The one on the water heater is required by many codes and is the most crucial one in my opinion. Not sure which you mean by the "second" one.

Some places require a separate PRV if supply pressure is high. Best to ask a local plumber or look up local code requirements.

I understand the one on the hot water heater. There is another pressure relief valve that was installed when the house was built. It's this other valve that's in question.

Yesterday, I disovered that valve was leaking. Discovered that the pressure reducing valve coming into the house was shot and repalced it. The pressure in the house was 90 psi. After replacing the PRV, this other pressure relief valve was still leaking (but not as much.) So, I have to replace or remove the offending valve.

Dave,

Sounds like you may be confused between a PRV (pressure reducing valve or pressure regulating valve) installed at the water service entrance and the T&P (temp & pressure) relief valve installed on the WH.

A PRV is installed at the service entrance to lower the street pressure to an acceptable PSI for the houses plumbing. Depending on water conditions they go bad and need to be replaced from time to time. They are usually set to around 60 psi.

A T&P relief valve on the WH is a safety device and is installed so the water heater doesn't explode if pressure problems arise. When these T&P valves leak they MUST be replaced and with EXACTLY the same specification part or maybe... BOOM.

The expansion tank should be check annually when you drain your water heater. You do drain your WH annually don't you?. To do that you put an air gauge on the air valve. If the bladder has failed then you'll get water out of the air valve. If you don't get water then you must drain all the water from the tank, which usually requires removal, and the air bladder needs to be pressurized to what the water pressure is when the tank is empty.

Nope...no confusion. There are three valves. The pressure regulator on the line coming into the house, the T&P relief valve on the water heater and a third pressure relief valve that has the vent line running outside. This third valve was leaking...I replaced it today. My question is do I need this third valve. If not, I'll unscrew the new one and put in a plug.

Is the third valve a T&P valve? If it is they are cheap and there's nothing wrong with having two. Might be code in your area to have two.

"Is the third valve a T&P valve? "

Probably cheaper and easier to find than a pressure only valve since every water heater requires one.

PRVs typically turn the house into a partially sealed system since they do not allow back flow.

This post was edited by brickeyee on Mon, Apr 15, 13 at 10:13

It's just pressure and I found one at the second hardware store I went to for about $8. Rather than take the risk that it was code, I just replaced it. It was easy enough to do. Next time, It will only take 30 minutes to swap instead of 1.5 hours. :-)