Recently installed a new electric water heater with an expansion tank. Water coming into the house is 52 psi, the line that runs to outside faucet is the same pressure but the faucet in the sink after my water heater is at 140 psi and the pressure release valve has a small constant drain and I have am at a lost as to what is causing this.
Sounds like you expansion tank is failed or not pressurized properly. I suggest draining the expansion tank and checking the pressure. If you get any water out of the air valve, your tank has failed.