shower mixer, odd fault

[ambrosia]

the problem is on a bathstore thermostatic mixer shower with huge shower head. When you turn it on starts off with good pressure, however about 4mins in pressure drops and will continue to drop till its a dribble, temperature remains constant.

there is 2 shower heads attached to this shower, the main one which is fixed overhead, and a 2nd more conventional one which can be detached and waved about.
the actual valve is the exposed thermostatic type, one side you adjust temp the other side you turn on off or direct it to the which ever shower head you want

the problem is only on the large shower head, the smaller head you can run all day without the drop in pressure

shower is fed from a combi boiler and the drop in pressure is not caused by anyone else in the flat using the water at the same time, hot and cold pressure is fairly even

I'm guessing its something to do with the thermostat not mixing the hot and cold very well, but why the drop in pressure and why only on the larger head

 

[Chalked]

Is it a storage combi?

 

[ambrosia]

no just a vaillant 24kw in a 1 bed flat

 

[Allgoode]

Sounds like a problem with the exposed thermostatic shower mixer is at fault.
Another way to test is to check if the flow from other taps reduces out as time runs by with combi running.
Could also be combi heat exchanger being partially blocked etc.
Try to eliminate one area at a time to be able to isolate issue area.
That you can do yourself & reduce potential costs .
Cheers,
Andy

 

[ambrosia]

thanks Andy
other taps are fine, its just weird
if it was the plate heat exchanger you would expect the temp to inconsistent not the pressure

 

[Plumbstar Tom]

sounds to me like the combo can't keep up with the shower. those big showerheads need quite a high flow rate