To replace an existing water heater, virtually all cities and counties require a building permit. Because standards have changed, a new water heater should be installed in accordance with current regulations for safety reasons. Many things can go wrong when a water heater is installed incorrectly. For instance, it is possible for a water heater to explode if it is placed without a T&P valve (temperature and pressure relief valve). Because people have died as a result of this, it is critical that a water heater be built and installed appropriately.