Rad aeration when HW and CH on at same time

[Secret_Sipper]

We have lived in our house for a few years and inherited the problems that it has, as most do.
Old baxi solo boiler, HW tank and mid position valve.
We can have the HW on without the CH and both on a the same time but not the CH on without the HW (Not without turning the HW stat down)

I've noticed since changing the CH pump and one of the rads that if both HW and CH is on, I get a huge amount of air in the rad closes to the HW tank so much so that it needs bleeding every night. If I put the HW on before the CH, or when like in summer - no heating on at all there is no aeration.
I can live with it at the moment but I'm more curious if it's something I can fix/low cost fix until it's all ripped out and we install a more efficient boiler/system.

Cheers

 

[stem]

I'm not an expert on the plumbing / aeration part, but if the problem has only just occurred since the pump was changed, was it was installed with the flow in the in the correct direction? If not it may be 'pumping over' or drawing air into the system.

Regarding the control aspect:

We can have the HW on without the CH and both on a the same time but not the CH on without the HW (Not without turning the HW stat down)

It could be that the 'Hot Water Not Required' (aka: hot water satisfied, or hot water off) signal from the programmer is missing. This closes off the motorised valve's hot water port when not required. This connection comes from two places. The hot water cylinder thermostat; that seems to be working, and the programmer which doesn't.