Water Heaters - Another water heater overflow question

Hi,
My gas-fired hot water heater is currently ejecting water out the overflow line. (About 1 quart at a time). I've read the other threads in this forum relating to this condition, but I'm still confused. How can I tell whether this is normal operation caused by too high a temperature or pressure or whether it is a defective release valve? I presume this is a pretty easy job and doesn't really require a plumber?
All suggestions will be appreciated.
Joe

It is not an overflow line but the drain from the Temperature and Pressure (T&P) safety relief valve.

No, it is not normal.

I'll bet that you have a pressure regulating valve (PRV) on your city water supply piping or at least a check valve somewhere between the cold water connection of your water heater and the city main. I'll also bet this occurs after a fairly large usage of hot water.

If I am correct in the above assumptions then what you are experiencing is most likely thermal expansion of the water in your heater. The water piping being a closed system (when water is not being used) will cause an increase in pressure when the water is heated. The result is when the expanding water raises the pressure to the set point (usually about 150 psi) of the T&P valve the valve opens to relieve the excess pressure. Due to the relative incompressibility of water only a small amount needs to be expelled to bring the pressure down to normal.

The cure is to install an "expansion tank" on the cold water feed to the water heater. If you already have such a tank it is likely defective and needs replacement.

"I'll bet that you have a pressure regulating valve (PRV) on your city water supply piping or at least a check valve somewhere between the cold water connection of your water heater and the city main. I'll also bet this occurs after a fairly large usage of hot water."

You are right on both counts.

"The cure is to install an "expansion tank" on the cold water feed to the water heater. If you already have such a tank it is likely defective and needs replacement."

No, I don't have an expansion tank currently installed. I'll look into it. Thanks for the device.
Regards,
Joe

I replaced the Relief valve and I still get a slow drip. The one thing I don't understand is why this just started to happen. I have been in my house for 7 years with no problem. I have read that maybe I need an Expansion tank and if so is this very difficult to install.

I htink Furd's favorite water heater answer is to install a expansion tank lol. But he is usually accurate in his judgement :).

They are easy to install. You just have to add a tee in on the cold line going to heater (looking up) and then add a female adapter (facing up). Buy a expansion tank and add them. Other way is too add just a relief valve outside on your incoming water line (this is allowed in most codes instead of expansion tank).

Expansion tank is best way to go because if needing replaced it is easy as snapping your fingers.

Reasons you may not have needed one before coud be traced to age of heater...thermostats...elements....changes to plumbing system over time. Also some house don't have backflow devices or PRV valves and counties when doing road work will come by and add them at a later date..sometimes straight to the meter and people have no idea what they are. They don't inform you, cause thei usually not plumbers, that you have to add a expansion device.

Thanks for the additional information.
Joe

Thanks for all the information this helps greatly

Jeff