Thermostatic Shower Valve, Hot water tank - Strange Behavior

[Plumbtastic]

Hi,

I have recently installed a thermostatic shower mixer that replaced an older one. I installed it without reading the instructions because I've fit many showers before and besides this was a like for like swap.

The shower exhibits behavior the likes of which I have only seen once before on a shower I didn't fit and therefore didn't have to solve the problem.

When the flow valve is opened, no water flows. If the basin hot water tao is then opened sill no flow. But finally when the basin hot water tap is closed the shower flow begins.

The instructions state that the shower should be fitted with its own 22mm supply directly from the cylinder and that it should be fitted using a essex or sussex flange and should have no branches supplying any other fixture. Neither of these instructions have been followed.

The hot water tends to be too hot and I believe that most of the time, if not all, there is no cold water flowing through the mixer.

The run from HW tank to shower is long and would involve making a mess of the house which I would like to avoid. Can anyone explain this phenomenon?

My thoughts are:

1. The cold water pressure is not enough to allow thermo valve to open when in presence of higher hot water pressure. Opening the basin HW tap reduces the hot water pressure to the shower but by too much to allow it to flow up through shower head. when closed again pressure is enough to flow through head.

2 Some sort of air lock... Someone mentioned dead legs? Could anyone explain the physics involved here.

Please help

 

[twgas]

MIs should be followed, and least then it may work, and if it didn't someone may be able to help you.

 

[Plumbtastic]

No points for that reply mate.

It's obvious if installed correctly it would work. What I'm curious about is what exactly is causing this behavior in the system.

If anyone has any theories please post them, even if your guess is just a guess....

Ta

 

[seco services]

where is the cold/hot taken from then ?

 

[blondini]

My guess:

The mixer is not suitable for low pressure systems and the system pressure is simply too low to pass the check valves. The action of turning off the hot basin tap off will momentarily raise the hot water pressure, (as the flow is arrested), and maybe that is just enough to kick start the mixer into passing hot water.

 

[Onetap]

It may be an airlock; the essex flange is usually there to stop air bubbles from the hot water collecting in a shower pump. Is there a pump in this installation?

There are spring loaded non-return valves built into the shower valve; these are there to prevent back-siphonage. There is a minimum head of water required to push these open, usually 2m or so.

There is no flow when the shower is turned on because the pressure is inadequate. When the basin tap is turned on and then off, the pressure surge (like water hammer) is sufficient to open the check valve. The flow then starts the pump (I'm assuming there is one) and the pump pressure then keeps the check valves open and water flowing.

I've seen this with a shower hose; The pump wouldn't start when the hand set was on the wall bracket. Removing it and lowering it to waist height started the water every time.

 

[Plumbtastic]

Thanks very much for your responses folks.

There is no water pump installed in the system.

The Cylinder is located in the cellar and the shower on the first floor.

The cold water storage in the roof approx providing approximately 3.5m head of water (raised to increase pressure in an attempt to solve this problem and give a better shower).

According to the manufacturers instructions this should be more than adequate - a min of 1m head is required.

To test the non return valve theory I shall remove them on Sunday and let you know what the result is.

In the mean time I noted previously that (with the mixer removed) the cold water pressure seemed much less than the hot. There are two cold water supplies entering the bathroom. One of them to feed the shower the other is feeding the WC and the basin. The pressure of the latter is much higher.

If installed correctly the feed to the shower should be from the CW storage in the roof space providing a balanced feed.

In another theory of mine, I assume there is a restriction on the CW feed to the shower. The lower pressure in the cold side is stopping the valve opening with potentially scalding outflow. Opening the HW tap on the basin lowers the HW pressure giving a better balance and finally when basin tap closed enough pressure to provide flow to the head...

By removing a section of floor in the adjacent room there is access to the pipes entering the room so to test this theory I'll swap the two cold water feeds entering the room. Again, I'll post the results.

The problem can be solved by following the instructions as twgas pointed out but I'm interested in the cause of the problem, not only the solution.

Thanks again for replies, if there are any more ideas floating around please keep them coming.

 

[Plumbtastic]

Thanks to all who replied.

Top marks go to 'onetap' who correctly deduced the fault being at the non-return valves.... Once removed the shower works great and the customer is happy.


Thanks again....

 

[blondini]

Thanks to all who replied.

Top marks go to 'onetap' who correctly deduced the fault being at the non-return valves...

Hey! I wuz robbed! http://en.wikipedia.org/wiki/Check_valve