Whether a water heater can be run without an expansion tank depends on the plumbing system. If a home has a closed plumbing system, then an expansion tank is needed. A closed system has a backflow prevention device, like a check valve or pressure-reducing valve, that stops water from flowing back into the city's water lines. When water is heated, it expands, and without an expansion tank, this can cause pressure to build up and damage the water heater and pipes. In open plumbing systems, water can flow back into the city water lines, so an expansion tank isn't required. To prolong the life of your water heater, consider installing an expansion tank water heater