Why is the TPR valve dripping on my new water heater?

Question

I recently install a Kenmore #58240 40 gal. electric water heater. I had a 50 ga. approx. 12 yr. old Rheem which started to pop a little so I just decided to get a new one. After I installed it at factory set of 120° F, I noticed the TPR valve was cracking open and dripping as the water was being warmed up after usage.

I had the house pressure checked overnight using an outside faucet to record the house pressure for 24 hrs. and the meter showed that the 150 P.S.I. occurred thru out the night. My incoming water line meter was replaced around 6 yrs. ago and I was informed that the meter has a double check valve in it (closed system now).

My old water heater never had the TPR valve open even after they replaced the meter with the check valves in it. I do not have an expansion tank on the incoming line and am curious why I am seeing this pressure spike all of a sudden.

The service man came and replaced the TPR as a shot gun approach but I checked it immediately afterwards and the same dripping from the TPR valve occurred.

Answer 1

Sounds like you need to install an expansion tank, and possibly a pressure regulator (or pressure reducing valve).

When a check valve is installed, it becomes a closed plumbing system. Because it's closed the pressure in the system increases, as the heater heats the water. An expansion tank that is properly charged (charged to the "normal" water pressure) will "absorb" the pressure increase, keeping the system at a steady pressure.

Follow the installation instructions provided by the expansion tank manufacturer. Typically the tank has to be on the cold water supply of the heater, and within a specific distance to the heater. You'll have to measure the pressure at this point in the system, to determine what to charge the tank to.

Expansion tanks typically have a maximum charge value, which should not be surpassed. If you measure the normal incoming water pressure (without the water heater running), and the pressure is above this rating. You'll have to install a pressure reducing valve (PRV), to prevent the incoming pressure from being too high. Plumbing code calls for a PRV, whenever the incoming pressure is greater than 80 psi (See this question for more info).

Answer 2

found this online at structure tech

If a recently replaced T&P relief valve starts leaking again, it probably means that the relief valve is only doing it’s job; it relieving excess pressure in the water heater. When this happens, the fix gets a little bit more involved. I’ll explain.

When a water heater heats up the water in the tank, the water expands. When this happens, the water typically ends up expanding back out the cold water inlet, all the way back to the water supply coming in to the house. The municipal water supply for the house acts as a gigantic expansion tank… and nobody notices.
Natural expansion What would happen if a one-way valve, or check valve, was installed on the water supply piping for the house? The water wouldn’t have anywhere to go. As the water heater heats the water, it expands, which builds up pressure in what is now essentially a closed system. When the pressure builds up enough, the T&P relief valve on the water heater just does it’s job and relieves the excess pressure by leaking a little water. The Fix When a closed system exists on the water distribution piping in a home, an expansion tank needs to be installed somewhere on the plumbing system. This is a fairly simple and straightforward fix; an expansion tank will give the water somewhere to go when it expands, and the T&P relief valve on the water heater will stop causing problems.