What device has replaced boiler fittings and tank fittings for air control in modern systems?

The correct answer is that air scoops have replaced boiler fittings and tank fittings for air control in modern systems due to their efficiency in removing air. Air scoops are specially designed to capture and expel trapped air from a hydronic heating system, thus preventing air pockets that can hinder system performance and efficiency.

By utilizing air scoops, modern systems benefit from reduced maintenance issues and improved overall operation as they help maintain optimal water flow and minimize corrosion by ensuring that air does not accumulate in the pipes or boilers. This makes air scoops essential components in contemporary heating systems compared to older fittings, which may not effectively manage air.

The other devices mentioned serve different functions: manual air vents are used to release trapped air but require manual intervention, compression tanks are primarily for absorbing pressure fluctuations rather than managing air in the system, and overflow valves are designed to prevent overpressure conditions rather than controlling air.