I recently ran into this Temperature Pressure Relief Valve Extension reduction while inspecting a home. I have heard of this being done but had never seen it. While a TPR Valve might look like a simple plumbing job they serve a very serious purpose and there are specific guidelines regarding their installation. These guidelines are in place to insure the appliance is as safe as possible. What was created here is a potential safety hazard. Never reduce the size of the TPR discharge!
A Temperature/pressure-relief or TPR valve is a safety devices installed on water heaters and other home appliances that are designed to automatically release water in the event that pressure or temperature in the water tank exceeds safe levels.
If the TPR Valve malfunctions, 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.
The TPR valve will activate if either water temperature or pressure exceed safe levels. The valve should be connected to a discharge pipe or TPR Valve Extension 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 pipe installation be made following the manufacturers guidelines. Typically, the extension should not be smaller than the diameter of the outlet of the valve it serves (usually no smaller than 3/4"), nor should it be reduced in size from the valve to the discharge point. Reducing the size of the discharge restricts the ability of the valve to vent pressure in the manner intended.