Air in central stem when hot water is switched on

[James English]

Replaced Grundfos Ups 2 C/H pump with a Ups3. when c/h only on no problem as soon as h/w is on as well gurgling and air in system experienced, also hot water coming out of 22mm vent into expansion tank ( doesn't happen when only c/h is on) so bad that h/w must be turn off. I had no problem before changing pump.
I would be grateful for your advice.
Regards,
James