Turn cold water tap off and water shoots out of mira shower head

[2wd]

As per long title but ill start from the beggining.
Had a Mira advanced atl thermostatic shower which went fauly, bleeping. LowHigh pressure flashing and on button flashing, research indicated a possible relay pcb fault so i had an old spare mira advance ATL
Bro in law who is a qualified plumber of 40+ years popped up, swapped it over, ran it and all seemed ok.
Later i turned a water tap on and when i turned it off a blast of water shot out of the shower for a second or two
Run the shower and all is fine until a tap is turned off/on then this one bleeps, but the difference is i can switch the shower off and back on and it runs fine....until a tap is switched off and on.
Appreciate any advice as bro in law now away all day

 

[Madrab]

It'll probably be water hammer. When there is relatively high mains water pressure and velocity, when a tap is turned off or a valve snaps shut (more-so 1/4 turn valves) it sends a shock-wave traveling back down the water in the pipe. Chances are the shower is one of the first thing that the shock-wave is hitting, pushing the solenoid open and letting a little water past.

Try turning the tap off very slowly and see if it still does it.

It will eventually damage the valve.

 

[2wd]

Thanks
Yes if i shut it slowly it doesnt happen
So does that mean we have high water pressure ,or my old spare one (donated by brother in law some years ago) has a faulty valve and possibly the reason why somone changed it.
If i do buy a brand new shower im looking at the Mira sport airboost

 

[2wd]

Just shut the water off and opened all the taps to drain water in pipes off and its much better now, just a little dip in water flow for a second

 

[undertrained]

Try priming the unit again for about 10 seconds. Also check your stopcock is fully open with a quarter turn back.

 

[Harry Bloomfield]

Our Mira, over the bath, has a steady drip when the bath is filled. Initially my theory was that it was heat rising from the bath causing the water in the flexi pipe to expand out of the shower head, but obviously not.

 

[Madrab]

It could be high pressure but not necessarily, could just be the water velocity mixed with the speed the valve is closing.

You'd need to contact Mira to confirm if this is indicative behaviour of their shower in these circumstance or not but I wouldn't think so.

 

[Harry Bloomfield]

Oh, it does cause a problem, just a curiosity.