HVAC Equipment

What Is an Air Bottle? Air Collection and Venting in Heating Systems

Learn the role of the air bottle that collects trapped air at one point in heating systems, how to install it and how it differs from an air separator.

ORFA Technical TeamUpdated: Oct 30, 20263 min read

For a hot-water heating system to run properly, air dissolved or trapped in the water must be removed from the pipework. An air bottle (air collection vessel) is a cylindrical chamber, usually made of steel sheet, that gathers the air accumulating in pipes and radiators at a single point. An automatic air vent fitted on top discharges the collected air.

This article covers what an air bottle does, its benefits, the correct installation rules and how it differs from an air separator.

What Does an Air Bottle Do?

Air that enters during initial filling, or is released from water as it heats up, rises through the pipes and collects at high points. The air bottle provides a calm volume where this air can gather. The collected air is then discharged continuously and automatically by an automatic air vent mounted on the bottle’s top connection.

Benefits of Using an Air Bottle

Even heating: Prevents air pockets in radiators and risers so that heating surfaces warm up fully.
Better heat transfer: Removing air reduces bubbles that impair heat transfer, supporting energy efficiency.
Quiet operation: Reduces gurgling and knocking noises caused by air in the pipes.
Equipment protection: Reduces the risk of air-related noise, flow reduction and efficiency loss in pumps.
Lower corrosion risk: Oxygen entering with air accelerates corrosion; regular venting reduces this risk.

Installation and Construction

Location: Install the air bottle where air naturally collects: system high points, tops of risers and points such as the flow header in the boiler room.
Vertical mounting: The bottle must be installed vertically so that rising air accumulates inside the chamber.
Air vent connection: An automatic air vent is fitted on top. A ball valve below the vent allows it to be removed for maintenance without draining the system.
Pressure and temperature: Both the bottle and the vent must suit the system’s operating pressure and temperature.

Tip: If water drips from the vent outlet, dirt has usually built up in the float mechanism. Close the valve below the vent and clean or replace the device.

Air Bottle vs Air Separator

An air bottle is a simple chamber that only collects free air rising to it. Air separators reduce the flow velocity and, thanks to an internal element, also separate the small bubbles (microbubbles) circulating in the water. In large closed systems, installations with persistent air problems or applications where high efficiency is expected, an air separator is the more effective solution.

Feature	Air bottle	Air separator
Operation	Collects rising free air	Separates bubbles from the flow
Microbubble removal	Limited	Effective
Typical location	High points, tops of risers	Boiler outlet, main flow line
Discharge	Automatic air vent on top	Automatic air vent on top

Frequently Asked Questions

Where should an air bottle be installed?

Vertically at the highest points where air collects, at the tops of risers and on the flow line in the boiler room, with an automatic air vent fitted on top.

Can an air bottle be used without an air vent?

The air bottle only stores air; an automatic air vent or a manual bleed valve is needed to discharge it. Otherwise, once the bottle is full, air passes back into the system.

Should I use an air separator instead of an air bottle?

In small installations an air bottle with an air vent is often sufficient. In large systems where circulating microbubbles must also be removed, an air separator is more effective.

Conclusion

The air bottle is a simple but effective component that collects air in heating systems so it can be discharged by an air vent. Mounted vertically at the right point, it supports even heating, quiet operation and equipment protection. For more comprehensive air removal solutions, see our air separators page.

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