> That thing is probably what is probably known as a T/P
> (Temperature/Pressure) valve. Its purpose is to get rid of excess
> pressure in the water heater if there is a problem. They usually are
> vented to the exterior of the structure, rather than just screened off.
> Are there just 2 plumbing lines going to the tank, cold in and hot out?
> If so, then it is probably a T/P valve. All the water heaters I have
> seen have that valve either mounted on the top or the side of the tank,
> rather than on the plumbing near the tank.
I agree, it's probably a temperature/pressure relief valve. And I have seen
them incorporated with the outlet fitting, on very old heaters.
"Code" normally requires that they be vented at least to a catch pan, preferably
a drain (or outside), but it's not unusual to find them simply not connected to
anything. You are also supposed to test them periodically, by opening the valve
by hand (should be a lever on it for that purpose).
> T/P valves usually leak because either the inlet pressure is too high,
> or the heater is set to a rather high temperature. Try turning the
> temperature down on the water heater and see if it stops leaking.
The "temperature" setting should be far higher than the water heater setting
will go, it's primarily for when the thermostat fails "on" and does not shut off
the heat. And, as long as the inlet line from the water main is not blocked
off, the pressure is limited by the mains pressure.
So, in my experience, a leaking T/P valve means it needs to be replaced. If
memory serves, that is even mentioned in the owner's manual that came with my
hot water heater.
http://www.factsfacts.com/MyHomeRepair/PressureRelief.htm
Randall
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