There have been too many times, during the course of an inspection, that I have discovered the discharge pipe for the Temperature Pressure Relief valve (TPR valve) missing or incorrectly sized on a property’s water heater. The valve is located near the top (or on top) of the water heater. It is designed with a lever that can be moved to open the valve, and a discharge pipe attached to it that runs from the valve straight down to the bottom of the water heater. The valve is designed to open if the water temperature, or water pressure, inside the tank gets too high. Without the TPR valve, the water heater would run the risk of exploding if the temperature or pressure got too high.
Water heaters have temperature-limiting devices that shut off the energy source if the regular thermostat fails and temperatures become excessive. This thermostatically controlled valve is the first line of protection against a water heater explosion. The TPR valve is the second line of defense.
Water heater explosions can occur due to excessive water temperature and structural failure of the tank. As long as temperatures inside the tank stay under 212ºF, excessive pressure alone will not cause a water heater to explode. Increased pressure inside the tank causes the tank to leak, which relieves the pressure. Water may drip, spray or pour from the tank but the structural integrity of the tank will remain. The industry standard for the TPR valve to open is set at 210ºF and/or 150psi.
A properly functioning TPR valve begins discharging water when there is excessive pressure or excessive temperature or both. As the water is released, the pressure drops and the water is replaced with water from the cold water line, which then lowers the temperature also.
What happens if the discharge pipe is missing? If the valve opens, the over-heated water can discharge rapidly through the opening in a scalding steam, risking injury (or worse) to anyone nearby.
A discharge pipe controls and directs the release of the water.
A controlled release requires a proper installation, also. The discharge pipe should extend to within 6″ of the floor. This allows for a safer emergency discharge. A pipe ending higher than 6″ increases the risk of spray throughout the area. A pipe ending lower opens the possibility of cross-connection issues. (A discharge pipe extending too close to the floor risks getting submerged, which could siphon dirty water back into the tank.)
I also don’t recommend extending the discharge pipe to a drain, or the exterior of the structure. In addition to plumbing codes frowning on it, a remote discharge means the owner will not know if there is a problem with the valve. When the valve releases, sometimes it is a blast and other times it is a trickle or perhaps just a drip. When discharge is observed, a plumber needs to be contacted to assess the water heater and TPR valve.
Whether it is a worn out TPR valve, or a failing water heater, the discharge pipe needs to be present and properly installed for the health and safety of the household.