by Nick Gromicko, CMI® and Kenton Shepard Show Temperature/pressure-relief or TPR valves are safety devices installed on water heating appliances, such as boilers and domestic water supply heaters. TPRs are designed to automatically release water in the event that pressure or temperature in the water tank exceeds safe levels. If temperature sensors and safety devices such as TPRs malfunction, water in the system may become superheated (exceed the boiling point). Once the tank ruptures and water is exposed to the atmosphere, it will expand into steam almost instantly and occupy approximately 1,600 times its original volume. This process can propel a heating tank like a rocket through multiple floors, causing personal injury and extensive property damage. Water-heating appliance explosions are rare due to the fact that they require a simultaneous combination of unusual conditions and failure of redundant safety components. These conditions only result from extreme negligence and the use of outdated or malfunctioning equipment. The TPR valve will activate if either water temperature (measured in degrees Fahrenheit) or pressure (measured in pounds per square inch [PSI]) exceed safe levels. The valve should be connected to a discharge pipe (also called a drain line) that runs down the length of the water heater tank. This pipe is responsible for routing hot water released from the TPR to a proper discharge location. It is critical that discharge pipes meet the following requirements, which can be found in InterNACHI’s Water Heater Discharge Piping mini-course, at www.nachi.org/education. A discharge pipe should:
Leakage and Activation A properly functioning TPR valve will eject a powerful jet of hot water from the discharge pipe when fully activated, not a gentle leak. A leaky TPR valve is an indication that it needs to be replaced. In the rare case that the TPR valve does activate, the homeowner should immediately shut off the water and contact a qualified plumber for assistance and repair. Inspectors should recommend that homeowners test TPR valves monthly, although inspectors should never do this themselves. The inspector should demonstrate to the homeowner how the main water supply can be shut off, and explain that it can be located at the home's main water supply valve, or at the water supply shut-off for the appliance on which the TPR is mounted. TPR Data Plate Information
Although a TPR valve might never become activated, it is an essential safety component on boilers and domestic water heaters. Guidelines concerning these valves and their discharge pipes reflect real hazards that every homeowner and home inspector should take seriously. More information about this subject can be found in InterNACHI's Water Heater Discharge Piping mini-course, InterNACHI's Plumbing Inspection course or by contacting a qualified plumber. Where should pressure relief valve discharge pipe drain to?Item 5 requires that the discharge piping serving relief valves, discharge to the floor, to the pan serving the water heater or storage tank, to a waste receptor or to the outdoors.
How far off the ground does a relief valve have to be piped by code?Discharge pipe shall discharge independently by gravity through an air gap into the drainage system or outside of the building with the end of the pipe not exceeding 2 feet (610 mm) and not less than 6 inches (152 mm) above the ground and pointing downwards.
Can you use PVC for the T&P discharge piping?The T&P discharge pipe must be rated for hot water; PVC pipe is not hot water rated and must not be used as a T&P discharge pipe. PVC pipe is only cold water rated and can melt or burst if used for pressurized hot water.
What size must a temperature and pressure relief valve discharge pipe be?The Temperature/Pressure relief valve mouth must point down, and a 3/4" I.D. (or greater) discharge tube must be attached to the T&P Valve's opening and routed down close to but not touching the floor level, typically near a floor drain and stopping 6" above the floor.
|