at a low RPM during the hours the heater is scheduled, a skimmer basket packed with oak leaves, a valve someone turned toward the spa. The tell is a heater that lights fine right after the filter is cleaned and stops again two weeks later. Clean the filter, run the pump at high speed, and try again before you assume anything else. If the pressure won't come down after cleaning, the filter media is the problem — the heater and filter repair page covers that.

Cause 2: Settings, mode, and automation

Heaters have a Pool/Spa mode switch and separate thermostats for each; an automation system layers its own schedules, "heater cooldown" delays (the pump keeps running for several minutes after the heater shuts off — normal, not a fault), and sometimes a seasonal "heater off" setting that got flipped in October and forgotten by April. A heater on an automation system also won't respond to its own front-panel thermostat if the system is in control; the display will usually say so. Rule this out before anything mechanical.

Cause 3: Gas supply

A closed valve at the heater or the meter, an empty propane tank, a propane regulator that's frozen or failed, or a gas line that was sized for the original heater and is starving a larger replacement. The starving-line case shows up as a heater that lights, runs weakly or with a lazy yellow flame, and shuts down — it's a common consequence of an upsized heater installed without upsizing the gas line, and it's a licensed-plumber fix. If any other gas appliance in the house stopped at the same time, the problem is upstream of the heater entirely. Your involvement here stops at looking at valve handles and the tank gauge.

Cause 4: Pilot, thermopile, or igniter

Older heaters use a standing pilot and a pilot generator, or thermopile — a small device the pilot flame heats to produce the millivolts that hold the gas valve open. As they age they produce less voltage, the pilot lights but the main burner won't, or the pilot itself won't stay lit. It's a cheap part ($60–$140) and one of the classic "old heater, easy fix" repairs. Newer heaters use electronic ignition — a hot-surface igniter or a spark igniter — and their failure looks like clicking with no flame, or a display error naming an ignition failure after three tries. The igniter, the ignition control module, or the gas valve's pilot solenoid are the suspects, in that order.

Cause 5: Flame sensor

On electronic-ignition heaters, a flame sensor (a metal rod in the flame path) has to "prove" the flame within a few seconds or the control shuts the gas off — the classic lights-then-dies symptom. Sensors get coated with oxidation and stop conducting; sometimes cleaning restores them, often they're replaced ($20–$200). If the heater lights, runs for a few seconds, and drops out, with the flow confirmed good, this is the first thing a tech will check.

Cause 6: Pressure switch or high-limit sensor failure

The safety switches themselves fail. A pressure switch can stick open (heater never fires even with good flow) or get out of adjustment; a high-limit or thermal-regulator sensor can trip early or fail closed. A tech confirms these by testing across the switch — a homeowner sees it as a heater that behaves as though flow is bad when it isn't. The parts are inexpensive ($40–$60 for a pressure switch); the diagnosis is what you're paying for.

Cause 7: Gas valve

The gas valve opens on command from the control and holds open while the flame is proven. When it fails, nothing happens after the igniter fires, or the heater works intermittently as the valve's solenoid weakens. On millivolt (standing-pilot) heaters, a "gas valve" symptom is often actually a weak thermopile — which is why it's worth having someone measure before replacing the more expensive part ($175–$600).

Cause 8: A scaled or corroded heat exchanger

This is the expensive one, and it's preventable. The heat exchanger is the copper (or cupronickel) coil the water runs through above the burners. Pool water with high calcium hardness and high pH — which is where a Redding pool drifts by the end of a four-month hot season if nobody's balancing it weekly — deposits scale on the inside of the tubes. Scale insulates, so the metal overheats, the water inside the tubes can flash to steam, and the heater short-cycles on its high-limit or sounds like it's boiling ("kettling"). Water that runs acidic does the opposite and corrodes the tubes from the inside until they leak — a heater that drips from the bottom when it runs. A lightly scaled exchanger can sometimes be flushed; a corroded or cracked one is usually the point where the repair-or-replace decision lands on replace. Weekly chemistry is the prevention; see what a proper weekly service visit includes.

Cause 9: Bypass valve stuck open

Many heaters have an internal bypass that lets some water skip the exchanger to regulate temperature rise; some installations add an external bypass valve for winter. If the internal bypass sticks open or the external valve was left open, most of the water never sees the burners: the heater lights, stays lit, and the pool creeps up a degree a day. The inlet-versus- outlet temperature check in step 6 of the diagnostic is how you spot it.

Cause 10: Control board or wiring

A heater that does nothing at all — no display, no click, no attempt — with confirmed power is a transformer, a fuse on the board, a wiring fault (rodents, sun-rotted insulation, a corroded terminal), or the control board itself. Boards on heaters more than about ten years old are where parts availability, not just price, starts to make replacement the practical answer.

Cause 11: Run time, evaporation, and no cover

Not a fault, but the most common reason a working heater "doesn't heat." A heater only runs while the pump runs; if the pump schedule is four hours a day, that's the heating window. And heat loss from an uncovered pool is dominated by evaporation — the same dry, breezy conditions that make Redding's summer evaporation a quarter to half an inch a day pull heat out of the water all night in spring and fall. A cover fixes more "heater problems" than any part on this page, and it does it while cutting the gas bill.

Cause 12: The heater is undersized

A heater that's too small for the pool will run for hours and gain little, especially in March or October when the air is cool. Sizing depends on surface area (not gallons — surface area is where the heat leaves), desired temperature rise, and wind exposure. If the heater is original to the pool and the pool has always heated slowly, this is the likely explanation, and it's a replacement conversation, not a repair.

Heat pumps: a different failure list

A heat pump pool heater moves heat from the air into the water, so it's a small air conditioner running backward, and it fails like one: a refrigerant leak (weak or no heating, frost on the evaporator coil), a failed fan motor, a compressor that won't start or trips, a defrost cycle that runs constantly on cool mornings, or the same flow switch and thermostat issues as a gas heater. Two things specific to Redding: a heat pump's output drops as air temperature drops, so it's slow in the shoulder seasons when you most want heat, and its coil fins clog with cottonwood fluff and pine pollen in spring — a gentle rinse of the coil is a homeowner task; anything refrigerant-side is licensed work.

Solar: not broken, just not sunny

Solar pool heating pushes pool water through roof panels and relies on the pump for flow. A solar system that "isn't heating" is usually a controller sensor (roof or pool sensor failed, so it never diverts), a diverter valve or its actuator stuck, air in the panels because the vacuum relief valve failed, a panel leak dropping the pressure, or simply a run of overcast days — solar gains nothing in December. A panel leak on the roof is worth catching early; see the signs you have a pool leak guide for how solar plumbing shows up in a bucket test.

Where the homeowner part ends

Cleaning the filter, adjusting the pump speed and schedule, checking settings, reading a display code, looking at valve handles and a tank gauge, and comparing inlet and outlet temperatures are all yours. The moment the answer involves the gas valve, the igniter, any sensor, the control board, or the heat exchanger, it's licensed contractor work in California under the CSLB's D-35 Pool and Spa Maintenance classification, whose official scope names gas heaters explicitly — and gas-line connections are never a handyman job. Bring the tech the symptom sequence you observed and the error code if there is one; it turns a diagnostic afternoon into a parts call.