Shower pump/mixer problem?

[johnnym76]

I’ve a strange problem with my shower pump (Salamander CT50). I have a cold water tank in my loft and a hot water tank in my airing cupboard along with the pump. This was installed by a plumber several months ago and it’s always been an issue which has got worse.

When it’s initially started it works fine for around 10 seconds but then when it gets up to temperature the pump cuts out but water still flows through at a reduced rate. If you lower the shower head the pump cut’s in and out but still won’t work properly. However, if the temperature is turned all the way up it works fine and if it’s turned down it works fine, but it doesn’t work at the bloody temperature you ever want it too! Also if you change the temperature it sometimes cuts in and out.

I had an old shower with a different pump and mixer all in the same position and it all worked OK. The plumber changed the hot feed into the pump after a week when I mentioned it to him (which didn’t fix things) and then wanted more money to mess around with the cold feed. I then ran out of patience with him (mostly for many other reasons which I won’t bore you with) and he couldn’t explain why it worked OK before but not since he changed the mixer and pump, which to my mind suggests it’s the pump or the mixer that’s the problem.

Any advice would be welcome before I try changing the pump or the mixer.

 

[stem]

Shower pumps usually have flow switches in the hot and cold outlets. (Unfortunately, I can't find specific data or a wiring diagram for the Salamander to confirm the CT 50 does have them or not, but most do) When the flow reaches a certain rate the switches operate and the pump motor starts.

Based on this, I believe that when you first turn the shower on, because the water is cold, the hot water flow is at maximum and the cold at minimum. Because the hot water flow is at maximum it is enough to operate the flow switch in the hot water outlet and the pump starts. Then, after 10 seconds, the hot water supply starts to run hot, so the hot flow is reduced and the cold water flow increases. Now the total flow is shared between hot and cold, and neither have sufficient flow to operate their respective switches, so the motor stops.

This would also explain why it works with the hot on full, or the cold on full, but when you return it to what you want i.e. a split flow, again there's not enough to operate the switches.

According to the manufacturers instructions, the requirement should be that the natural flow (ie when the pump is not operating) from the shower head MUST be at least 1ltr in 30 sec per side or 2ltr in 30 sec mixed.

So what could cause the problem? Is the pipe to the pump of the correct diameter? It should be 22mm if longer than 2 metres. Is it too long or have lots of bends.

Are the distances between the pump and the header tank in line with the manufacturers instructions?

If there are isolation valves fitted, are they fully open, and if so are they the "full bore" type, or are they the sort that restrict the flow even when fully open?

Finally is the shower mixer or head restricting the water flow because of being an unsuitable type or being blocked.