A ventilation duct is a vertical conduit that collects air trapped in the pipes and radiators of heating systems located in the upper floors, then expels it via an automatic or manual air vent. Trapped air in the system accelerates corrosion, causes pump cavitation, noise, and heating problems on the upper floors. A ventilation duct safely collects and expels these air bubbles, thus balancing circulation and improving efficiency and comfort.

It is best to install ventilation ducts at the top of radiator stacks, at the top of risers in central heating systems and boilers, and at the highest point of collectors. Generally, they should be installed at the highest point where air naturally accumulates and is warmest; this facilitates the separation of insoluble gases. The air vent should be easily accessible at the top, and a small vent hole should be provided at the bottom if necessary.

A simple purge removes air locally at its connection point; a vent creates a "pocket" that collects rising air bubbles and allows the purge to proceed uninterrupted, making it more efficient. Air separators (microbubble separators) also isolate microbubbles from the flow using dedicated stages within the housing, thus ensuring continuous air management near the boiler/chill unit. In large, multi-story buildings, using vents at the highest levels and an air separator in the boiler room definitively solves the air problem.

The choice of air tube depends on the pipe diameter, flow rate, operating pressure, and temperature range. In most applications, the tube diameter is chosen to be equal to or larger than the nominal diameter of the column to which it will be connected; the goal is to reduce flow and promote bubble retention. The connection type (threaded, welded, flanged), body material, and coating must be compatible with the water chemistry and project standards. If the pipe diameter and height, layout, and desired vent type are provided during the quotation process, the appropriate body size and fittings can be determined quickly.

The vent must be installed vertically, with the flow direction indicated. A mini ball valve must allow for the removal of the automatic vent valve located at the top. The vent valve must be accessible and free of any plaster, paint, or insulation. Sharp bends that could disturb the air pocket should be avoided. A significant amount of air will escape from the system during the first few days after commissioning, so frequent vent checks are necessary. Subsequently, the frequency of these checks should be adjusted according to the system's performance.