Radiators get warm when running hot water

[Allybate]

Hi there – first timer here and looking for some help.
I have read a number of posts on this subject but none seem to have a suitable answer for my particular problem.
I have a Glow-Worm Compact P100 combi boiler. When we run hot water the radiators upstairs get warm and I can hear water movement in the pipes that feed the downstairs radiators.
It is most obvious when we have a shower – I guess because the water is running for some time.
When we do use the shower (mixer tap type) occasionally it will suddenly go cold and to get it hot again we have to stop and re-start it.
I have read that a sticky diverter valve is an obvious culprit, but when I looked at the instruction manual I cant find any reference to one in the boiler??
Please help!!
Allybate.

 

[ollski]

There isn't one, it's a twin pass heat exchanger. There is a bypass which dumps excess heat from a hw demand into the ch system.

 

[Allybate]

Hi - thanks for reply. So I can stop looking for something that isnt there.
How do I solve the problem? I'm not a plumber (obviously) but before I get soemone in I wanted to at least understand what they should be advising me on.

 

[Allybate]

Can anyome please tell me if this is normal for this type of system or should I try to do something about it??
Thanks.