You most likely need a new 'thermocouple' . -A very simple job for an expert, but don't try it if you're not. -It's less than 1 hour of work for a competent gas fitter - so call one.
Thermocouple. Keep pilot knob depressed for 30 seconds after lighting pilot.
If the pilot light is on, the heater should come on by itself. If the pilot light is off then it has to be lit. If it will not stay on, the thermocouple is bad.
High winds, pilot generator. Is the heater located next to a wall? Is the heater door closed? Does the heater have a draft hood or what they call a stackless heater? Ken
Gas not turned on to water heater. Gas valve on water heater not set to pilot setting. Faulty gas valve. Igniter for lighting pilot not working.
Gas water heater has an orifice for main burner and for pilot light, if not electronic ignition.
Have you turned the control from Pilot to On? Have you increased the temperature setting?
Yes
The water continues to run into and out of the water heater. There is just no flame to heat it up.
A number of problems can cause a swimming pool heater not to work. The most common I have found is lack of water flow. A heater needs a certain amount of water flowing through it to work correctly. The next most common problem is due to the pilot not lighting or remaining lit. This is usually due to the pilot generator not working. When you light a pilot light and have to hold it for a length of time before releasing the button you are allowing the pilot generator to heat up and then stay open to keep the pilot lit. Usually you need a licensed person to do work with changing items on a gas unit. The thermostat can be a problem for any water heater. Most of the heaters I have worked on were gas. If you have an electric heater waterflow still needs to be checked and the electrical connections can become corroded in the presence of chemically treated pool water.
Right below the thermocouple if applicable
The flame detector on the pilot assembly is probably defective. If you do not know how to fix this, call you gas supplier and have it repaired.
Same as any hot water heater. Turn the gas valve to off.