help with water heater

When the water heat is on and heating water there is water dripping out the pressure relief valve. Does this mean it is broken?

Maybe, maybe not. If you have an expansion tank, it may be shot. If you don't have one, maybe you need one. Sometimes when the valves get old, they start to leak, but it is designed to release water if the pressure gets too high, or the temperature gets too high.

Now, you ask why the pressure would be high...heating water expands it. Since the pipes are basically pretty strong, there's no place for it to go except push back out to the street, or if it can't do that, out the weakest point which is usually the relief valve.

To provide a place for the water to expand into, you use (ta-da) an expansion tank. This is basically a tank with a flexible membrane where one side is filled up with pressurized air, and the other is connected to the cold water supply. When the WH runs and the valves are off, it expands into the expansion tank, keeping the pressure fairly constant. When you open a valve, the bladder pushes that water out, and the water flows normally.

A pressure reduction valve will block off the water from going back out to the street as will a check valve. Some water meters have one in them as well. So, if one of these exists in your system, when the water expands, there's no place for it to go, and the valve leaks until the pressure is reduced.

Change the valve first, if it still leaks, turn the heat down or check the incoming pressure.

I do have an expansion tank but it is connected to the boiler. When I knock on it it sound pretty solid.

Are you talking about a hydronic heating system or are you talking about domestic hot water?

Its for a regular home hot water/ heating system. I think it's a closed system.

Jim covered it. Faulty pr. valve or t&p or both........Some expansion tanks can be drained and some cant. If the bladder is shot you have to replace exp. tank......

I will change the pressure valve first. If I need to change the expansion tank, how do I do it? Do I just turn the knob on top off and then just unscrew the old tank and put in a new one?

Yes,shut the valve off, then unscrew the tank to remove it. The new one should be pressurized to about the same as the normal incoming water pressure. Note, if the pressure is good in the air bladder, the tank should be nearly empty. If it sounds full when you knock on it, it is probably shot. THere is an air valve like the one on your tire. Check it with a tire pressure gauge. If you get water out, replace the tank. If not, see what pressure there is in there. It should be about the same as your normal incoming waterpressure.

Note, the hot water heating system's expansion tank is separate and distinct from any you might have on the normal potable water system; they are separated by a backflow preventor device. If you domestic water system is closed because of a check valve, you need one there as well as in your boiler circuit.