Hot water flow to bathroom

Hi there, the problem that I have is that when the kitchen hot tap is turned on the flow from the bathroom hot taps dries up completely. the hot water is supplied from a year old vaillant turbo max combi housed in a bedroom airing cupboard. I want to fit a thermostatic mixer shower to replace my existing electric one but I'm worried about what will happen if I'm in the shower and the downstairs tap gets switched on. Any advice much appreciated.

I presume only one tap "dries up". This is often due to a fault in the tap valve.

You should really only be using one set of taps at a time unless you have exceptional water pressure and a very high power combi.

If you have a turboMAX 824 then you only have 9 litres or under of usable hot water at any point in time.

Opening a hot tap downstairs will seriously modify the flow and the answer is that your shower will go cold if you open the hot tap downstairs. Water will always take the easiest way out; faced with a wide bore kitchen tap and a shower mixer with pinholes in the head, most of the flow will always go via the kitchen.

If stopping others from running the hot tap is going to be a problem I suggest you keep the electric shower.

Thanks for the good advice I'll have a think about the best option much appreciated