I have a gravity fed system hot tank in airing cupboard top of stairs/cold tank in loft, due to low pressure off the hot i had a shower pump put in, but y-day as i was having a new sink fitted in the kitchen i had to turn off all the water, when this was all put back on i now find the shower pump will only kick in when the shower temp is on cold and as i turn the temp up the pump gets louder and then turns off