I have someone coming out tomorrow afternoon. The valve is soldered to a series of copper pipes that go up into the ceiling and then out. No DIY job here to replace the valve.
Kirk, I misread, I thought it was at the base of the water heater, but you wrote base of the house,
the pipe that is plumbed out is the PRV as Tony says, and it does sound like it is starting to leak...
You can purge it and hope it reseats, if you're feeling lucky.
A plumber can replace it, but how old is it?
My 12 year old at the lake just quit from the IRMA Outage, so I'm going tonight to try and see if I can reset it on the internal thermostat, or decide if I'm going to replace the heating elements and thermostats... $35 vs $500, but 12 years on a well is about all you can hope for. Its a shorty under the house, PITA!