What device must be installed if a check valve or backflow preventer is in a water system?

The correct choice is the thermal expansion tank. When a check valve or backflow preventer is installed in a water system, it creates a sealed environment that can trap water within the system, especially when the water is heated. As the water heats up, it expands. Without a mechanism to accommodate this thermal expansion, the pressure in the system can rise to dangerous levels, potentially leading to pipe bursts or leaks.

A thermal expansion tank serves as a buffer that absorbs the increased pressure from expanding water. It typically consists of a bladder that expands to accept excess water pressure when thermal expansion occurs, allowing the system to maintain a safe and balanced pressure level.

In contrast, while an expansion tank also mitigates pressure changes, it does not specifically address thermal expansion as effectively as a thermal expansion tank does. Other devices like pressure regulators and safety valves may serve their own important functions in the water system but do not specifically manage the pressure increase caused by thermal expansion when a backflow preventer is involved.