I have a problem with a Salamander CT50 shower pump. It's running a bath/shower mixer. The pump is sited underneath the bath and the run to the HWC is approximately 4-5 metres in 22mm on the same floor. The pump is the first device in the pipe runs from the CWS and HWC. The pipe runs continue to feed the toilet and basin and tees off through the ceiling to feed the kitchen. The pump will only start if the mixer is turned to cold. Once running the mixer and temperature controls work fine. There is a samll inverted loop where the feed pipes come out from under the floor at the tap end of the bath and run along the wall at approx 8" high and then drop back down again to the pump at the back of the bath. Any ideas?