What device is essential between a hot water boiler and the expansion tank?

The device that is essential between a hot water boiler and the expansion tank is the shut-off valve. This valve plays a crucial role in the system by allowing for the isolation of the boiler and expansion tank from the rest of the heating system.

Having a shut-off valve in place facilitates maintenance and repairs, as it enables technicians to safely separate components without needing to drain the entire system. It also allows for the control of flow and pressure when necessary.

The other devices listed serve different purposes; for instance, the pressure gauge monitors the system's pressure but does not directly control flow. The safety relief valve is critical for preventing overpressure situations by venting excess pressure, while the float switch is used in certain applications to control water levels but is not essential in the context of connection between the boiler and expansion tank. These roles are supportive but do not replace the essential isolation and control function provided by the shut-off valve.