This is usually caused by one of two things - high main water pressure or a back flow prevention/check valve. The other reason for the TPR valve to run water is high pressure in the water heater tank. It must terminate 6″-12″ above the floor, and the end cannot be threaded or have a fitting which permits connecting a plug or cap. The pipe size must match the opening size of the TPR valve discharge (usually ¾ inch). This includes most rigid wall copper, iron and, in most places, chlorinated polyvinylchloride (CPVC plastic not regular PVC) pipe. Note: If you also replace the discharge tube, it must be made of a material that’s rated for both high temperature and pressure. Wrap some teflon tape on the threads of the new valve when you replace it. Don’t drain a lot of water until you’ve broken the valve loose, so the weight of the water helps keep the heater from moving while you pull on the wrench. You only need to drain enough water to get below the level of the valve. Turn off the gas or electricity and cold water supply to the tank. You may need a long handled wrench with a cheater. These valves are cheap and there’s only one problem replacing them - sometimes they’re hard to unscrew. In our experience if you open an older valve it’s likely it will never close properly again – it will begin weeping when it wasn’t before. Some people will advise you do this periodically to be sure it’s working properly. There’s a lever on the valve that lets you open it deliberately.

Or, perhaps a particle from the tank gets stuck in the seat holding it partly open. It may be if it has released small amounts of water over time this water has built up deposits in the valve that begin to interfere with it closing. As the valves get older they sometimes begin to leak. If your TPR valve suddenly started leaking when it didn’t use to, and you haven’t had any plumbing renovation work done recently, the valve may simply be defective. the pressure in the tank it exceeding the relief point.These valves can begin to run water either because Water heaters can become bombs if the pressure gets too high and these valves fail to work. This safety valve releases water (and thus relieves pressure) if either the temperature or pressure in the tank gets too high. The name is Temperature Pressure Relief Valve (TPR valve).

Hot water heater overflow pipe code