biasi m90e.28s air in system?

[presston]

Hi
Central heating works fine no noise from pump but when i turn hot water on it sounds like there is air in the system which i cant seem to bleed. Occasionally the hot water wont heat up at all but this is down to the primary circuit flow switch not closing the microswitch so turning the pump off. What i cant undestand is why the primary circuit flow switch works fine when central heating is being used but sometimes fails when hot water is turned on
Thanks
Mick

 

[presston]

Hi guys
If the d.h.w. heat exhanger was getting blocked could this cause the gurgling noise in the pump when hot water is selected and so sometimes not close the primary circuit flow switch?
Thanks
Mick