How many expansion tanks are required for a system with six boilers?

In a hydronic heating system with multiple boilers, the purpose of an expansion tank is to accommodate the expansion of water as it heats up, preventing pressure build-up that could damage the system. Generally, one properly sized expansion tank is sufficient for a system that contains multiple boilers if they are all connected to a common header. This is because the expansion and contraction of the water in the system can be managed by a single tank that serves the combined volume and thermal changes of water from all the boilers.

The design and size of the expansion tank are crucial to ensure it can handle the cumulative thermal expansion of the entire system. As long as the tank is designed to accommodate the total water volume and the expected thermal expansion, having just one expansion tank provides an effective solution.

In systems where each boiler operates independently with separate piping, it might necessitate additional tanks, but in typical configurations where the boilers share a common system, one expansion tank is usually adequate.