Air in the radiators

If opening the thermosiphon purge valve the air comes out initially and not water means that there was still air.

The cause of this lies in the boiler or in the emptying of the system during work.

This is not a problem, just purge all the heaters with a little patience. If this operation is not carried out, in fact the yield will drop considerably.

In fact, you will notice the presence of air even if you touch the radiator, in fact the upper part will be colder because; occupied by the air bubble and not by the hot water coming from the boiler.