Watts 0121344 3/4 IN Lead Free Self Closing T and P Relief Valve, 175 psi, 210 degree F, Test Lever, 4 IN Ext Thermostat

Yes. The 0121344 has a 3/4 inch male NPT inlet that screws into the heater's 3/4 inch tapping, and a 3/4 inch female NPT outlet for the discharge line. The shank measures 1-1/8 inches, so the thermostat probe reaches through a standard tank boss and into the water. Nothing else is needed to make the connection beyond thread sealant and a 3/4 inch drain pipe.

Not usually. The 0121344 is set to relieve at 175 psi, which suits tanks rated for that working pressure, typically commercial or high-pressure heaters. Most standard residential water heaters are built around a 150 psi relief setting, and the valve on a heater must never be set above the tank's rating. Check the heater's data plate for its maximum allowable working pressure first; if it reads 150 psi, order a 150 psi LF100XL instead of this one. The temperature side is standard either way, relieving at 210 degrees F.

Size it by BTU input, not gallons. The 0121344 is rated for 200,000 BTU per hour of pressure steam discharge and 105,000 BTU per hour on the temperature relief side. The temperature figure is the one that governs on a water heater, so the heater's input rating has to be at or below 105,000 BTU per hour. A 150,000 BTU commercial heater needs a larger valve.

The body is a lead free copper silicon alloy, which is what lead free plumbing codes for potable water require, and the thermostat is copper. It is made in the United States. That construction is why the LF prefix is on the series designation.

The 4 inch copper probe reaches past the tank wall and any insulation or lining so it senses actual water temperature rather than the shell. Use the 4 inch probe where the heater tapping sits in a thick wall or where the manufacturer specifies an extended element; a short-probe valve in that spot can read low and fail to relieve at 210 degrees F. The threaded shank ahead of the probe is 1-1/8 inches.

It covers both in one body. The 0121344 provides temperature and pressure protection together: it opens on pressure at 175 psi and on temperature at 210 degrees F. Water heaters need both, which is why a straight pressure-only relief valve is not a legal substitute on a tank. A pressure-only valve belongs on a boiler or a system line where a separate temperature control handles the heat side.

Yes, a manual test lever is fitted, which is what lets a maintenance tech exercise the valve on the annual walk without pulling it. It is also a self closing valve, so it reseats after the lever is released or after a relief event, rather than staying open.