Water Heaters - Changing out a gas thermostat

Our water heater will only heat the water as long as the temp. is set on the highest point. Then it must be turned down to low within 20-30 minutes or the pressure relief system spews hot steamy water everywhere. Of course, the water is tepid by nightfall. If it is set on a med. number, it will kick on, but barely heat the water (lukewarm) before automatically shutting off. The local heating/plumbing people said the Gas Valve Control and Temp. adjustment dial had gone bad.

In order to replace a bad Gas Valve Control and Temp. Adjustment dial with a new one, do you have to disconnect the inlet/outlet pipes, after draining the hot water from the tank? I was told you must do this. I have natural gas, so any info would be greatly appreciated, since my mechanical skill are fair, but only if I know what I'm doing. Thanks in advance.

You do not have to disconnect the water piping but you do have to disconnect the gas supply piping to the valve and the burner piping from the valve to the burner. When reassembling you need to use the proper pipe sealant on the gas supply piping.

If you have never done any work on gas appliances this is NOT a good place to start. It is easy to get what seems to be a minor gas leak but no gas leak is ever minor.

If this water heater is more than five years old it may not even make any economic sense to try to replace the thermostatic gas control valve. They are not cheap and the "average" life of a gas water heater is between eight and ten years. You may be better off replacing the entire water heater.

I will certainly take all you said under advisement.