Flow rates and pressure for showers

[Adam2]

Currently designing the main plumbing aspects and need to determine if we have enough flow rate & pressure for the 2 concurrent showers + someone putting on a washing machine or flushing a loo. Would assume min of 14L/min showers (so a small drop would still be OK).

 

I'll go measure the flow rate with a timer and a jug from the house supply / standpipe. That I guess can tell me if the shower flow will be looking OK. I was looking at getting a simple pressure guage like this - let me know if this is the right thing! 

 

Planning 32mm from meter into house, 22 to manifolds and 15 to showers, 10 to wc/basins as per other threads and this  very helpful schematic that I'll try and reproduce for my setup.

 

The flow / pressure measurements will be at our planned ground floor level and the main shower will be at 1st floor BUT the supply will go via the lower ground. Expecting a UVC - not sure if any other aspects are critical to work out if we'll need to plan for an accumulator or not so please let me know if more info is needed. If we do need an accumulator that will need some space planning and will influence water supply route so need to work this out quite early to ensure we've planned for it.

 

cheers

[markc]

This isnt straight forward, if you stick a pressure gauge on the end of the pipe it will give you the max pressure available, but you are looking for potential pressure drop with both showers on plus a washing machine etc. will the showers be electric or boiler supplied?

[Adam2]

28 minutes ago, markc said:

will the showers be electric or boiler supplied?

 

UVC - so mains pressure

[Dan F]

You can simulate things, based on schematic, with this tool.  You'll need to get the density/roughness etc all correct and ensure you use internal pipe dimensions..

 

https://www.engineeringtoolbox.com/water-supply-system-design-d_2157.html

 

Interested to hear if/how well it works, as I'm trying to do the same...  don't have a schematic yet though...

[Dan F]

BTW, I've been looking at this over the last week too, and here are some other useful resources:

 

- https://www.marshflattsfarm.org.uk/wordpress/?page_id=925  (and various linked pages)

- https://www.houseplanninghelp.com/hph202-designing-the-ventilation-heating-and-hot-and-cold-water-systems-for-bens-house-with-alan-clarke/ (see plumbing schematic)

- https://www.garykleinassociates.com/writings.html

 

I did get a pressure gauge and have got readings between 4.5 and 6 bar.  I'm not using this to calculate anything but think I'm right in assuming that this means that, along with a with 32mm supply pipe,  that the DHW system will almost certainly be up at the limited value of 3 bar and no accumulator is needed.

 

In terms of the sizing of the specific radial runs, I was trying to use that calculator and model everything... but I think the best approach for sizing the radial runs is  just to ensure that each radial run has a maximum of 1bar pressure drop for the desired flow rate and a maximum velocity of 2m/s.  From the UVC to manifold currently assuming probably 28mm as it will be very short, but I do like the idea of a seperate smaller manifold for basins to avoid needing to heat a larger manifold and improve time to hot water (although I'm not sure TMV is really required).

 

[Adam2]

Thanks @Dan F I'll take a look. Will also have a chat with my neighbour as he has just finished building a similar house!

[PeterW]

3 hours ago, Dan F said:

 

- https://www.houseplanninghelp.com/hph202-designing-the-ventilation-heating-and-hot-and-cold-water-systems-for-bens-house-with-alan-clarke/ (see plumbing schematic)

Thats a really poor schematic, and will cause a number of issues and has some errors. 
 

The UFH (which he states they aren’t using) will cause short cycling in the boiler as there is no buffer; the cold feeds to taps will be unbalanced so it will cause issues with any mixer taps or showers as should be off the control block; WC feeds are not shown (but should be where all cold is shown); the hot water will be throttled as there is a 22mm supply but a 15mm outlet (..??!), and 12mm to showers is far too small. He also off balances the showers using 12mm hot and 15mm cold. 
 

Also, his heating schematic shows two bedrooms without radiators - this is the issue with PHPP in that he only wants a small heat input (~5kW from a quick add up) yet he discounts UFH. The better choice for this would have been UFH with a 8kW ASHP which would be more cost effective and simpler to install. 

 

 

[Dan F]

To be fair it's hard to know if those are final construction issue designs or just a draft that have been added as artwork but agree it raises a number of questions.  Few novice questions/comments to see how much I do/don't understand this though:

 

46 minutes ago, PeterW said:

the cold feeds to taps will be unbalanced so it will cause issues with any mixer taps or showers

This seems logical as best practice, but surely even if feeds are both the same bore isn't it unlikely that, in most typical install, this will necesarilly mean that the presure is balanced just because cold/hot piping may have different lengths, different numbers of fittings and are used differently.

 

46 minutes ago, PeterW said:

the hot water will be throttled as there is a 22mm supply but a 15mm outlet

Surely this depends on the incoming water pressure and the pipe lengths though?  If lengths are short and the flow rate can be achieved then don't see why this would be an issue in itself. 

 

46 minutes ago, PeterW said:

12mm to showers is far too small.

Again, doesn't this depend on pipe length and desired flow rate.   The proposed shower (hot feed) flow rate is only 7l/min and the furthest shower seems to be max 10m away from manifold so,  using 12mm OD (8.8mm ID) MLP pipe, that gives a pressure drop of 0.5bar which isn't much if the system it at 3bar.

 

That said, I wouldn't personally plan for 7l/min showers and would want to ensure 2-3 showers could be used concurrrently (which would almost certainly mean >22mm supply and >12mm for showers).  Neither would I use boiler and radiators in a passive house.

 

 

 

Edited January 8, 2021 by Dan F

[PeterW]

47 minutes ago, Dan F said:

This seems logical as best practice, but surely even if feeds are both the same bore isn't it unlikely that, in most typical install, this will necesarilly mean that the presure is balanced just because cold/hot piping may have different lengths, different numbers of fittings and are used differently.

Nothing to do with pipe size, this is because he takes his cold feed off before the control block and not from the balanced cold port. This will affect balance as you could have 6bar cold and 3bar hot. 
 

47 minutes ago, Dan F said:

Surely this depends on the incoming water pressure and the pipe lengths though?  If lengths are short and the flow rate can be achieved then don't see why this would be an issue in itself. 

It’s really bad practice ..! You have put a restriction where one is not required and 99.9% of tanks come with 22mm as a minimum in and out. You will struggle to get full flow into the manifold at 15mm, even with decent pressure. Also, look at his layouts and the tank and manifold are miles from the cold supply - classic under the kitchen sink and this is a new build so why not the utility and run everything from there ..??

 

47 minutes ago, Dan F said:

Again, doesn't this depend on pipe length and desired flow rate.   The proposed shower (hot feed) flow rate is only 7l/min and the furthest shower seems to be max 10m away from manifold so,  using 12mm OD (8.8mm ID) MLP pipe, that gives a pressure drop of 0.5bar which isn't much if the system it at 3bar.

Pressure drop may be 0.5bar (if you’re lucky) however he’s got it mixed with a full mains 15mm supply. So off balancing will mean your 12 l/min cold will swamp your 7 l/min hot and you won’t be able to balance it correctly. 12mm MLP has a bore of 10.2mm, and at 3bar and 10m length my calculator shows a flow of 4.6l/min hot and 15mm cold at 4.5bar giving 12.6l/min. That is a huge discrepancy. At 3bar even 15mm is only giving you 8.8l/min so this is why showers need decent sizing. 
 

47 minutes ago, Dan F said:

Neither would I use boiler and radiators in a passive house.

Nope neither would I and he discounts ASHP as expensive imported tech which it isn’t.. and I wouldn’t call Vaillant reliable unless you install all their kit and controls, as they don’t play nicely with other systems ..!! There are better products. 

[Dan F]

2 minutes ago, PeterW said:

Nothing to do with pipe size, this is because he takes his cold feed off before the control block and not from the balanced cold port. This will affect balance as you could have 6bar cold and 3bar hot. 

Right, I didn't see that, but that makes complete sense now!

 

4 minutes ago, PeterW said:

You will struggle to get full flow into the manifold at 15mm, even with decent pressure.

 

15mm to manifold will work for one shower, but I agree I can see that being an issue, even with decent pressure, if you want to use much more than one shower.   Seems a bit silly for a new build with 4 bathroom!

 

29 minutes ago, PeterW said:

12mm MLP has a bore of 10.2mm, and at 3bar and 10m length my calculator shows a flow of 4.6l/min hot

Out of interest, how do you get to 4.6l/min? 

 

 

[PeterW]

41 minutes ago, Dan F said:

Out of interest, how do you get to 4.6l/min? 

 

I use Copley’s calculator as it allows you to change the values via the graph

 

https://www.copely.com/tools/flow-rate-calculator/

[Rich123]

The shower flow rates will be lower than 14 l/ sec, I would suggest 8 l/ sec . High shower flow rates are not very energy efficient for one reason. And the flow rate is split Hot Cold with about 65% Hot so the cold flow is quite low.

32 MDPE will be fine for the supply to the house.

I would supply 15 mm to all outlets. It is normal practice to allow diversity to all of the outlets and generally a 22 main to a bathroom Is more than adequate.

The mains incoming pressure will vary, but you can ask the local water authority to check the pressure if you can’t do it . It’s unlikely it is less than about 1.5 bar so you should get 1 bar at the shower heads which is adequate for a mixer shower. 
I would also add a pressure control valve on the inlet.

A decent shower valve and don’t forget it is a requirement for a thermostatic mixer on the baths , I would use a TMV3 standard valve for safety.

Re hot water , a combi of minimum 35 kw to get 2 showers.

Edited January 9, 2021 by Rich123

[PeterW]

5 hours ago, Rich123 said:

 

32 MDPE will be fine for the supply to the house.

I would supply 15 mm to all outlets. It is normal practice to allow diversity to all of the outlets and generally a 22 main to a bathroom Is more than adequate

32mm is only needed for high flow high use properties where 3 showers are required concurrently. 25mm at 2.5bar is more than adequate

 

15mm off a 22mm main is very traditional and requires point of use isolation and it’s better to use manifold isolation if possible plus 10mm to basins as it reduces cold leg and increases efficiency. 
 

8l/min is on the low side of acceptable for flow - most mains flow shower heads are designed for 12 l/min which is what consumers expect these days.  

[Rich123]

5 hours ago, PeterW said:

32mm is only needed for high flow high use properties where 3 showers are required concurrently. 25mm at 2.5bar is more than adequate

 

15mm off a 22mm main is very traditional and requires point of use isolation and it’s better to use manifold isolation if possible plus 10mm to basins as it reduces cold leg and increases efficiency. 
 

8l/min is on the low side of acceptable for flow - most mains flow shower heads are designed for 12 l/min which is what consumers expect these days. 

OK I assume this was traditional connection. I’m not sizing the system agree I would normally allow 25 max on domestic houses.

interesting about the showers , anything over 10 l/ sec is seen as low efficiency ,  in commercial, i would only use 6 l/ sec, and I have that at home also and the shower is fine .

 

 

[Dan F]

13 hours ago, PeterW said:

I use Copley’s calculator as it allows you to change the values via the graph

 

https://www.copely.com/tools/flow-rate-calculator/

 

Do we know what assumptions this calculator uses though?  I get 24 l/min (using the Copely pressure loss calcutor) assuming: 3bar supply, 10.2mm ID, 10m length, 60C water, roughness of 0.007mm (hep2o) and assuming that the fitting needs 0.5bar.  The velocity at 24 l/min is 5m/s though,  so you wouldn't want to do more than 10 l/min with this pipe diameter.

 

That's a rather different result to 4.6 l/min!    Wondering if I've done something very wrong, or that flow rate calculator is mis-leasding. 

 

Interested because the flow rate calculator/graphs also suggests that 10mm (8.8mm ID) can't be use for runs longer than 1.8m if you want to acchive 6 l/min, which doesn't seem to map to peoples experiences using this approach.

 

 

 

Edited January 9, 2021 by Dan F

[PeterW]

Depends if you want to get into Darcy-Weisbach calculations here, and the turbulent flow characteristics of small pipes ..??

 

As a rule of thumb (which John Hearfield explains well) then the following are pretty good for flow rates with little noise

 

pipes

	10 mm	15 mm	22 mm	28 mm	35 mm	42 mm	54 mm
1.5 m/sec	0.08	0.22	0.45	0.82	1.3	1.9	3.1
2.0 m/sec	0.1	0.3	0.6	1.1	1.7	2.5	4.2

 

That gives your 10mm pipe a max flow of between 5.4 and 6 l/min - you couldn’t put 5m/s through a 10mm pipe without significant noise and pressure loss. 

[Dan F]

29 minutes ago, PeterW said:

That gives your 10mm pipe a max flow of between 5.4 and 6 l/min - you couldn’t put 5m/s through a 10mm pipe without significant noise and pressure loss. 

 

Completely agreee.  Main point though, was that I'm not sure I would trust that calculator for design for domesitc hot supply suply (vs. hoses) as it looks like it's baking in some assumptions which gives rather different results.

 

A better concrete example is that it suggests that the max length of 10mm pipe (8.8mm ID) to achieve 6l/min with 3bar is just 1.8m.  If this was the case then using 10mm for basins (as a lot of people do) would be a terrible idea.

 

[Rich123]

Seems to be a big discussion about flow rates and pipe sizes here.

The whole system should be calculated and the pressure drop assessed. Sizing on velocity can be risky in some circumstances ie low pressure.
I don’t really understand why you would pipe in 10 mm , it’s not any cost saving .

just run 15 mm and reduce at the outlet .

I would add that if you are using flexible connections the bore on some is reduced substantially. These can delaminate over time so better to flex 15 and reduce after the flex in my opinion.

I also note some of the flow rates and wonder how you are complying with the  requirement of Part G of the building regulations .

I am not up to speed on this as I’ve only used the Code for Sustainable homes in the past.

[PeterW]

57 minutes ago, Rich123 said:

I don’t really understand why you would pipe in 10 mm , it’s not any cost saving

A massive reduction in cold legs on hot water feeds to basins..?? Faster hot water delivery ..??

 

A 15mm pipe holds 2.5 times the water a 10mm pipe does. So it is a significant difference when you think of taps on basins in WCs and bathrooms as the hot is delivered significantly quicker. 

[Rich123]

On 09/01/2021 at 19:03, PeterW said:

A massive reduction in cold legs on hot water feeds to basins..?? Faster hot water delivery ..??

 

A 15mm pipe holds 2.5 times the water a 10mm pipe does. So it is a significant difference when you think of taps on basins in WCs and bathrooms as the hot is delivered significantly quicker. 

Yes possibly there is an issue but I wouldn’t position a manifold that far away that it’s an issue.

A local bathroom manifold seems the sensible answer served from a suitably sized main. 

 

 

 

 

 

 

 

 

[Dan F]

11 hours ago, Rich123 said:

A local bathroom manifold seems the sensible answer served from a suitably sized main. 

 

If you have a local bathroom manifold then you need 15 or 22mm to the bathroom, which increases the delay for hot water, unless you rely on secondary circulation.   The goal of central manifold and 10mm radial feeds to basins is to:

- Avoid need for secondary circulation but still get short delay for hot water.

- Reduce heat losses (due to less water in pipes).

- Central isolation point is also useful.. and for some people this is preffered over finding suitable locations for manifolds in bathrooms.

 

In our case our basins are at 4, 5, 6, 12, 13 and 14m.   With 10mm it'll take between 2.5 and 8 seconds at 6 l/min to empty these runs yet with 15mm that goes up to 6->20 seconds.  (With 22m it's 13->42 seconds). Given the maximum 8 seconds is a guest en-suite, we'll likely not bother with any secondary circulation at all.

 

Planning to use Uponor 12mm (8.8mm ID) as the Hep2o 10mm only has 6.7mm ID which would mean 2.8m/s velocity at 6 l/m (vs. 1.6m/s)

 

 

Edited January 11, 2021 by Dan F