Pumping blended shower outputs

[Kyrt]

Hi,

I have done quite a lot of searching but couldn't find an answer to my specific questions.

I am about to refit my bathroom and en-suite, including replacing one shower and fitting a shower over the bath. I have a gravity fed system with what to me looks like a fairly large tank mounted raised up in the loft and HWC in the airing cupboard. The airing cupboard backs onto both shower/bath locations. I am undecided on whether to go for one or two pumps, and single or twin.

I know I can fit two twin impeller pumps, and also split the hot/cold outputs from a single twin impeller pump into both mixers. I guess I have to weigh up the pros and cons of each option (simultaneous use, tank size, pump failure).

But since all the pipework is exposed in the airing cupboard, the Stuart Turner technical info seems to suggest I can use a single impeller pump to propel the mixed output from a mixer. Is this correct, and are there any gotchas (would an Essex flange still do its job for example, and is the pressure effectively halved)? Assuming this would work well, what about using a twin impeller pump to propel both mixers' blended outputs - do the impellers work independently? This would seem to be an elegant solution - one pump, but no drop in pressure when the second shower is turned on.

Thanks in advance!

 

[Onetap]

The problem is cavitation, excessively low pressure at the pump inlet which will shred the impeller.

If you read the installation instructions on any shower pump, there are details which are intended to avoid cavitation (22mm inlet pipes, full-bore ball valves, minimum static head, etc). Pumping the shower outlet, i.e., putting the shower mixer on the pump inlet, causes an additional loss of pressure due to frictional losses through the mixer valve and this makes cavitation more likely.

I'd stick with a conventional twin impeller pump.

 

[kevplumb]

do the impellers work independently?

only if the shaft has sheared

 

[Kyrt]

OK thanks guys. I will scratch the idea of a twin impeller pump to propel 2x mixed outlets I realised actually one of them is a bath/shower mixer with two outputs so it wouldn't work anyway.

So like using a twin impeller for the bath and a single impeller for the shower could work if inlet pressure is OK, but it might be more trouble than it's worth? I think I will go for twin impellers, now just need to figure out whether it's better to split after the pump or use two pumps...

I'm also having trouble working out what the flow rate will be in my application once installed. Is "head" in this link the vertical distance from surface water in the tank to the pump?
http://www.stuart-turner.co.uk/media/5167-Performance-Monsoon-Standard-3.0-bar-Twin.pdf
Mine would be about 4 metres, which seems a ridiculous 50+ l/min, and it doesn't make sense to me that a shorter distance (and thus lower pressure in an unpumped environment) would give a higher flow rate.

Is it pipework length instead?