Dedicated cold water feed for shower.

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Hello All
Newbie at plumbing, being stupid^Wbrave, fitting thermostatic shower with dedicated 22mm feeds from HW and CW tanks to twin-chamber shower pump. Need pointers on where to pickup feeds from.
Bungalow, vented HW tank, large CW tank in loft. Standard stuff.
HW I think I've got figured out. Using a Surrey Flange (or equivalent if I can get one to fit the unused immersion tapping rather than messing with the existing HW out).
CW I'm confused about. Currently the large CW tank has a single 22mm output - which goes straight to the HW tank input. There are no other outlets from this tank. (Ie, toilet and all taps are off rising main).
Where is the best place to get my feed?
Straight out the tank? I'm scared of fluffing up a connection here - is it easy or hard to make a good waterproof hole? It's a modern circular plastic tank and very large. (~50-100gallon, bigger than yer "normal" galvanised tank anyway)
Or better to spur off the existing 22mm feed to the HWT? (I really want a good flow rate, would doing this throttle it down that much?)
Also - shower pumps. As has been pointed out, I need some head to play with. Is this head, when taken from an indirect hot water tank, measured from the bottom of the tank or the top? Or even from the bottom of the CWT that feeds it?
Thanks!
--
Simon Avery, Dartmoor, UK
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The immersion tapping is likely to be too low the tank to provide useful hot water for a long enough period, unless it is one of those top mounted down pointing types. Don't know if you can get flanges for them.

Add another outlet to the tank. This involves buying a tank connector (strangely) and drilling the appropriate sized hole in the tank. Easy enough if plastic. Remove the swarf if you expect the tank not to leak afterwards.

The problem here is that it will prevent the hot water coming out properly and might cause the hot water system to supply air, damaging your pump.

The last, luckily. The head requirement is so that at least a tiny amount of water flows when the shower is turned on. This activates the flow switch. If a problem, it can be overcome by either installing an instananeous switch to kick start the pump, or supplying the bath taps also. These being large bore and low down give enough kick.
Finally, you may have an alternative that involves much less work. On my previous house, I installed a cheap single impellor pump on the hot side only. As the hot water cylinder was supplied from a tank directly above in the loft with a large bore flowed bend supply, there was no need for a flange in practice. The cold water was taken from the mains. A pressure balanced thermostatic mixer (100 quid) performed beautifully. The pumped hot also supplied the bath taps. Your mileage may vary.
Christian.
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Actually, reading again, not quite. It is from the water level of the cold water tank, not the bottom.
Christian.
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On Mon, 1 Dec 2003 15:12:15 -0000, "Christian McArdle"

It depends on which manufacturer's information you use.
There are two factors.
1) Having enough head to operate the flow switch
2) Having enough head to achieve the pump's specified pressure and flow rate.
I dug out the notes from my Stuart Turner pump and they use the bottom of the CW tank as the reference. This is going to be more conservative than the top.....
.andy
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A bit silly really. They can't know how high your tank is, and physically it is only the water level that can make any difference. I suspect they choose the bottom so it still starts reliably when the tank is mostly depleted.

I can't really see how a few centimetres will affect a high pressure centrifugal pump, though.
Obviously, the allowed head is often quite critical, as it is very common to have a shower head near a ceiling, with the cold water tank on the loft floor above. Also, the figures they give don't normally allow for check valves, which will usually prevent the pump starting in a low head situation.
The alternative is to use a separate low head start (switch or bath tap) or to fit one of those intensely annoying hose tethers instead. Given that one of the main purposes of the shower is to clean the bath, I can't see how people can live with them...
Christian.
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On Mon, 1 Dec 2003 15:41:47 -0000, "Christian McArdle"

I think that that is probably the point.

I guess that it depends on which one. Possibly the lower power 1 bar models would show a difference?

In practical terms, I could only envisage a difficulty (perhaps) if the shower head were a drencher type fitted in the ceiling.

.andy
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But 1 bar is equivalent to a five storey house. I can't see 20cm difference making much of an inroads to flow rate in comparison.

I would have thought there was less problem with a drencher, due to the reduced resistance of that type of shower head and the fact that it doesn't use a narrow hose. My conventional showerhead wouldn't start reliabily with my the pump. However, I am pretty tall and it was a modern house with low ceilings, so the head was jammed against the ceiling to less me squeeze underneath. Coupled with double check valves, the bath tap was needed.
Christian.
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On Mon, 1 Dec 2003 16:38:07 -0000, "Christian McArdle"
g.

I suspect that it was mainly the double check valves that did it in.....
.andy
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Christian McArdle wrote:

I thought 1 bar was about 10m/30ft of water, if so thats only a couple of stories...
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Doh! I was probably thinking of 1.5 bar = 5 storeys. In any case, the pressure differential of the pump is so much greater than the 20cm water height, that it would have a tiny effect on flow rate.
Christian.
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On Mon, 01 Dec 2003 14:15:14 GMT, snipped-for-privacy@digdilem.org (Simon Avery) wrote:

You need a second fitting on the tank. It should be fitted at a level below the existing one, so that if the tank empties while showering, the hot will stop before the cold. This is an important safety issue. If the existing fitting is very close to the bottom, rejig the pipework to make this your new feed and put in a new fitting at a level above the existing. One pipe depth is enough of a difference.

It's reasonably easy.

Make sure you get a tank adaptor with a rubber sealing washer, and two projecting metal tabs on the back of the plate for a spanner. Make sure you have a spanner which fits
http://www.screwfix.com/app/sfd/cat/pro.jsp?id 810&ts607
although it looks like this one comes without a sealing washer.
Drain the tank :-)
Measure the diameter of the tank fitting and use a hole cutter a mm or so larger but not too much more. I have a complete set of cutters that work with a arbor and drill but you can buy them separately in the required size as well.
for example
http://www.screwfix.com/app/sfd/cat/pro.jsp?id 022&ts239
(It may not be this size.)
If you are going to be doing various hole boring work, a set is worthwhile. B&Q have a similar product.
The other kind of cheap holesaw consisting of a backing plate with concentric grooves and thin metal cutters that slot in are not nearly as good IME.
Once you have drilled the hole, clean away any material left around the hole with a new utility knife blade laid flat on the surface. Use a gentle sawing motion.
Fit the washer to the fitting. I generally put a smear of silicone sealant each side of the rubber seal, a little on the threads and on the outside around the hole. This is more belt and braces.
Screw up the fitting nut with spanners/wrenches inside and out.
WHen you come to do up the compression fitting, put a spanner on the inside of the fitting again, otherwise you will undo it. Finally, when plumbed up, tighten the fitting nut once more with spanner on inside and out.

Don't do this. a) You'll screw up the flow rate and (b) it's not safe.

It's from the bottom of the CW tank to the operating level of the shower head.

.andy
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Hello Andy

Understood, and thanks for a very clear answer.

Hehe, yeah... :)

Right. I wasn't keen on messing with perfectly functioning plumbing anyway.

Um, shower head? Surely the bit after the pump isn't /that/ important? I was meaning the priming head these things seem to need.
--
Simon Avery, Dartmoor, UK
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Yes, but you need to be able to turn the pump on. This normally operates using flow switches. For the flow switches to work, you need a slight dribble out of the shower head when you turn on the mixer. This will only happen if there is sufficient unpumped pressure to cause the water to flow.
The alternative is to have an alternative way of kicking the pump into life. Once started, there is no problem in keeping the flow switch happy. Some pumps have the option of installing a momentary switch to kick the pump. Unfortunately, that means having yet another skanky tangle cord. My prefered option is just to run the pumped lines to the bath taps. These large bore items are much lower down, don't require double check valves, and reliably create enough flow to start the pump, although you don't get the opportunity to breed new types of fungal and bacterial lifeforms on them, like you do with the pull cord.
Christian.
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On Tue, 2 Dec 2003 10:25:40 -0000, "Christian McArdle"

AIUI, even though bath taps (on their own) would not require check valves, if there is a shower fitting with a hose that can be dropped into the bath water etc. then check valves are required.
I talked to WRAS some while ago on this, and even though having a roof cistern with ball valve protects the mains, there is also a requirement to protect secondary services in the house. Where a dedicated cold supply from a roof cistern to a shower is used, there is no way for back siphoning to occur, but since the hot is taken from a common point with services to the rest of the house, there is a siphoning risk - e.g. somebody leaves shower head in the bath and somebody else turns on a hot tap. Hence check valves are required for this case as well.
.andy
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Yeah. I've always put double check valves on the hot and cold of anything resembling a shower.
The OP appears to have a thermostatic mixer, so the bath taps will be separate. Of course, if it is a separate shower cubicle rather than an overbath shower, then using the bath taps to jump start won't be very useful!
Christian.
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Hello Christian

Gotya - a mornings' hammering of google has proved most enlightening. Most pumps need 0.5l/min flow and >400mm twixt tank and shower, which I've got easily (nearer 800mm).
$scary order to screwfix now submitted. I estimate it'll only be a few minutes before I realise I've forgotten something.
I wish screwfix stocked surrey flanges though, PITA having to go somewhere else just for one item.
--
Simon Avery, Dartmoor, UK
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On Tue, 02 Dec 2003 09:47:47 GMT, snipped-for-privacy@digdilem.org (Simon Avery) wrote:
.

Running everything in 22mm almost to the shower valve is a good plan as is minimising the pipe length and especially the number of sharp bends. An elbow is equivalent to at least 0.5m of pipe and bending the pipe is a much better solution.
If you don't have one, a pipe bender is a worthwhile hire/investment (again trade off on amount of use is needed). Pipe bending springs seem to be a dead loss. I used them satisfactorily is a heating installation that I did many years ago, but more recently have had problems - and I don't think I'm losing my touch. I believe that the properties of the tube have changed at some point in the last 20 years.
If you would prefer to use plastic, then it is easy to form a gentle bend. You can get formers for doing this from most of the plastic system manufacturers. They clip onto the pipe and hold it with a neat bend of several radii.
The main critical thing with the head is that there is enough to operate the flow switch of the pump. Once the system is full of water, priming is not the issue.
.andy
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Hello Andy

As I'm doing it in speedfit, (Sigh, 28 metres needed, Screwfix only stock amounts of 20-25M...), I'm expecting to be able to swoop bends quite smoothly, so bends shouldn't slow things down. It's a fairly straightforward run, I think. Through airing cupboard, into underfloor void, under landing into bathroom. 8-9M per stretch.

They've gotten much cheaper, but don't know how closely that's related to copper price or manufacturing methods. Whatever, I'm using plastic. :)

I think I understand it a little better now, thanks. Going to be a PITA job as I'm probably going to have to find a plasterer to make a wall straight after I've removed the existing bath so the tiles don't look as shite as they do at the moment. Large tiles and wonky walls don't really work.
--
Simon Avery, Dartmoor, UK
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Yep, lower in height than the feed to the HW tank (when the cold mains fails whilst you are in the shower the HW feed runs out first so you get a cold shower rather than a scalding one).

I fitted a new plastic 50 gallon cold tank last year with three outlets. Follow the instructions, don't overtighten, fill it slowly and let it stand for ten minutes with minimum water in the tank to cover the new joint. Slightly tighten as required if it weeps.

Circular? I never read that properly. My tank is rectangular so the "face" is flat. Should still be okay though.

Bad move. Separate supply required.

Out my depth on that one Simon; good luck otherwise.
Mungo
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The head is the vertical distance between the shower head and the surface of the water in the header tank. The position of the hot water cylinder is irrelevant, because the gravity flow is driven by the pressure exerted by the water in the header tank.
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