The Critical Role of Brass Automatic Air Vents in Heating Systems and a Selection Guide

In modern district heating and individual heating systems, the efficient transmission of thermal energy depends not only on the capacity of the heat source but, more critically, on the purity and flow characteristics of the heat-transfer medium in the distribution network. Air pockets that accumulate within the pipes act like “blood clots” in blood vessels: they create air resistance that can result in localized underheating and also accelerate oxidative corrosion of the metal piping.

In modern district heating and individual heating systems, the efficient transmission of thermal energy depends not only on the capacity of the heat source but, more critically, on the purity and flow characteristics of the heat-transfer medium in the distribution network. Air pockets that accumulate within the pipes act like “thrombi” in blood vessels: they create airlocks that result in localized underheating and accelerate oxidative corrosion of metallic piping. As a core component for addressing this persistent problem, the brass automatic air vent is often referred to as the “lung” of the heating system. This paper provides an in-depth analysis of its operating principle and offers engineering professionals a comprehensive selection guide.

I. Brass Material: The Optimal Balance of Corrosion Resistance and Thermal Conductivity

Among the materials used in the manufacture of automatic air vents, brass has come to dominate the market primarily due to its outstanding physicochemical properties. Compared with cast iron, brass exhibits superior corrosion resistance, effectively withstanding the corrosive effects of dissolved oxygen and chloride ions in heating system circulating water, thereby extending the service life of the valve in humid environments. At the same time, brass’s excellent thermal conductivity enables it to respond rapidly to changes in the medium temperature; when paired with an internal float made of stainless steel or polypropylene, this ensures stable operation across a wide temperature range from –10°C to 120°C. Particularly in regions with harsh northern climates, brass valve bodies demonstrate exceptional environmental adaptability, making them the material of choice for ensuring the safety of pipeline networks.

II. Core Mechanisms: Removal of Dissolved Air and Prevention of Air Lock

The primary function of an automatic air vent is to “vent air only, without draining water.” It typically employs either a float-lever mechanism or a vertical float design. When the system is filled with water or during operation, gases accumulate in the upper part of the valve body, causing the liquid level to drop. The float then descends under gravity, actuating the lever to open the vent port. Under the resulting pressure differential, air within the system is rapidly expelled. As the air is vented, the water level rises, the float returns to its original position, and the sealing surface presses tightly against the vent port, automatically closing the valve. This mechanism effectively prevents air locks, ensures smooth circulation of hot water through radiators or underfloor heating pipes, and boosts thermal efficiency by more than 15%.

III. Selection Guide: Principles for Matching PN and DN

In engineering practice, proper selection is the prerequisite for ensuring that the vent valve functions effectively.

Selection of Nominal Pressure (PN): The operating pressure in heating systems typically ranges from 0.6 MPa to 1.6 MPa. For standard multi-story residential buildings, air vents with a PN10 or PN16 rating are generally sufficient; however, for high-rise buildings or fire sprinkler systems, where static pressure is higher, products with a PN25 rating or even higher must be selected to prevent valve body rupture or seal failure.

Matching the Nominal Diameter (DN): The DN of an air vent directly affects the venting rate. DN15 (4-point connection) is typically used at the ends of residential radiators and distribution manifolds, while DN20 (6-point connection) and DN25 (1-inch connection) are more commonly employed at high points in main pipelines or in large HVAC systems. When selecting an air vent, it is important to note that a too-small DN will result in poor venting, whereas a too-large DN may lead to installation difficulties and unnecessary cost overruns.

Connection methods: Currently, the market mainly offers vertical (upright installation) and flush-end (horizontal installation) types. Vertical valves must be installed at the highest point of the pipeline to collect gas by leveraging the principle of gas buoyancy, while flush-end valves are suitable for horizontal pipelines where space is limited.

In summary, although the brass automatic air vent valve is small, it serves as the “gatekeeper” of system energy efficiency. When designing and selecting such valves, engineers should thoroughly consider system pressure, medium temperature, and installation conditions to ensure that the valve’s performance is perfectly aligned with system requirements.