Pool Heater Not Working: The Quick Troubleshooting Checklist

If your pool heater won't fire up or runs but can't raise the water temperature, start with these five areas in order: water flow, filter condition, thermostat and settings, fuel or power supply, and water chemistry. Most heater failures trace back to one of these, and you can rule out the majority of them in under 20 minutes without any special tools. If you work through all five and still have nothing, then it's time to call a tech - but most people find the problem before they get there.

Why Pool Heater Troubleshooting Has an Order

Pool heaters - whether gas, propane, or heat pump - are built around safety switches that shut the unit down if anything looks wrong. That's a good thing, but it means a heater that "won't work" is often a heater that worked exactly as designed and shut itself off to prevent damage. Before you assume the heater itself is broken, you need to rule out the conditions that trigger those shutoffs. Checking things in the right order saves you from chasing symptoms instead of causes.

Step 1: Is Water Flowing Properly Through the Heater?

Every pool heater has a flow switch - a small sensor that confirms water is moving through the heat exchanger before the burner or heat pump is allowed to run. If flow is too low, the switch stays open and the heater won't fire at all. This is the single most common cause of a "dead" heater that is otherwise perfectly functional.

Check these flow-related items first:

• Is the pump running? Obvious, but worth confirming - especially if you're on a timer.
• Is the filter dirty or overdue for cleaning? A clogged cartridge or sand filter can drop flow below the heater's minimum threshold. Check your filter pressure gauge - anything 8 to 10 psi above your clean baseline means the filter needs attention.
• Are all valves fully open? A partially closed bypass valve or return valve can starve the heater of flow without you knowing it.
• If you have a variable-speed pump, is it set to a speed that produces enough flow? Some VS pumps run a very low speed during heating cycles. Most heaters need at least 25 to 30 gallons per minute - check your heater's manual for the minimum.

Step 2: Check the Thermostat Setting and Control Board

This sounds too simple, but it happens constantly: the thermostat is set lower than the current water temperature, so the heater sees no call for heat and does nothing. If your water is already 82°F and the dial is set to 80°F, the heater is doing exactly what it's told. Turn the set point up 5 degrees above the current water temp and wait a few minutes to see if it fires.

Also check whether the unit is in "pool" mode versus "spa" mode if you have a dual system. A heater stuck in spa mode won't heat the pool no matter what you do to the thermostat. Look at the control panel and confirm the mode matches what you actually want to heat.

Step 3: Gas Heaters - Check Fuel Supply and Ignition

If you have a gas or propane heater and the unit tries to ignite but won't stay lit - or you hear clicking but no flame - the problem is almost always in the fuel supply or ignition system.

1. Check the gas valve at the meter or propane tank - it should be fully open, not partially closed or accidentally bumped off.
2. If you're on propane, check the tank level. A tank below 20% can cause pressure problems that prevent a heater from lighting reliably.
3. Look at the pilot light or igniter assembly. On older standing-pilot heaters, a dirty or corroded thermocouple is a very common failure point - it senses the pilot flame and holds the gas valve open. If it's failing, the gas valve shuts immediately after ignition.
4. Check for error or fault codes on the display panel. Write down the code and look it up in the manual - ignition fault codes will tell you whether the issue is the gas valve, igniter, or flame sensor.

Step 4: Heat Pumps - Check Power and Ambient Temperature

Heat pumps have a requirement that gas heaters don't: they need warm enough air to operate efficiently. Most pool heat pumps won't run below 50°F ambient air temperature - some models cut out at 45°F or even 55°F. If you're trying to heat your pool on a cold morning or late in the season, the air temp alone might be the issue. Check the spec sheet for your unit's minimum operating temperature.

Beyond that, check the breaker for the heat pump at your panel. Heat pumps draw significant amperage and are more likely than gas heaters to trip a breaker, especially on a hot day when the unit is working hard. Reset the breaker once - if it trips again immediately, stop and call an electrician or HVAC tech, because that's a wiring or component issue.

Step 5: Water Chemistry That Can Cause Heater Problems

Water chemistry doesn't usually cause an immediate "heater won't start" failure, but it causes slow failures that eventually show up as fault codes or a heater that runs but doesn't heat well. Low pH - below 7.2 - aggressively corrodes copper heat exchangers. High calcium hardness - above 400 ppm - causes scale buildup inside the heat exchanger that insulates the water from the heat source and drops efficiency fast. For your heater specifically, keep pH between 7.4 and 7.6 and calcium hardness between 200 and 400 ppm. If you're not sure where your chemistry stands, AquaDoc makes a water testing kit built around the parameters that actually matter for equipment health, including calcium and pH. Good chemistry is cheap maintenance compared to a heat exchanger replacement.

If you want to understand how many BTUs your heater actually needs to keep up with your pool size, that calculation also affects whether your heater appears to "not work" when it's really just undersized for the conditions.

When to Call a Professional

Call a pool tech or HVAC tech (for heat pumps) if: the heater fires but produces an error code you can't resolve with the manual, you smell gas near a gas heater, the unit trips its breaker more than once, or the heat exchanger is visibly corroded or scaled. Some diagnostics - like checking gas pressure at the manifold or testing the capacitor on a heat pump compressor - require equipment and training that most homeowners don't have. There's no shame in drawing that line. The goal of this checklist is to get you to the right call faster, not to turn you into a pool equipment technician. Experienced pool professionals who service equipment regularly will have seen your exact fault code before and can often diagnose it in minutes.

Frequently Asked Questions

Why does my pool heater turn on but not heat the water?

The most common cause is low water flow. The heater fires but water moves too slowly or too quickly through the heat exchanger, triggering a safety shutoff or simply not transferring heat efficiently. Check your filter pressure, make sure all valves are fully open, and confirm your pump is running at the correct speed for your heater's minimum flow requirement.

Can low chlorine or bad water chemistry stop a pool heater from working?

Not directly in the short term, but bad chemistry causes heat exchanger damage that leads to failures over time. Low pH (below 7.2) corrodes copper heat exchangers. High calcium hardness (above 400 ppm) scales the heat exchanger and chokes efficiency. Keep pH between 7.4 and 7.6 and calcium hardness between 200 and 400 ppm to protect the unit long-term.

Why does my pool heater keep shutting off after a few minutes?

Repeated short cycling almost always points to a flow switch issue, a clogged filter reducing flow, or a high-limit safety switch tripping because the water temperature at the inlet is too high. Clean the filter first - it's the most common cause - then verify your pump is producing adequate flow for the heater's minimum requirement.

How do I reset a pool heater?

Turn the power off at the breaker for 30 seconds, then restore it and try again. Most units also have a reset button near the control board - check your manual for its location. If the heater faults again within a few minutes of reset, there is an underlying problem that a simple reset won't fix.

What causes a pool heater to show an error code?

Most error codes fall into four categories: flow errors (not enough water moving through the unit), ignition errors (gas or spark problem on a gas heater), temperature sensor faults, and high-limit trips from overheating. Look up your specific code in the heater's manual - the code will point you directly to which component or condition to check first.