I have just installed a new thermostatic shower valve (Screwfix -
Moretti) and connected it with new 15 mm pipe work (around 5m to the
boiler) back to my 18 month old potterton combination boiler. Upon
testing the shower I found it runs from cold to luke warm even with
the temperature knob as high as it goes (there is no other temperature
The hot pipe is very warm and I have left the shower running for a
couple of minutes with no difference, even when hot water arrives at
the taps in 10-15 seconds and as I said the pipes get very warm.
When fitting the therom control it suggested adding the supplied flow
restrictor (plastic) valves if the pressure was greater than 0.5 bar,
my system is so I fitted them.
When turning off the cold supply completly the shower runs very hot,
with the cold open a 1/4 of the way it is warmer but not as warm as I
think it should be. I also removed the flow restrictor from the hot
supply and think this made a little difference (i.e. it was a little
The hot supply seems to have a lower pressure than the cold and I am
not sure if that is the problem and if there is anything that can be
Basically I am out of ideas, I am not sure if it is an issue with
differing pressures, a faulty theromstatic valve or something else.
Any advice is greatly appreciated.
If you need and more info please let me know.