Disabling the ignition or disconnecting the gas source is a good start.
If the tub is not getting hot water the issue is probably with the hot water heater. The hot water heater may have tripped a breaker or the pilot light went out.
low back, spillage, or a bad thermocouple.
Central heating systems usually heat on a different system from the hot water heater, unless the main heater is a combination furnace/hot water heater. If this is the case, it could be because the outflow of hot water is greater than the ability of the furnace to heat the water.
it could be from the hot water heater if it's not turned off. hot water expands pushing out more water when cool the heater can suck in air to fill the voide.
There are a few possibilities why. Troubleshoot asking yourself these questions: Is the hot water heater switched on electronically? Is the stop cock switched on? Are there air locks in the line? Is the hot water heater connected?
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.
Thermocouple. Keep pilot knob depressed for 30 seconds after lighting pilot.
If you have a gas water heater and your pilot light will not light, the most common problem is a faulty thermcouple. A licensed plummer can easily and cheaply fix this or you can find information to do it yourself online.
It's possible hard water deposits have built up inside. To prevent this and the early demise of your water heater, drain and flush the heater at least once a year.
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.
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?
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.
Yes
it sometimes takes around 45 minutes to have hot water
The water continues to run into and out of the water heater. There is just no flame to heat it up.
Right below the thermocouple if applicable