Confused about water pressure - what is going on?

[scubapete]

I'm in the process of planning a new bathroom and looking at all the various (cheap and costly) options and getting very confused about pressures and flow-rates etc

Current setup: gravity fed vented system in 1939 house with one bathroom. As far as I can tell a 15mm rising main feeding all cold taps (1 in kitchen, 2 in bathroom). Cold cistern is sat directly on joists about 1.5m above bathroom basin taps and 2m above bath taps. 22m feeds hot water to bathroom taps.

My rough and ready flow rate calcs show 12l/min from cold taps (measured one at a time) and 40l/min from bath hot tap. Kitchen tap is a ceramic disc mixer. Bath tap is a spindle mixer.

I want to get some snazzy, designer ceramic disc taps for the bathroom, but there are disclaimers in the brochures: "although work with low pressure, recommended for use with minimum 0.5bar". Now, by my calcs, I have 0.2bar to the bath tap, which would not be good enough (?). But, there seems to be bucket loads of water coming out. What's going on?

In addition, I thought my onlu option for a good shower was to install a pump. But if I raise the cistern to about 2m head (to shower) and install custom 22mm feed pipes, I should get the same bucket load of water coming out as the bathroom tap. Or am I barking mad?

 

[oldplumb]

hi
lift the cold tanks as high as poss above the shower head
use 22mm pipes to give good flow.
oldplumb.

 

[scubapete]

Hmmm, seems I got my calcs wrong...sorry

Actual flow rate from the hot bath tap is 18l/min.

Is greater pressure needed for ceramic disc taps because the hole at the disc is small, needing a greater force of water to push it through?

Am I stuck with spindle taps for the bathroom?

 

[ChrisR]

Bucket o water tipped over head = low pressure, high flow
Pinholed mains pipe, = high pressure, low flow (in litres/min)

If you use a low pressure mixer like a Mira 88 you'll just about get a reasonable shower, say 6-7 l/min.
My pump's 4 bar...

If it says it needs 0.5 bar, it really needs over 1 bar.

 

[scubapete]

So, if I stay unpumped I need to find 3/4" washer taps and feed them with 22mm.

Alternatively, I can pump the entire bathroom supply and get snazzy ceramic disc. And lots of noise.

Saw this in some pump literature:

"Check pressure requirements of shower/bathroom equipment to be used in metres head"

How do I calculate this?

 

[chrishutt]

"Check pressure requirements of shower/bathroom equipment to be used in metres head" How do I calculate this?

Any reputable tap or shower manufacturer will state the minimum head required for the fitting to operate satisfactorily. Typically this will be 0.1 bar, 0.5 bar or 1.0 bar (1 metre head, 5 metres head, 10 metres head respectively). But note ChrisR's comment above about these figures.

 

[scubapete]

Thanks, guys.

I've seen comments in other posts about the lack of transparency about these required pressure figures from manufacturers and after looking at the websites I can see why.

There is a vacuum of technical information.

I got a glossy from Eastbrook that lists flow rates @ pressures for some of the advertised mixer showers. Now, that's useful (if they are true).

As for taps, I haven't found anything, except this 0.5 bar requirement in *some* literature.

So frustrating.