baxi duo tec air vent on top of combi boiler

[giarc00007]

is the air vent on the top of the boiler (heat exchanger i think) supposed to be open or closed, i know you open it to fill up system, but are you then supposed to leave it fully open half open, or fully closed. Its just a little black screw on the vent.

 

[ferret]

open fully even take out the scew🙂

 

[giarc00007]

why do you get the option to close it if you are supposed to leave it open thats why i was a bit confused