Expansion Tanks FAQ

Q: What are expansion tanks?
A: Water expands when heated, creating increased pressure in a water heater or boiler. In the absence of an expansion tank the water will look for a way to escape. In older homes the hot water will back flow through the cold water pipe through which it came. Today, the national plumbing codes (Safe Water Drinking Act) mandate a backflow preventor. Because of the backflow preventor, the water has nowhere to go except the pressure relief valve, dumping the water onto the floor. The expansion tank provides a pressurized air cushion that absorbs expanded water, keeping pressure in the system within safe levels. Over time, the consumer will realize money savings – no longer will hot water be wasted by either back flowing into the cold water line or by dumping out onto the floor. Also, if left unchecked, the constant fluctuations in water pressure will damage the heater and associated piping.

Q: Why are there different expansion tanks for water heaters, boiler systems and radiant heating systems?
A: Thermal expansion tanks are designed to work with common household water heaters. Hydronic expansion tanks are used in common boiler systems, which are normally found in areas of the country where the heating load greatly outweighs air conditioning (Northeast, Mid Atlantic, North Central, Mountain and Northwest). The types of heating systems found in these homes will be baseboard or radiators.

Q: Why can’t you use the same tank for each system? What makes them different?
A: Water heater expansion tanks contain a special plastic liner to prevent corrosion and carry a higher working pressure than boiler expansion tanks. Boiler expansion tanks have a higher temperature rating for heating system use.

Q: Do water heater manufacturers specify a need for an expansion tank?
A: Yes, all water heater manufacturers state a similar requirement to maintain warranty coverage if the system contains a one-way device like a backflow preventor.