Viessmann combi starts when cold, mixer, tap is opened

[Plumdim]

Recently had a Viessmann Vitodens combi boiler installed, quite often when the adjacent cold mixer tap is opened the boiler starts for a few seconds!
Can anyone tell me what is causing this?
And how to cure?

 

[ShaunCorbs]

Needs a non return valve fitting on the cold to the boiler and a shock arrestor

 

[Plumdim]

Needs a non return valve fitting on the cold to the boiler and a shock arrestor

Thank you Shaun for your reply and solution.
Might changing the tap from a mixer to two singles also cure the issue? {they are 45 years old!)

 

[ShaunCorbs]

Thank you Shaun for your reply and solution.
Might changing the tap from a mixer to two singles also cure the issue? {they are 45 years old!)

No as the issue is when you shut the tap a shock wave goes back through the pipes to the boiler and moves the flow switch