What may be installed in the supply line but not in a return line?

A check valve is specifically designed to allow fluid to flow in one direction and prevent backflow in a piping system. Its installation in the supply line ensures that the flow of oil or fuel moves towards the equipment, which is essential for the proper functioning of heating systems. In contrast, installing a check valve in a return line could prevent the system from properly returning the fluid back to the tank or boiler, leading to potential operational issues or damage.

In contrast, a shut-off valve and a safety valve can be used in both supply and return lines to control flow and safeguard against over-pressure, respectively. Additionally, a pressure regulator is essential for managing the pressure of the fluid in either line but does not have the one-way functionality of a check valve. Therefore, the distinct functionality and the role a check valve plays in maintaining flow direction is why it is permitted in a supply line but not in a return line.