What Is HVAC T&P Port?

The T&P Port and Valve

• A T&P port is a location on a water heater or hot-water tank for the installation of a temperature- and pressure-relief valve. These valves are typically calibrated to relieve if the water exceeds a temperature of 210 degrees Fahrenheit, or a pressure of 150 pounds per square inch; most thermostats have a working ceiling of 140 degrees Fahrenheit, and systems seldom have design pressures of more than 75 PSI. The valve allows water to be discharged from the over-stressed system, thereby protecting it from damage.

Operation

• The T&P valve is a safety feature. In normal operation, the T&P valve should do nothing; no water should ever be discharged. The valve exists to relieve pressure during abnormal or emergency states only. Such states commonly occur as a result of a thermostat or heater failure. To read the conditions inside the heater or tank effectively, the valve must be installed in such a way that its temperature-sensing element is immersed in the top 6 inches of water. The T&P port is located so this is possible.

Thermal Expansion

• Forty gallons of water expands by approximately 1/2-gallon between its cold state and the typical household thermostat setting. This extra volume translates into a pressure inside the system that must be discharged; if a combination of circumstances occurs where the heater or tank would otherwise explode, the T&P valve releases the pressure and the explosion is averted.

Discharge

• A discharge line should be connected to the T&P valve’s outlet; there should be no shut-off valve installed in the discharge line, and no cap at its end. The discharge line must run to a place that is safe for venting scalding water. Local building codes may mandate a specific pitch at which a discharge line must descend from the T&P valve to the vent; check with your local code enforcement office before installing the line. If the discharge line vents into a bucket or other container, the home-owner can check occasionally to see if a problem is indicated; this is not possible if the line discharges directly into a drain.

Maintenance

• Frequent relief of pressure through a T&P valve can cause mineral deposits to build up on the valve seat. This will render the valve inoperative. An inoperative valve will be unable to relieve system pressure in a true emergency situation. Conversely, those same minerals can block the waterways inside a T&P valve that is never used; to prevent this, the external valve lever should be operated at least once every year by the home-owner. Turn the water heater off before doing this, check that the discharge line is connected and open, and stand in a location where there is no risk of scalding.