Valve? What is this, how it works, why does it leak?

[adrian_ros]

Hi guys.

What exactly is the device in the attached picture? It is mounted on one of the pipes of the Megaflo unvented cylinder.

As can hopefully be seen on the photo, it leaks minute amounts of water. This is only noticeable when heating is off, otherwise the water evaporates.

The tiny leakage might explain why over many weeks the pressure drops enough that the boiler (Vaillant) won't fire without manual pressurization. This is a weekend/holiday house, hence could be a big issue if the heating doesn't kick in during a very cold period.

A flurry of questions then: what it is device, how it works, does it needs replacement, link to a site selling those or having technical details, etc?

I tried myself to identify it with Google Lens, no success .

Many thanks.

Adrian

 

[Mottie]

Looks like an automatic air vent valve to me.

Something like this?

 

[adrian_ros]

Looks like an automatic air vent valve to me.

Something like this?

Yes, looks very similar, the small black cap unscrews. It's an air vent then - mystery solved - thanks.

Why the cap though? It should have a purpose, when taken out it shows a small pin.

 

[adrian_ros]

Actually after Mottie identified the thing Google can finally help with the rest.

It tells "It should be considered at the design stage where air vents will be located to give access for inspection maintenance and repair. During operation the upper cap must be loosened. It is not advisable to fit the valve in places which could be subject to freezing."

I just tightened the screw days ago, not knowing what the thing is and how it works .