Installing a pressure relief valve (PRV) before your tankless water heater is not only required in some regions depending on where you live, but is also a very important safety feature. Always be sure that your tankless water heater is able to relieve both the water temperature and pressure at a given point in order to avoid damage and possible injury.
Pressure-relief valves are an essential component of tankless water heaters. The valves allow you to manually release air from the heating chamber when filling the chamber with water for the first time, or during 
periodic maintenance.
The pressure relief valve is similar to an emergency stop valve. This valve is designed to offer an escape route for water from the heater if the temperature rises beyond a certain point, allowing an unsafe temperature or increased pressure build-up inside of your tankless water heater. The increased water pressure opens the pressure relief valve and water can escape to the exterior. This action immediately lowers the pressure inside the water heater tank.
Although this is not very common, it can still occur. If there is ever an issue with your tankless water heater, the pressure relief valve is installed to serve as protection for the home and the homeowner. The heater should automatically adjust itself back to normal after the extra pressure (and water) has been relieved.
- As you can tell, the pressure relief valve is crucial component for to protect homeowners. Being that the pressure relief valve is a crucial component to a tankless water heater set-up, most water heater manufacturers often recommend that you should hire a licensed plumber to check it at least once per year and replace it if it is ever faulty.