The 108 Series valve itself is designed to be installed in the flow and return pipe work from the heat pump and installed on the external pipework.
By opening at temperatures below 3C, the valve allows system content to be dripped/discharged safely, preventing the system from freezing. At temperatures above 4C, the valve then shuts again to stop unnecessary draining.
Overall, it does exactly as it implies, it prevents the freezing of heat pumps and pipework. With the valve designed specifically for heat pump installation, Caleffi has ensured it is compact and easy to install, whilst remaining a top-quality product.