I know few. if any, systems that do not require an auto air vent above the
boiler on the flow pipe. In days gone by when there was no system boilers
with integral pressure vessels and all components had to be external, an
auto air-vent on the flow over the boiler was mandatory by all
manufacturers. You didn't think, you just put one in.

I'm not saying they're not an excellent idea and extremely widely used, just
that not all systems have them.

Christian.