Water heater gas water heater blows water from the pressure relief valve

See post below.

If the valve is "blowing water out" you could have a legitimate problem causing the heater to over heat or high water pressure issues. If that's the case, I would have a professional look at the heater as soon as possible.

If it's just a small steady drip or stream of water, over time sediments and chemical residue build up and take a toll on the Temp/Pressure relief valve causing corrosion. Also, the spring mechanism can become weak over time.

You can try opening the valve and letting it run for a minute or so to flush it out. More than likely, this will not fix it though. PRV's simply screw out of the heater and can be replaced pretty cheap. You would need to drain the heater to below the PRV and then cut your PRV line and you can make quick work of replacing it.

As water heats, it expands and the excess has to go somewhere. If you have a back flow preventer, but not an expansion tank, that leaves the PRV.

The water utilities are doing a better job installing the back flow preventers, than seeing to it that the then required expansion tank is installed.

Labman's correct. If you have a sprinkler system and no expansion tank installed on the heaters cold water supply line then you've found your problem. Regards, Tom