What's the low down on mixer flow control and shower pumps?

[FoxtrotOscar]

Hi all

I've just installed a Salamander CT50+ and everything is great, but for one niggle...when I turn down the flow control (Mira Pace EV thermostatic mixer) very low for when I'm shaving in the shower, there's a banging noise from somewhere (which I guess is what they call water hammer) and the pump cycles. I haven't tested beyond this initial incident as I don't want to damage anything.

Is this because the outlet flow is too restricted for the pump's impellor speed? Does the pump stop itself due to the low flow and then straight away start again due to the head of water? I would have thought I should be able to reduce the shower strenght to a dribble without any issues, but it seems not.

Many thanks.

 

[echoes]

I think you're right about the low flow turning off the pump. When the pump cuts out, the pressure difference across the pump is sufficient to cause enough flow to restart the pump, and the cycle continues.
It's also possible that when your pump stops, the mixer valve tries to compensate, and that results in some flow to restart the pump.
Cycling can also be caused by air between the pump and shower, but having seen your pictures, I can't see where or how that would occur.

 

[ChrisR]

the pressure difference across the pump is sufficient to cause enough flow to restart the pump,

Eh? If the flow stops, there's NO pressure across the pump.

I suspect trapped air or loose pipes. A theromostatic valve would have a the water in the pipe to respond to so I can't see it being quick enough to hammer. You might have sticky flow switches in the pump.

 

[FoxtrotOscar]

So maybe I'm paying for those slack PEX pipes eh ChrisR?

But anyway these pumps should definitely be able to contend with low flow rates? It doesn't say 'variable' in the specs, is this what variable refers to in the context of shower pumps?

 

[echoes]

the pressure difference across the pump is sufficient to cause enough flow to restart the pump,

Eh? If the flow stops, there's NO pressure across the pump.

Yes, I stand corrected.
But there's some transient increase in flow, caused by the changing pressures when the pump stops, enough to activate the pump's flow switch; caused by trapped air / loose pipes / cavitation etc as you suggest.

Doe the 'hammer' correspond to the cycling? At what approx. frequency?

 

[FoxtrotOscar]

The cylinder 'stat is set to 55 degrees so I don't see how cavitation could be a problem, unless it's broken. I'll test the water temp tonight with a sugar thermometer, just to rule it out.

Likewise with air, as you say I don't see how it could be a problem looking at my pipe runs, and I obeyed the commissioning instructions for both pump and mixer, i.e. run hot and cold independently of pump and each other for a few minutes each.

The only remaining possibilities seem to be:

- Loose pipes as per ChrisR.
- Pump not designed for very low flow scenarios.

Doe the 'hammer' correspond to the cycling? At what approx. frequency?

Not sure, I'll check this tonight. As soon as it happened I increased the mixer flow to maximum as I was slightly concerned about causing damage.

I wonder if the problem point is the point at which the natural flow from gravity exceeds the flow being allowed by the mixer.

In the mean time I've emailed Salamander so let's see what they say.