The Caleffi Series 5080 Hygroscopic Automatic Radiator Air Vent (MPN 508041) is a ½" BSP male-threaded automatic air bleeding valve for panel and column radiators. It operates on Caleffi's hygroscopic cellulose disc principle: when system water is present, the cellulose discs expand by 50%, sealing the vent outlet; when air accumulates, the discs dry out and contract, releasing trapped air automatically. The nickel-plated brass body with white POM resin knob and PTFE-sealed thread is rated to 10 bar and 100°C. The hygroscopic cartridge is field-replaceable without draining the radiator, with a recommended service interval of 36 months.

10 bar

Max Pressure

100°C

Max Temp

½" BSP

Male Thread

36 months

Cartridge Life

Auto + Manual

Dual Mode

How the Hygroscopic Mechanism Works
Inside the vent body is a cartridge of cellulose fibre discs. These discs swell by 50% on contact with water, sealing the vent outlet against water loss. When air accumulates and the discs are exposed to air (dry conditions), they contract — opening the vent and allowing air to escape. Drying time decreases with higher water temperature: at 100°C the discs re-seal in as little as 15 minutes.

Cartridge Replacement Interval
The hygroscopic disc cartridge must be replaced at least every 36 months, and more frequently on systems with hard or unfiltered water. The cartridge design allows in-situ replacement without emptying the radiator.

Installation Orientation
Must be installed vertically upright. The hygroscopic discs require vertical installation to function correctly. Do not install at an angle or horizontally.

Key Features

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Passive Hygroscopic Venting

No float, no electrical components — just cellulose discs that respond to moisture. Automatically seals on water contact and opens when air displaces water, with no mechanical parts to jam or corrode.

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Manual Override

Rotating the white POM knob approximately one turn anti-clockwise activates manual bleed mode — essential during initial system fill or when large air volumes require rapid purging.

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Replaceable Cartridge

The hygroscopic cartridge assembly can be removed and replaced in situ, without draining the radiator. A significant practical advantage over non-serviceable vent valves.

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Quality Construction

Nickel-plated brass body, white heat-resistant POM knob, and PTFE-sealed thread — consistent with Caleffi's wider manual radiator valve range for a uniform radiator connection finish.

Technical Specifications

MPN	508041
Series	5080 — Hygroscopic Automatic Radiator Air Vent
Connection	½" BSP Male (external thread)
Body Material	Brass EN 12164 CW614N, nickel plated
Knob	White POM (acetal resin), heat-resistant
Thread Seal	PTFE
Max Working Pressure	10 bar
Max Working Temperature	100°C
Venting Principle	Hygroscopic cellulose fibre discs — auto and manual modes
Disc Expansion	+50% volume when immersed in water
Auto Mode	Knob fully closed — vent opens when dry, seals when wet
Manual Mode	Knob rotated ~1 turn anti-clockwise
Cartridge	Replaceable; service interval minimum 36 months
Installation	Vertical upright only
Other Sizes Available	508011 (1/8"); 508021 (1/4"); 508031 (3/8")
Manufacturer	Caleffi S.p.A., Italy

Frequently Asked Questions

How is a hygroscopic vent different from a float-type automatic air vent?

A hygroscopic vent uses cellulose discs that physically swell and shrink with moisture content to open and close the vent — no float, no moving mechanical parts beyond the knob. Float-type vents (such as the Caleffi Robocal) use a rising float to seal a valve outlet. Hygroscopic vents have a smaller body profile suited to radiator bleed points, while float vents are typically installed in-line on pipework high points.

How do I know when the cartridge needs replacing?

Signs of cartridge failure include water dripping from the vent outlet without air being present (discs no longer sealing), or the system failing to auto-vent trapped air. Caleffi recommends proactive replacement every 36 months regardless of apparent condition — more frequently if the system uses hard or unfiltered water.

What thread does my radiator need for this vent?

MPN 508041 has a ½" BSP male (external) thread. The vast majority of UK panel and column radiators have a ½" BSP female bleed port at the top-end of the radiator — this is the standard direct fit.

Is this suitable for pressurised sealed heating systems?

Yes. The 10 bar maximum working pressure rating is well above the typical 1–2.5 bar operating pressure of UK sealed heating systems.

Can this vent be installed as a permanent automatic vent without manual intervention?

Yes — with the knob fully closed (automatic mode), the vent operates passively. Manual intervention is only needed during initial fill or when rapid large-volume air purging is required.

Radiator Valves and Accessories

Caleffi Series 5080 Hygroscopic Automatic Radiator Air Vent 508041 Half Inch BSP