I've got a Salamander CT50 Twin shower pump that provides pumped hot and cold water to the shower. The hot water output is also piped up to the hot water tap in the bathroom (the cold water tap runs off the mains directly).
We've always had an issue with this pump whereby sometimes when you turn the shower/hot water tap on, the pump will hum but not actually pump and all you get is a trickle coming out from the tap/shower head. If you open and close the tap enough times it will sometimes pick up and work well for a while.
Our setup is similar to the one shown below (taken from the manual):
Rather than their being one pipe connected to the hot water outlet, there are two, with one going to the shower hot water and the other going to the hot water tap in the sink. The other main difference is that the 600mm gap that they recommend is actually more like 400mm.
My own suspicions is this gap is the reason why we are having problems, and that the pump gets filled with air and is unable to prime.
What's the reason for this behaviour? Would buying a negative head pump solve the issue? Alternatively, would placing non-return valves on the outlets of the pump resolve the issue?