In a combination heating system utilizing a water heater, where should isolation valves be installed?

In a combination heating system that utilizes a water heater, it is essential to install isolation valves in both the supply and return lines of the heating loop. This setup allows for effective maintenance and serviceability of the system without draining the entire system or disrupting the operation of the water heater.

Having isolation valves on both lines enables technicians to isolate specific sections of the system, which is particularly useful if repairs or adjustments are needed. It also helps in troubleshooting and ensures that the heating components can be accessed for inspection or cleaning without affecting the rest of the system, thus maintaining efficient operation.

The other options present limitations in maintenance flexibility. For example, installing valves only in the return line would make it challenging to service the supply side, whereas having them solely in the supply line would do the opposite. Placing isolation valves in the water heater alone does not provide adequate control over the entire heating loop, thereby reducing the effectiveness of the system management and maintenance.