Radiator Bleed Key / Entlüftungsschlüssel für Heizkörper

The cold seasons are coming and it's getting cold at home because the radiators are full of air! Didn't have a radiator air release key around, but what do we have 3D printers for?
It works with 5mm square nuts in a 10mm hole.

Heat up slightly with a lighter, press over the nut carefully if it doesn't fit right away.
Works well.