Boiler HW short cycling due to mixer tap

[Dom_DV]

Hi. My boiler (a Vaillant Ecotec Plus 8xx) short cycle HW every few seconds and, because of the he priority it means no central heating. If I close the hot water circuit (turning the valve underneath the boiler) then the CH works fine. So, I can either have hot water or central heating.
The boiler engineer says there is nothing wrong with the boiler and the problem is caused by one of the taps in the bathroom or kitchen with a mixer (the shower is also a suspect).
My doubt is that the boiler short cycle happens even if no tap at all is open. It's not like we open the cold water and the boiler thinks there is a demand of HW.
Is that possible at all? It would be great if someone could confirm before I call other engineers and a plumber to find a faulty tap
Many thanks

 

[snowhead]

If isolating the Hot water circuit stops the cycling then the problem lies within that circuit.
It can only be the Hot could be flowing into the cold via a thermostatic mixer or possibly the shower mixer if the Cold supply is from a storage tank.

Another possibility is a leak in the Hot circuit.

 

[Dom_DV]

If isolating the Hot water circuit stops the cycling then the problem lies within that circuit.
It can only be the Hot could be flowing into the cold via a thermostatic mixer or possibly the shower mixer if the Cold supply is from a storage tank.

Another possibility is a leak in the Hot circuit.

Thank you for the prompt reply. That makes a lot more sense now. I fear it might well be a leak. Anyway, out to find a good plumber now.
Thanks again.