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): enter image description here

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?


  • I have never worked on this kind of system. What you describe sounds like the pump is loosing prime. Are the supply lines from the cold water storage tank large enough? With under sized lines and 200mm reduced head pressure this could be the cause. The other thing I see that could be a problem is the valve on the cold water line to the heater is this fully open? There is also most probably a check valve on the inlet of the heater but that is a guess could this be plugged or obstructed? You did not mention anything about the cold side is it running ok?
    – Ed Beal
    Oct 30, 2016 at 15:29

2 Answers 2


Turns out the whole air/negative head/priming thing was a complete red herring. I found the manufacturer's FAQ and in it they state that if the pump is humming then it is likely physically not turning.

So I disassembled the pump and on one side, found that a small retaining spring pin had come loose and had jammed into the impeller, breaking a few of the teeth. Once the pin was put back in its proper place and the pump reassembled, it works again.


The CT50 pump is shower specific and should not be used on single taps. The pump does not incorporate a bypass tube like the whole house pumps. The pump has a single drive shaft so both sides of the pump turn at the same time, so if you are running the hot side of the pump the cold side will also turn. With no bypass tube there is nowhere for the increase in temperature or pressure to be released when the pump is operating at closed head. This will damage one-side of the pump and may lead to impeller breakdown or the body O-rings failing and the pump will start to leak.

The CT50 is also positive head so relies on a flow of water to lift the floats and activate the pump. It requires a natural flow of mixed hot and cold from the outlet of at least 4 litres/minute to start the pump, as the pump is pumping hot only against cold mains pressure there may not be sufficient flow to allow the float to lift. Normally an ESP50cpv would be more suitable as it works when a drop of pressure is detected, so as soon as the hot is opened the pump will kick in so you are not relying on natural flow to lift the floats.

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