Shower pump location

[Roger Mills]

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 ?urge?.

The instructions for the Monsoon say ?The pump must, for optimum performance, be sited as close as possible to and never more than 4 metres from the HOT WATER cylinder?.

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

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 wasn?t 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?

[Tim+]

I fannied around for years with pumps, and then upgraded to an unvented cylinder. Wish I?d done that to start with.

Unless you have poor mains pressure in which case your options are limited, I?d suggest an unvented cylinder (or a thermal store).

If you still want to go with a pump could you not put your pump by the HW tank and run the cold feed to the shower back up to the loft and tie it in there?

Tim

[Adrian Caspersz]

Our downstairs shower is somewhat aggressive at full bore from the gravity head in the loft, where the tank is raised near the roof apex.

If ye dare step in as full bodied person, ye'd come out as sparkling bones of a skeleton...

However, the upstairs shower we found was weedy until we installed a better bar mixer. The previous was a cheapy probably suited more to direct water supplies rather than tanks.

[Roger Mills]

In my holiday flat I've got an unvented cylinder allowing mains pressure hot and cold to be supplied to a bar mixer - but it doesn't have anything like as much urge as I would like. It's worse than my current rather feeble "power shower" at my main home.

Possibly, but it's a bit messy bringing the cold down and then up again. No-one has yet explained why my preferred solution wouldn't work.

[Roger Hayter]

Apart from flow rate/cavitation which, as you say, depends much more on pipe diameter than length, the only problem I can think of is running cold and then overshooting too hot as the hot water comes through. The solution, of course, is not to get in the shower till it has stabilised.

[David]

I think the general theory is that sucking sucks compared to blowing.

The pressure drop on the input is likely to be the main problem, and the further the pump is away from the tank the greater the pressure drop is likely to be.

Also if you have a cold and a hot feed going by very different routes then there might be an imbalance on the input side of the pump. This seems less likely. However if there is an imbalance then this might cause the mixer valve problems if the outputs are unbalanced.

A long time ago, but IIRC there was a separate dedicated cold feed from the loft down to the side of the hot water tank and some kind of flange near the top of the hot tank. So in theory hot and cold were drawn at the same spot and at the same pressure.

You could possibly have two single pumps but that sounds complicated to set up, and assumes the cold feed only feeds the shower. Although thinking about it you would have to have a dedicated feed to a pump, otherwise you would be sucking on any cold taps or toilet cisterns sharing that feed when the pump was on.

Is it practicable to add a large cold feed to the airing cupboard and then run a new cold feed to the shower alongside the existing hot? A lot more work but removes a lot of the unknowns. Are you prepared to do this if you try your approach and this fails?

Memories also, we managed to empty the hot tank quite quickly and had to have a larger one fitted. We then (more occasionally) managed to empty the cold tank in the loft if we had both showers running at the same time. Rare enough not to warrant fitting a bigger cold tank.

HTH

Dave R

[Roger Mills]

But that's exactly how the existing power shower is fed, and that works ok apart from needing a bit more urge.

The cold feed is dedicated to the power shower - with the basin and toilet having mains feeds. However, the hot feed (unboosted) is shared with the basin - with the power shower being after the basin take-off point.

That would be a major upheaval! Floor to ceiling fitted wardrobes have subsequently been fitted over the part of the bedroom where the hot feed runs, and I don't fancy disturbing those - nor the kitchen ceiling below, for that matter.

Boosting the hot pressure before the basin would introduce another problem because the pump would run when the the basin hot tap was opened but there would be no cold flow through it, which it may not like. So why not use boosted stored hot and cold rather than mains to the basin, I hear you cry? For the simple reason that I use the basin cold for a supply of drinking water, so it needs to be mains.

[Tim+]

Clearly you have mains pressure/flow restriction in that flat then. Does this apply to your main residence though?

I?m sure it would work, after a fashion. I?m sure cavitation issues are why ST recommend fitting close to the hot tank. I doubt you?ll get a definitive answer. You may just have to try it and report back. ;-)

Tim

[Tim+]

Well exactly. It may be at the limit of what you can do by sucking.

And this is when you say ?Sod it, let?s put in an unvented HW system. ;-)

Tim

[Tim Lamb]

Not really been reading this but the *builders* fitted a double monsoon shower pump to the farmhouse. Gravity system with about 2.0m head from loft tank. Tanks to pump distance within 3-4m.

Initial experience was that the pump exceeded the top up feed to the hot tank. An Essex flange was fitted but there were still occasions when an air lock stopped hot water flow to the main bathroom.

I believe there is a reed switch energising the pump which must rely on gravity flow. Is this linked to their pipe run limit?

[Roger Mills]

Almost certainly. The en-suite is at the end of a long and tortuous run of 15mm pipe and there is a significant reduction in flow when water is also being used nearer to the point of entry.

[Roger Mills]

I don't see why. If there is sufficient gravity flow between tank and shower to trigger the flow switch, why should it matter where in the run that switch is placed?

There are actually two types of Monsoon pump - standard and universal. The standard ones need a positive head - and enough gravity flow to trigger the flow switch. The universal ones can work with a negative head. They also have a pressure vessel and a pressure switch. Presumably the pressure drops when a tap is opened, and triggers the pressure switch.

[newshound]

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.

[David]

Indeed. Also, I believe the universal ones have better-rated seals, to cope with the possibility of the pump running dry. For that reason, if my current 9-year-old standard ST Monsoon ever needs replacing, I will probably replace it with the universal version.

[Muddymike]

I installed one under the bath at the opposite end of a big bathroom to the hot water cylinder as a temp measure to boost the existing shower with poor flow. It worked perfectly despite the instructions saying it must be only be installed with a dedicated supply direct from the hot tank and no more than 2 meters from the tank.

In fact it worked so well when later updating the bathroom I left it as is and ran pipes to a second shower in an adjacent room.

Both worked perfectly even when both showers were in use. Mind you STWNFI did moan a bit about the noise and vibrations when she was taking a bath whilst either shower was being used:-)

Mike