Plumbers/heating engineers - what is this called and why is it dripping?

[Andr3w]

I've noticed that this 'overflow pipe'?? Has started dripping....quite a lot.

I seem to remember asking a plumber once and they said it should never drip.

It is dripping a lot - at least 2-3 drips per second.

Any ideas what could have caused this? My googling failed me.

https://imgur.com/gallery/Rw5uh

Edit: I've found out its called a tundish. It has dripped before but never this much.

Edit 2: is this at all dangerous. Should I turn my boiler off to stop it heating the water?

 

[Andr3w]

I know it's a hot water tank but the water is heated up by the boiler, which causes pressure.

 

[Andr3w]

Jesus, just a few things that could be faulty then!

I have the details of the guy who installed the system, I'll see if I can get him to come over this week.

Those of you that had a similar fault how much did it cost to rectify?