About Boiler Feed Valves
"Fill" or "feed" valves, as the name suggests, are intended for supplying the boiler and the heating system with water. While fill/feed valve generally implies only the pressure regulating valve, the make-up water package nearly always include a backflow preventer, which stops the system water from going back into the drinking water supply. These two, often sold as a factory-assembled combo are very common for new boilers installs.
Most boiler fill valves are similar in design and specs (but could use different quality of materials - hence the price difference), and have a 10-25 psi adjustable outlet pressure, often factory set to 12 or 15 psi. They are simply pressure reducing valves designed for boiler feeding applications. Some models may utilize a built-in check valve.
Dual check backflow preventers with atmospheric vent are devices intended to prevent the reverse flow of boiler system water into the potable supply. How it works - under normal operating conditions, supply/inlet pressure keeps the vent closed, but in the event of zero supply pressure or backsiphon, the vent is opened, allowing contaminated water to drain through the vent.
