New shower mixer intermittent

[archibas]

I have fitted a new shower thermostatic mixer to supply a ceiling rainfall shower. The hot is supplied from a vented hot water tank with an inline pump in the loft, the cold from the water tank also in loft. When switching on mixer there is sometimes no water flow. I turn the cold fully on the cold flows then turning back slowly the hot starts and pump kicks in. Once set it is fine until you leave for a while then nothing again... Any ideas !! I have checked without shower being connected to shower outlet (thinking pressure drop going to shower head) but its the same.

 

[Gasguru]

Sounds like insufficient head to trigger pump....look at the manual and have a measure of the height differences.

 

[archibas]

The hot pump is in the loft about 1.3M above valve. Pump has a max head 6M. Shower about 1.2M above valve and cold tank 1.4M above. Valve operates low pres 0.1 to 1. Just don't understand why its intermittent

 

[archibas]

 

[Gasguru]

I doubt there's sufficient head to start the pump and air will be pulled down the hot safety vent pipe and why the imbalance of hot and cold pressures?
Have a read of Salamanders guide...
https://www.google.co.uk/url?sa=t&r...9-ONLINE.pdf&usg=AOvVaw3dnFu_gqW7vzZLKFQu64bK