Make sure the pilot light is switched on.

Double-check that the pool heater is switch on and that the gas supply valve is on. If everything looks good, make sure the thermostat is set higher than the water temperature. 

Most of the time, this means that the thermostat is set too low.

However, it’s possible that your heater is too low for your pool or that you don’t have enough gas to power it. In some cases, the high limit switch may be defective, causing a gas pool heater to shut down before reaching the desired temperature. When it’s too cold outside, you might turn on the heater. If that’s the case, you’ll have to wait until the weather warms up.

A dirty filter, a closed valve, or even reversed water connections could all be to blame.

It’s possible that the thermostat is to blame, and you’ll need to replace it.

Condensation that forms when heating freezing water is a common cause.

It could, however, be the result of a faulty internal bypass or excessive water flow. To reduce the water flow into the gas pool heater, you may need to install an external bypass.

The same chemicals that cause exhaust to corrode can also cause your heater to corrode.

If you notice rust in your pool while the heater is on, it’s most likely due to the gas pool heater. To help prevent corrosion, replacing any corroded heater parts is needed and keeping your pool water balanced is important.

You should check this on any gas appliance first when you encounter any issues with their performance.

The gas pressure may be low, or the air supply may be inadequate, resulting in the gas pool heater not venting correctly. Double-check that there is no obstruction on any of the vents, that the gas supply is on and then try start the gas pool heating system again.