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Unless they are connected to the same system, boilers and water heaters will both need their own expansion tanks. The only case they wouldn’t have separate expansion tanks is if the boiler is pulling double duty and managing a heating system while also supplying hot water for a home. In which case, you may not need a hot water heater.

No, boilers and water heaters are not interchangeable because they serve different primary functions. Boilers are used for central heating systems, providing hot water or steam to heat your home. In contrast, a water heater's purpose is to supply hot water for daily household use, such as for faucets and showers. A water heater alone cannot supply the warm water or steam needed for a central heating system.

While they are not interchangeable, it is common to have both a boiler and a water heater. In fact, a conventional boiler often works best when a water heater system is already in place. The two systems can be combined to provide both heating and hot water, either by pairing a boiler with an indirect water heater or by installing a premade combination boiler-water heater unit. For home heating, some homeowners may also opt for a furnace instead of a boiler.

While not all homes require a hot water heater expansion tank, they are necessary in most newer homes and are often mandated by local building codes. Codes typically require an expansion tank if your plumbing system is a 'closed-loop' system, which often includes a backflow preventer or pressure regulator on the main water line. Some municipalities have specific requirements. Installing an expansion tank on a closed system is crucial, as it prevents excess pressure that can damage your water heater and other appliances, potentially shortening their lifespan. However, tankless water heaters do not require an expansion tank.

If your gas water heater is over 20 years old, it should be replaced. Gas water heaters can last up to 12 years and while they can exceed this expectancy if well-maintained, a heater that's 20 years old would require replacement. The heater is unlikely to run as efficiently as a new model and could be more expensive to do so.