On 09/04/2021 22:22, Roger Mills wrote:
I am planning to re-furbish an en-suite bathroom, and am considering
installing a Stuart-Turner Monsoon twin impellor pump. I currently have
a Mira Vigour wall-mounted power shower with built-in pump but would
like to replace it with something with a bit more €śurge€ť.
The instructions for the Monsoon say €śThe pump must, for optimum
performance, be sited as close as possible to and never more than 4
metres from the HOT WATER cylinder€ť.
In my case this would be very difficult to achieve because the hot and
cold feeds to the en-suite take different paths and only meet when they
get there (see drawing at
https://app.box.com/s/20x0sg5z9rpwsh27mi2wwsth3d7fk5g0). The hot runs
under the landing and bedroom floors, and the cold runs through the
roof-space. If I install the pump in my preferred location inside a
vanity unit in the en-suite, the pipe run from the hot cylinder to the
pump would be about 10 metres. I have spoken to a droid on the S-T
support helpline, who simply reiterated what it says in the instructions
but wasnt able to explain the rationale behind this. I am struggling to
understand which law of Physics would be violated if the distance
between hot cylinder and pump is more than 4 metres. How does the pump
know how far it is from the cylinder? If it's a cavitation issue, isn't
head and pipe size more important than horizontal distance? What are the
likely consequences of installing it in my preferred location? I accept
that I may need an Essex flange (or similar) in the cylinder to avoid
sucking the vent pipe dry.
Am I missing something?
I think your theory is right, and the droids are just quoting a "rule of
thumb". I'm sure you could envisage layouts (15mm pipe, lots of
non-swept elbows) that might provide enough resistance on the suck side
to give you cavitation at the impeller. You didn't say whether it was
the 2 bar or 3 bar monsoon, or if it was a "universal".
I don't think the formal calcs for pressure drop are all that difficult,
and there are formulae and calculators on the web.
That said, my ST just sits next to the bottom of the DHW cylinder,
taking its feeds from the 15mm rising main and 22mm from an Essex
flange. The shower is directly underneath the cylinder, plumbed in 15mm.
So I have never had to do the calcs for this case myself.