Pipe Diameter Advice

[ChrisR]

The sums can be done, chaps.
Saying something like
"That's why I checked with Grohe that our gravity fed HW system would be OK with these taps and they said yes. "

Says nothing. WHat one person calls Ok and what another thinks is not,
can be totally different.

Grohe and Hansgrohe "technical" are thick as...

Point of reference -
1)a standard bath tap with the cistern on the floor in the loft, (3m head?), and 22mm pipe, and 3/4" taps, supplied me 22 litres/minute.

2) A 22mm copper pipe has something about 6 times less resistance than 15mm copper.
( It goes with diameter to the power 2.5)

3) a shower fed by 2 x 15mm pipes, something like 6m long, THROUGH A LOW RESISTANCE MIXER ( Mira thermostatic) gives a shower 10 litre/minute. I know cos my old man put that in his house and I measured it.
more...
A big bathful is 100 liters, so at 12 litres a minute hot, plus the cold, that's 5 minutes to fill a bath. Not too bad.

My bath has a pump on it. It fills at 40+ litres/minute. It's fed by 15mm plastic pipe, bore about 12mm. But I'm a plumber so I have to have a F O pump. (4.5 bar). The shower gives the needles I wanted to hurt me. If you want fast needles, you need pressure at the shower head, and lots of resistance at the shower head, with small holes.

Pipe's like a resistor, except that it's a non-linear one. Similar for taps etc.
(( they have Cv values)). If you measure a certain flow, and you have a certain length of pipe, you can work out the pressure drop across it, then (try to ) work out what the flow would be if the pipe were so big it was no significant resistance.
There's loads of pipe-resistance charts online - try the Copper Development association (CDA) site.
They do vary, but are consistent within themselves...
( some charts give double the drop others do, but in the end they're all close enough)

So - does the 15mm pipe matter?
Imagine you have 10 units of resistance. If the pipe is only one of them, then it's not making a significant difference. (ie less than 10%)
Pipe chart I have here shows that at 0.2 litres/second, (near your 12l/min)
head loss in metres per metre run is
0.035 for 22mm
0.22 for 15mm.

I think I read you have 2.5m of 15mm in dispute.
so you'd get a drop in the 15mm of
2.5 x 0.22 = 0.55m
and 22mm:
2.5 x 0.035 = 0.0875m

That's some difference , you'd win 46cm head.
Your head is 2m, so your flow would go up to by a factor of about
246/200, something like 23%
Maybe 20%, maybe 30% ( I said the charts were flaky) but it's not enough to be too significant.

Pipes have scale inside, bends etc which make them worse than theoretical, but even so, most of your resistance is elsewhere.

There are figures for everything, even the hole in the side of the tank has a resistance. If it didn't, you'd have infinite flow with almost no head.

So most of the resistance is in the taps/valves/mixers.
Look at it another way: all the hot and all the cold has to go through the same shower hose, in most houses, which is often 10mm or so diameter, 1.2 or 1.5 metres long. Far bigger effect than two parallel lengths of 15mm!
FLow loss through 10mm pipe, 20l/min, is huge, near 1m head loss per 1m run!

Just for the exercise 2m head, 0.2 litres /sec (=12 l/min) means that you have effectively,
2/0.22 = 9meters of 15mm in there. Or
2/0.035 = 57 metres of 22mm!

My old bath (22 l/min gravity, 22mm pipe, 3, head) had 8m of pipe.
The chart says it would have been 30m of pipe - so there's LOTS of losses in the fittings.
When I was doing domestic shower stuff, I used 22mm plastic - big slow bends, big enough diameter, quick, usually accessible...

Edit 1)_ I checked the CDA chart - page 21
It makes the resistances higher than the IOP, so you'd see more of a difference increasing the pipe size. Still not going to gush, exactly.

Edit 2) I misread your flow rate - now changed to 12 l/min.

 

[Terry lambert]

So....

Shall we tell him to change the tap, upgrade the pipe or do as mrs securespark says?

 

[doitall]

Look at the data that came with the taps, does it need a check valve to prevent back flow, and has one been fitted on the hot.

 

[ChrisR]

Go for the restrictions. Yes look for an nrv, but you DO need one ifyou have a mixer. You could replace it with an inline bigger one, but they still lose you 0.3m head or so.
See if the tap is connected with flexible connectors. If so, take them out and use something less restrictive. Again, 22mm hep is fairly convenient under a bath, with compression connectors. 15mm tap connectors themselves will often have small holes in the middle though.


But with taps meant for high pressure, you'll never get sensible flow enough to fill a bath with gravity. European mains pressures are about the same as ours, a couple of bar or more, which is 20 metres head.
2 metres head might be "OK" for some goon at Grohe but no-one sensible.

At the end of the day, when all's said and done, the bottom line.... you need more head for "An exciting showering experience". Raise the cistern in the loft a few feet, or stick a pump in.

 

[securespark]

Don't think the bath mixer has flexi on it.

It is rated at 0.2 bar, their website and the literature that came with the tap says so. So it should run at 2m head, no?

But going back to Mrs Secure, if we can't change it, as I'd like to, is 12 litres per minute acceptable to you as plumbers as a flow rate for the hot fill on a bath or not.

Obviously it could be bettered, but would you accept it as a bare minimum?

 

[doitall]

12Ltrs/min hot is better than a combi so yes it's acceptable.

But you need to check the check valve has been fitted, as you have mains cold, that will make a big difference, should say it may not be necessary if the taps are approved, you need to read the paperwork that came with the taps.

 

[ChrisR]

If the bath tap's a mixer, then you need an NRV, somewhere, period. It may have one, in which case take it out and put a bigger one in series, in the pipe. The internal ones are tiny.

12 l/min is not bad, you'll have water at that temperature from your HW cylinder, at 55-60 or whatever, which is a lot hotter than a combi (esp in the winter), as well as a reasonable flow.

When you add the cold ( which will be at about half the rate, cos we all bath at about 44) you'll have a mix going in at about 20l/min so the bath will fill in 4 mins or so for an average one.
(Though some can be 120l)

It is rated at 0.2 bar, their website and the literature that came with the tap says so. So it should run at 2m head, no?

No, it's like Ohms law. That's like saying a light will come on at some voltage or other . The current may be so low the wattage will be dim glow, maybe. All it guarantees is that water will come out, NOT that it'll be fit for your purpose.


NB - that long post a couple back was based on me misreading your flow rate. I've corrected it now for 12l/min hot.
It shows changing the pipe size for that bit would make a noticeable, but not huge difference. You put the flow up and the other resistances go up so you don't get as much change as you hope. Comes out to something around 2 litres per minute more.

 

[securespark]

Thanks again, guys.

The check valve and NRV - are they the same thing? And, I don't understand the importance of this thingy: what does it do?

Chris, thanks for the long calculations: they're a bit heavy for me but I do understand the basics of what you're saying.

So, with the fact that the majority of the HW feed to the bath being 22 and only the last coupla metres or so being 15, the difference will be negligible?

2 litres I can live with...as long as it's not Weston's!

When the bath tile grout and sealants are dry I will fill the bath to the correct temperature and check how long it takes to fill and empty the bath. I have to bear in mind that although the new bath is the same length, it is wider and taller than the old one and the tub is much bigger.

The old bath took in XS of 5 mins to empty and I'm keen to see how much difference the changes to the waste pipes make.

The old waste was 1.5" to 1.25". The new is 40mm - 50mm.

The shower work has been stopped at the moment. I want the pipework upgraded to 22mm. As I said, he is adamant it wil work well, but I want it upgrading. I don't want the walls tiling and stuff only to find the flow worse than a dribble.

 

[doitall]

Yes check valve and NRV is the same thing.

The importance is it stops the mains cold back feeding into the hot pipes/cylinder.

 

[Terrywookfit]

Unless the mixer is Bi flow, you will not be able to run hot and cold together due to the pressure imbalance, which will stop/ reduce the hot supply. Especially with a NRV fitted.

 

[securespark]

Here are the instructions for the Grohe Eurodisc Wall Mounted Bath Mixer:

 

[doitall]

You just have the taps without the shower I believe.

You need the NRV on the hot, you may also need a pressure reducing valve on the cold reading that.

 

[Norcon]

It takes 10 seconds to fill a 2 litre jug at the bath.

I've a gravity system with 22mm pipes. About 3.5m so a short run.
A 2 litre jug from the the hot fills in 7 seconds. The cold being gravity is similar.
Both together its 5 seconds. Bath tap is a late seventies model gravity compatible. Head is 2m.
Just to give you some idea.
At 10 seconds you're not that far behind and would be fine for me.

 

[securespark]

doitall:

Yes, we do. The shower tap was a minimum 1 bar, so I had to go for the one without.

Norcon:

Thanks for that.

How are these valves fitted?

Could they (for example) be retro-fitted under the bath to the pipes feeding the tap?

Perhaps I should ring Grohe again to see if the tap is Bi-wotsit, ooh-err missus.

 

[ChrisR]

How are these valves fitted?

Check valves? They look like over-long compression couplers, so pretty easy.

The most common sort is a "DCV" = a double one, which has two in series inside, to meet regs elsewhere.
You only need a single, which would have less resistance.
A 22mm with reducers each end to 15mm, would also have less resistance.

here ya go:
http://www.screwfix.com/search.do;j...8y7vp!-110413257?fh_search=single+check+valve