...or why a Hans Grohe iBox is such a silly design.

So, you learn something new every day.  Fitting a new ensuite as part of the whole house refurb.

Finally, all systems go. Time for a shower.  

Scenario...our house has two separate pumps for cold and hot water.   The cold pump (historic) is in the cellar.  The hot water pump is new and installed upstairs next to the HW cylinder.  I can hear it when it kicks in and once the hot water is pressurised, it stops.

So into t'shower, turn it on (separate control) and wallow. Have shower.  Turn it off.  Only ....only ..

only the H/W pump does not stop. Thinks...this should not be happening. I've tested it before it was all tiled up.  There are no leaks. It should not be doing this. Switch off the H/W feed to the ensuite at the manifold.  Pump stops.  Sh*t. I must have a leak somewhere.  I have an inspection hatch.  No leak.  WTF ?

Engage brain. Where can it be pumping to?  'cos it is - otherwise it would pressurise up and stop.

Back up the cold water feed ?  Surely not ?  Switch off cold feed at the manifold and the H/W pump stops.

And that is when I realised that OFF on this Hans Grohe cock-a-mamie iBox is only off as far as the shower head is concerned.  Both cold and hot feeds remain 'live' relative to each other and so if there is an imbalance in pressure then one pumps into the other.  The 'fix' is to turn off the temp control on the iBox but what a faff...annoying, to be sure....but even then there is still a slight bleed of water through back into the cold water pipes and so the H/W pump kicks in...on and off.

I think a checkvalve in the cold feed will sort it.

I can assure you that the only design fault is the Hans Grohe iBox.  The rest of the house has a multitude of mixer taps, thermostatic shower controls etc.  All work perfectly well.  When you turn them off, they do turn off.  Cold and hot both pressurise up and the pumps stop.

The i Box has no such luxury.  The only on/off is on the output of the mixer.  So hot and cold circuits commune together quite happily because they are nit turned off.  I can understand that the iBox will work perfectly well if it's installed in a house with, say, a twin Stuart Turner pump because both hot and cold will be of equal pressure...more or less.

But if there is an imbalance between the two of sufficient magnitude then whichever circuit has the higher pressure will force back into the other.

It's simply a duff design and I'm surprised that Hans Grohe have designed it this way.

Well, I'm not an expert on pumps.  You're right in that if one pump is running then the water must be going somewhere.  My conclusion is that it's flowing back through the cold water pump into the clean tanks.  Cos there isn't any leak anywhere.

Because you mentioned that the pump should be protected by a double-check valve (it isn't...none of them are..we have five....all here before I came) then it must be feasible for water to be forced back through a pump.

Good point.  The double-check valve arrives tomorrow which should sort it out.  I say 'should'.

I have four separate plumbed areas in the house - each fed from a manifold via a gatevalve.

The one on the extreme right is feeding the ensuite in question.  There are three 'outlets'...the WC, handbasin and the Hans Grohe shower control.  

So just to recap...all works in the ensuite until I come to turn the shower off.  The shower stops flowing but the hot water pump still keeps running. If I turn off the cold gate valve (extreme right in picture) then the h/w pump stops.  If I turn off the Hans Grohe temperature to its coldest then that (sometimes) will also have the same result.  

So this evening I thought...if my theory is correct then if I adjust the Hans Grohe temperature control to, say, mid-point, turn off that gate valve then if I flush the WC then the H/W pump should kick in ...but it doesn't !

I really have no idea now what's going on !   AllI know is that I have no leaks !

You're making assumptions that are incorrect.  For example, the cylinder is fed from a cold tank in the loft.  Historic reasons.

Anyway problem solved.  Hans Grohe are supposed to have fitted backflow preventers inside the shower controller.  They are missing and so they are sending me a set.