Pool Heater Not Working? What to Check

The short answer

If your pool heater is not working, the most common causes are:

• the thermostat or mode is set wrong
• low water flow from a dirty filter, clogged basket, or failing pump
• a tripped breaker or power issue
• a gas supply or ignition problem on a gas heater
• a bad pressure switch, sensor, thermostat, or control board
• scale, soot, or internal wear in an older heater

Start with the easy checks you can do without opening the heater. Make sure the pool pump is running, the filter is not badly dirty, valves are open, and the heater is actually set to heat the pool or spa.

If the heater still will not start, keeps shutting off, smells odd, trips the breaker, or shows an error code you do not understand, it is time to get help. Problems with gas, electricity, combustion, and internal controls are usually not DIY jobs. A licensed and insured pool pro can test the system safely, explain the scope in writing, and help you compare whether repair or replacement makes more sense. You can also review typical pricing at [/costs/] and get free help finding a pro at [/get-matched/].

Safe checks you can do first

Before you call for service, go through this short list. It can save time, and sometimes the fix is simple.

1. Check the settings
Make sure the heater is on the right mode. Many units have separate settings for pool and spa. Confirm the set temperature is higher than the current water temperature.

2. Confirm the pump is running
Most heaters will not fire if there is not enough water moving through them. If the pump is off, air-locked, noisy, or losing prime, the heater may shut down to protect itself.

3. Look at filter pressure and water flow
A dirty filter can reduce flow enough to stop heating. Empty the skimmer and pump baskets. If your filter pressure is much higher than normal, the filter may need cleaning.

4. Check valves and bypasses
Make sure return and suction valves are open the way they should be. If your setup has a heater bypass, check that it is not sending water around the heater instead of through it.

5. Verify power or gas basics
For electric or heat-pump systems, check for a tripped breaker. For gas heaters, confirm the gas valve is open and other gas appliances are working, if you have them. If you smell gas, stop there and follow utility safety guidance.

6. Read the display or error code
Many heaters show a short code that points to low flow, ignition failure, pressure switch trouble, or a sensor issue. Write the code down before you reset anything.

7. Check the outside conditions
Heat pumps often struggle in cold air. Some units will not heat efficiently below certain temperatures. Strong wind can also affect some heater performance.

Do not take apart burners, wiring, gas lines, or sealed panels unless you are qualified to do that work. Pool equipment mixes water, electricity, and sometimes fuel. That is not the place to guess. If you want a simple overview of the system parts, see [/guides/pool-equipment-explained/].

What the symptoms usually mean

The pattern matters. Here is what owners commonly see, and what it often points to.

• Heater will not turn on at all: power issue, breaker, timer setting, bad controls, or failed igniter on some gas units.
• Heater starts, then shuts off quickly: low flow, dirty filter, pressure switch problem, overheating, scale buildup, or a bad sensor.
• Display is on, but water stays cold: wrong mode, thermostat issue, bypass valve problem, undersized heater, or heat loss from weather and evaporation.
• Unit says "low flow" or similar: clogged baskets, dirty filter, closed valve, weak pump, air in the system, or a failing pressure switch.
• Gas heater clicks but does not light: gas supply issue, igniter failure, dirty burner area, or flame sensor problem.
• Breaker trips when heater runs: electrical fault, short, failing motor or fan on a heat pump, or internal component failure.
• Rust, soot, or water around the heater: internal damage, venting issues, leaking exchanger, or age-related wear.

Age also matters. If the heater is older and has had repeated service calls, a repair may not be the best use of money. A typical heater repair might run about $150-$700 depending on the part and labor. A full heater replacement is often around $2,000-$5,000+ installed. The real price depends on the heater type, size, fuel or electrical setup, access, code needs, and your area.

If a pro suspects low flow is part of the problem, they may also inspect the pump and filter side of the system. In some cases the heater is fine, but another part of the circulation system is causing the shutdown. That is one reason it helps to hire a pro who handles broader equipment repair, not just one symptom.

When to stop troubleshooting and call a pro

Some situations are clear stop signs.

Call a licensed and insured pool pro soon if:

• you smell gas or see scorch marks
• the heater trips the breaker more than once
• there is burning smell, smoke, or buzzing from the unit
• the display shows repeated error codes after basic flow checks
• the heater leaks water from the cabinet or internal area
• the unit lights, then bangs, rumbles, or shuts down hard
• you suspect wiring, ignition, sensors, control board, or heat exchanger trouble

Ask for three things:

1. Diagnosis in plain English
What failed? Is it a flow problem, a bad part, a gas issue, or age and corrosion?
2. Price and scope in writing before any deposit
You want the part, labor, and any recommended extra work listed clearly.
3. License and insurance details
Verify them yourself. Do not rely on a verbal promise.

PoolSteward is a free matching service. We do not repair heaters or send our own crews. We help you get connected with licensed, insured local pros so you compare options, you choose who to hire, and you hold the final payment. Start here: [/get-matched/].

What to do next and how to avoid another failure

Once the heater is working again, a few habits can help it last longer.

• Keep water chemistry in range. Bad chemistry can shorten heater life, especially internal metal parts. A pro often doses chemicals more safely. Store chemicals safely, never mix chemicals, and follow label directions. For a simple owner overview, see [/guides/pool-water-chemistry-basics/].
• Stay on top of circulation. Dirty filters, full baskets, and weak flow make heaters work harder.
• Do not ignore small symptoms. Short cycling, slow heating, error codes, and rust stains often get worse.
• Use regular service if you are struggling to keep up. Typical weekly pool maintenance is often about $30-$90 per visit or $100-$350 per month, depending on the pool, what is included, and your area.
• Winterize or close correctly if your climate requires it. Freeze damage can ruin plumbing and heater components.

And one more thing: if the reason you need heat is because children will be swimming, remember that pool safety comes first. Drowning is fast and silent. Never leave a child unattended near water. Use layers of protection like fences, self-closing gates, alarms, covers, and close supervision. Follow local safety and building codes. A warm pool is not a safer pool by itself. Review the basics here: [/guides/pool-safety-basics/].