not filling up

hi everybody new to this forum but not to pulsar coils
i drained down unit fitted off peak immersion and attempted to fill it got to a point where the header tank is full but in my opinion dose not seem full as it is not working ie pump is working all the time off peak boost on off peak on but no temp. at all no fault codes

dose the fill into the tank go in at the bottom and the vent come off the top the reason i ask is that i connected the mains to the feed into the tank and water came out of the vent as if it is full

any help would be most apreciated

No you should have a header tank.
On top of the unit or located elsewhere.
Run the heaters dry and they will be no good.
Mike