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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.

No. Expansion tanks must be installed on the supply side (where heated water flows out) of the boiler. There’s also a list of other requirements for most boilers, including being above the pressure relief valves, being on the inner side of any circulator, and more. Your installation manual should have more information on expansion tank installation requirements and what sort of tank the system is made for.

You can use multiple tankless water heaters, but that doesn’t mean you should. Adding multiple tankless heaters adds considerable cost to your home in both parts and installation costs. Also, you’re adding many more points of failure in the system, making plumbing issues harder to diagnose. More heaters mean more maintenance, and insurers may want a word regarding the increased risk of water damage. A single, larger unit makes more sense in most situations.

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.