A _________ valve, also called an automatic fill valve, is needed to keep a hot water heating system with a closed expansion tank automatically filled and under proper pressure.

A pressure reducing valve is indeed essential in a hot water heating system that features a closed expansion tank. This type of valve serves to maintain the proper water pressure in the system by automatically filling it with water from the supply line whenever necessary.

When the water in the closed system cools, it contracts, leading to a decrease in pressure. The pressure reducing valve detects this drop in pressure. If the pressure falls below a predetermined level, the valve opens, allowing water to flow into the system until the pressure returns to the optimal range. This automatic function is crucial for ensuring the heating system operates efficiently and safely, helping to prevent issues such as cavitation or insufficient flow through the heating elements.

In contrast, other types of valves such as shut-off, check, and isolation valves serve different functions. A shut-off valve is used to stop the flow of water entirely, a check valve allows water to flow in one direction to prevent backflow, and an isolation valve is primarily for maintaining or servicing parts of the system without needing to drain the entire system. These do not possess the capability to manage pressure levels automatically, which is why the pressure reducing valve is the correct choice here.