Why is My Swimming Pool Heater Not Working?

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Raypak pool heaterRaypak pool heater
photo: Raypak

It’s a beautiful day, and you decide to have a swim to freshen you up after a long day at the office. You dive in expecting a cool refreshment, but it is way too cool - the pool is freezing cold!

Within a few seconds, you are back out drying off, and when you investigate your curse, you will notice the heater is not working. It’s just ruined your day. So what is the problem, why your swimming pool heater is not working?

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We’ll run through a few things you can check and easily troubleshoot before calling in a technician, and hopefully, you’ll be up and running again quickly.

There are 3 main methods of heating pools. Electric pool heaters are cheaper to run, but they won’t heat your pool as fast as gas will. Gas pool heaters are incredibly fast at heating your pool, but their energy efficiency is low, so they will cost you more to run. Finally, heat pump pool heaters are very economical to run and efficient, provided the air temperature is above 50 F.

Most common pool heating issues & what to do

Everyday pool heater issues are common to electric, gas, and heat pump heaters for your pool.

Here are common issues and what to check:

  1. Power supply
  2. Gas flow
  3. Filters
  4. Variable rate pump
  5. Thermostat
  6. Pilot light
  7. Thermocouple
  8. Heat exchanger
  9. High-Limit switch
  10. Bypass valve
  11. Specific issues

Power supply

The first and most obvious thing to check is the power supply, and is the heater switched to the ‘ON’ position? Has someone disconnected it or switched the breaker off for some reason and forgotten to reconnect it? It sounds really dumb, but it happens a lot. If that’s the issue, it’s quickly sorted, and now you can concentrate on finding out who did it and chucking them into the cold pool as a punishment.

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Gas flow

If everything is connected properly, ensure you have a proper flow of gas to your gas-fired heater. Is your propane tank empty or low? Is there a problem with your city natural gas supply? They occasionally do have service issues, and if the pressure drops, it can cause your heater to shut down. Check with your local provider to see if there are any local problems.

Water valves open and clear

The next thing to check is that your water valves are all open and that you have good water flow. Check your pump is working because most heaters of all types have a minimum gallons per minute (GPM) flow rate requirement before the heater will start. If, for any reason, your heater detects a low flow rate, that could be what’s preventing your heater from starting.

If your bypass valve is defective, it will prevent the heater from working or, if set to bypass, the water flow won’t go through the heater so it won’t operate.

Filters and baskets free of debris

Check that all your filters and baskets are all clean and free from detritus. If you use a sand filter unit, check the pressure gauge, and if it’s reading high, then it’s time for a backflush or perhaps a change of sand. If your filter is blocked, it will increase the pressure of the water going through the system, and increased pressure lowers the water flow, causing your heater to work erratically.

Each setup is different, so the best thing to do is to check the PSI reading with a clean system. Then use that as a baseline or ‘normal’ setting, clean again if the pressure rises above that reading.

Variable-rate pool pump

Do you have a variable rate pool pump? If you do, then if the pump is set at the lowest level, it may not be pumping sufficient water to fire the heater. Turn it up a notch and see if it allows the heater to start.

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Check your thermostat. Has someone moved it down? If so, check the pool temperature, then reset the thermostat above that temperature and see if the heater turns on. If it does, your problem is fixed, and the culprit is heading for the cold pool too!

If its set as normal, then increase the temperature and see if the heater starts. If not, you could have a faulty thermostat, which you can check with a multimeter.

The pilot light on? (gas heaters)

If you have a gas heater that won’t light, then check that the gas is fully on and that you have a good supply pressure. Check the pilot light is on and working. These tiny units work very hard and can clog up with debris, and along with the thermocouple unit, they are common weak links in gas heaters. They are cheap and easy to replace.

The pilot needs fresh air to work, and if the airflow is poor, it may keep going out. If there is low gas pressure or any source of water like rain guttering or sprinkler run off, this can cause the flame to be extinguished.

Thermocouple unit faulty

The thermocouple unit is a sensor that keeps the gas line open when the pilot is lit if the pilot goes out then the gas is turned off. You can purchase a cheap thermocouple tester, which will allow you to check if your thermocouple is working, and if not, it’s cheaply and easily replaced.

Heat exchanger damage or cracks

Your heat exchanger can be damaged by corrosion or have cracks, which will cause it to leak. Damage to your heat exchanger can be caused by incorrect water chemistry, which is why it’s critical to keep your pool water balanced. Issues with your heat exchanger can cause the heater not to come on that includes corrosion, cracks, and leaks, which all may stop the heater working. Heat exchangers are a major cost to repair.

High-Limit switch

Occasionally a heater high-limit switch will fail (your system may have two). These are designed to monitor the air temperature passing over the heat exchanger and will shut it off to protect you and your system from overheating and causing a fire or other serious damage. If they fail, they will prevent the heater from coming on at all or do so intermittently. If you do have two in your system, you can isolate each one in turn and see if the heater comes on. They are easily swapped out and cheap to replace.

Bypass valve defective

The bypass valve is there to divert water from your heater when winterizing the pool or when you need to have the heater clear of water flow for maintenance. With no water flow, the heater won’t work to avoid damage. If your bypass valve is defective, it could reduce water flow to the heater, which will cut out. 

Heat pump specific issues

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Is the air temperature around or below 50 F? If it is, then your heat pump probably won’t come on or work efficiently. Heat pumps are very efficient in high temperatures, but they struggle in air temperatures lower than 50 F. Bad electrical connections, or rusted links can cause it not to start or run intermittently.

Your heat pump pool heater needs a steady water flow to ensure it keeps running, so make sure filters and skimmers are clean and your heat pump is well maintained.

If you discover a pool of water under your pool heat pump, switch it off, leave the pool pump running for a few hours, and then return. If the water has dried up, everything is fine, however, if there’s still a pool under the heat pump, test it with a test strip and see if there’s any chlorine in it. If its normal condensate, it won’t have chlorine, but if it tests positive for chlorine, it will likely be a leak from your heat pump, which needs to be investigated.


The installation and setup of a pool heating system are complex and need to be done professionally; then, it will give you and your family many years of pleasure. If it’s badly installed or not properly maintained, then it can be a nightmare, especially as it gets older. 

Always ensure the basic maintenance is done regularly, it’s a chore but nothing to the pain of a pool you can’t use because of an issue that would have been caught with regular maintenance.

Checking and cleaning filters, baskets, and valves is an easy job that needs to be done regularly, and the system will perform as it should for years to come.  Check for leaks, cracks and obvious corrosion, investigate and repair as soon as you can.

The most critical element you have control over is your pool chemistry, as it will affect not only the experience of the users of the pool but also the components in your system. The chemicals used can damage heaters, pumps, and filter units if incorrectly balanced, so it’s in your own best interest to ensure it is always correct.

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