Vacuum relief valves are critical components of tanks that need to stay in the right pressure range, such as water heater tanks. They prevent low-pressure conditions ultimately. If you want to buy one for a tank system, use this guide for a successful valve investment. Make Sure Precise Performance is Provided  Since the main role of a vacuum relief valve is to allow more air inside your pressurized tank to bring pressure levels up, you need this valve to work in a precise way consistently.