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Discuss Flow rates with this combination of pipes/equipment in the Fittings & Pipes area at

  1. MrGatch

    MrGatch New Member

    At the moment I have a very old 15mm copper water supply entering through a wall in the cellar. This services a 15mm network of cold water and a combi boiler (Worcester 37kW) on 15mm copper throughout.

    The house is over 4 floors with a lot of outlets. 5 toilets, 4 showers, 2 baths. Family of 4.

    Don't have any major general flow problems, even on the top floor. I measured the pressure at the highest shower and it was about 2 Bar. Flow rate is acceptable (not measured it), unless another tap is opened in which case the flow drops to a dribble.

    I'm having a new water main fitted. Standard 25mm MDPE pipe entering in a brand new ground floor location, directly off the main in the street.

    Converting the combi setup to a system boiler with 250L megaflow (Super Oso) with a balanced supply. Proposing to run 22mm polypipe from the stopcock directly to the cylinder and a balancing valve. Then proposing to run 22mm polypipe after the cylinder (tee'ing off in 15mm a few times) until it connects with existing 15mm pipework (as it goes upstairs).

    I'm paranoid about flow rate with the new setup and am hoping that you guys can tell me I don't need to worry!

    I just want to ensure that a few taps can be opened simultaneously and that the bath fills up at a good rate.

    If I make an assumption that the source pressure will be 3.5 Bar and flow is good then would an ageing 15mm copper network deliver fast flow-rate water from the megaflow and from the new main?

    Am I wise to run some of it in 22mm?

    Do I need to enquire about 32mm rather than 25mm MDPE?

    Thanks in advance!
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    22mm copper to the unvented system which needs to be installed by someone qualified

    22mm plastic ends up being like 15mm copper (internally)
    • Like Like x 1
    • Agree Agree x 1
  3. Riley

    Riley Plumber GSR Top Contributor!!

    As I think has been mentioned before look at a secondary return on the hot water to ensure swift hot water delivery
  4. rpm

    rpm Trusted Plumber Top Contributor!!

    Assume nothing.
    • Funny Funny x 1
  5. Riley

    Riley Plumber GSR Top Contributor!!

    And as I also said before speak with the water company as they will be able to give you indicative flow/pressure figures to allow you to plan accordingly. I do find this all a bit suspect that you do not have an engineer in designing all this for you in what could be a pitfall laden project water wise
    • Agree Agree x 1
  6. MrGatch

    MrGatch New Member

    Good advice on the 22mm copper from stopcock to cylinder.

    I can't run a secondary return as I don't have access to any of the pipework above ground floor level.

    I have to make a few assumptions as the water supply is being completely replaced. I'm fairly sure about the pressure being acceptable for the megaflow - its more the flow rate I want to maximise.

    I called the water company a few days ago and they guarantee 1.5 Bar and flow of 15-20 L/min. They couldn't provide specific details on my street but did say that where I live in South Manchester would generally have very good pressure and flow and that it would be adequate for a unvented cylinder. I'm getting 2-3 Bar now.

    Do you guys think I will notice a considerable difference between the old and new setups I've described?

    I've read so much stuff on the internet that recommends 32mm MDPE or higher and it sounds unnecessary to me, but I do have the option to do it in 32mm with extra cost and faff if there will be considerable benefit.
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Most places have 25mm alk so what I would do is get the main installed and then test your flow rates at your new stop tap
    • Like Like x 1

    CHRISX Active Member

    Hello MrGatch,

    As Member Shaun has stated the `Standard` pipe size for a new Mains Water Supply from most Water Suppliers is 25mm.

    I would recommend having a 32mm diameter Main installed - this should give about 28% extra Supply Volume over the 25mm diameter pipe.

    We cannot tell You on this Forum how your Mains water supply`s to various outlets on various Floors will or will not be affected by other outlets being opened - however I would definitely get the 32mm Mains pipe installed - it gives You a better chance of being able to use more than one or two Mains water outlets at the same time as some Hot Water outlets.

    To try and ensure that there is very good Volume available at each Floor You would have to install a new `Rising Main` throughout the House - starting in 35mm pipe and gradually reducing as the pipe goes up - 35mm - 28mm - 22mm - that will add to the expense but just having an Incoming 32mm Mains pipe supplying the existing Rising Main pipe would not be advisable.

    IF there is still a problem after having the 32mm Mains installed and upgrading the Rising Main pipework it would have been worse with a 25mm Incoming Mains pipe.


    • Winner Winner x 1
    Last edited: Mar 16, 2018
  9. Riley

    Riley Plumber GSR Top Contributor!!

    I think as Chris brilliantly alludes to, I really think if you want top performance from your system that you should not rely on the builder. It just sounds like you are doing so much research on this matter that you really want to do it properly, with results your family can all enjoy. I fear that the avenue you are heading down is going to lead to unsatisfactory results and you left with a bad taste in your mouth after parting with your hard earned cash. I really implore you to get a second opinion on your plumbing design
  10. justlead1

    justlead1 Trusted Plumber GSR

    When you increase the pipe size on hot supply, it also increases the volume of system. Which means you will need to pull more cold water off before hot water occurs at tap/shower.(high water bills)
    If you do not increase the pipe size on the hot you won't achieve much. So your between rock and hard place. From the new cylinder installation run a 22 mm up through the building and tee off at each floor, whilst doing that run a 15 mm secondary return from the highest point leg and back to cylinder ( if there are no long horizontal runs it may well work on gravity, saving the bronze pump costs)
    It is also worth remembering that when comparing sizes of pipe, it done on Sq area.
    Eg. 15 mm pipe 176 mm Sq
    22 mm pipe 380 mm Sq
    28 mm pipe 615 mm Sq
    32 mm pipe 804 mm Sq
    You can see a 22 mm pipe is more than double to that of a 15 mm pipe etc etc etc.
    You will always be limited when working of mains supply as you have no control over available supply.
    When the use of CWSC (cold water storage cisterns) were employed. One had a constant to work with, albeit low pressure the pipes could be sized according to the flow rates required. As the storage in houses reduces over time ( the installation of direct main systems, combi, unvented etc) draw lots of water from main system, which were not designed to provide such a demand. That is why you can expect lower provision during high demand times.
    • Agree Agree x 1
  11. CHRISX

    CHRISX Active Member

    Hello justlead1,

    I am trying to find out if / where I am going wrong regarding calculating the cross sectional area of the Copper pipe sizes that we have both mentioned in our posts:

    I am not trying to be pedantic - just the opposite - I noticed that the figures in your message did not seem anywhere near figures that I have often Calculated / used when calculating pipework volumes so I did the Calculations again.

    I also realise that the difference in these Calculations don`t really have much significance for this thread - but just to try and find out why they differ so much ?

    My figures don`t come anywhere near yours.

    I know that these internal diameters below are not exact - but rather than use something like `0.9mm wall thickness` / `13.2mm internal diameter` [15mm Copper tube] for example I have rounded up the wall thicknesses to 1mm - 1.25mm and 1.5mm -but they are not incorrect enough to have caused the difference in our figures - just for ease of calculations I have worked out my figures based on these internal diameters:

    15mm pipe = 13mm internal diameter

    22mm pipe = 19mm internal diameter

    28mm pipe = 25.5mm internal diameter

    35mm pipe = 32mm internal diameter

    My Calculations then give:

    15mm pipe - 6.5 x 6.5 = 42.5 x 3.142 = 132.74mm sq.

    22mm pipe - 10 x 10 = 100 x 3.142 = 314.2 mm sq

    28mm pipe - 12.75 x 12.75 = 162.56 x 3.142 = 510.771 mm sq

    I know that You used a notional 32mm pipe but just to complete my Calculations - a 35mm pipe - internal diameter 32mm:

    35mm pipe - 32mm internal diameter

    35mm pipe - 16 x 16 = 256 x 3.142 = 804.352 mm sq

    As I have typed this message with the Calculations I can now see that your figures are for the radius of the external diameters.

    E.G: 15mm pipe - 7.5 x 7.5 = 56.25 x 3.142 = 176.737 mm sq

    However your comment about 22mm Copper pipe being over double the cross sectional area and volume of a 15mm Copper pipe is still valid and well made to the OP.

    I have continued to post this only because it took me so long to type this message - because I type with only one finger.

    I assure You that I have not posted it to be `nasty` or to contradict you for any kind of `one up man ship` or any similar motive.

    I hope that You will take this message in the way that it is meant - it is meant in a Friendly manner.


    Last edited: Mar 16, 2018
  12. CHRISX

    CHRISX Active Member

    Hello again MrGatch,

    I apologise that there is an error in my previous message to You [quoted above] regarding the Calculation for comparing the cross sectional areas & volumes between a 25mm Incoming Mains Poly pipe and a 32mm Poly pipe.

    I realised that I had made a mistake after I did some other Calculations in my last message above.

    Actually a 32mm Poly pipe is about 78% larger in internal cross sectional area / volume than a 25mm Poly pipe.

    Sorry for my previous error.


    Last edited: Mar 16, 2018
  13. justlead1

    justlead1 Trusted Plumber GSR

    Good morning Chris
    I applaud your questions regard plumbing system design. It gives me the opportunity to have an opinion and rant to an audience.
    What is a fact surrounding your proposed system, is without changing pipe sizes. But removing combi and installing an unvented mains feed hot water cylinder will give better performance than the existing. Base on the fact that the frictional residence with combi units is high and water temp. is low.
    However its nice to know when plumbing what performance will be achieved before carrying out the work, hence pipework sizing.
    Head, pipe length, pipe size and discharge are the fundamentals. (Boxes formula) In the 60 is the arithmetic was time consuming using logs, anti logs etc. with calculators or spread sheet on Excel they are almost instant. An give a great overview of cost, performance etc.
    My bitter and twisted attitude derives from those day when the code of good practise wanted 4 gallons per min at a domestic bath tap and 10 gallons per min if a public bath. Every time i see a combi i remember the hours of calc's spent pipe sizing. Coupled with the man hour wasted in the UK. People spending hours waiting for the bath to fill with warm water. Its what cheap buys.
    On a brighter note, Once you have installed the cylinder and connected to existing, you can tweet it on a suck it and see approach. It may take up a few Saturdays but a good excise for proving theory and practise.
    Good Luck with project.
  14. CHRISX

    CHRISX Active Member

    Hello again justlead1,

    Are You mistaking me for the OP of this thread ?

    I am not the person who is trying to achieve much better Mains Cold water and Unvented Hot water volumes at the outlets in his Home.

    I previously gave an opinion on this thread about upgrading the Incoming Main pipe to 32mm Poly and also upgrading the Rising Main pipe starting at 35mm then reducing as the demand reduces to 28mm and then 22mm.

    As You know in an `All Mains water` property the more Volume of Mains water that is available throughout the Home the better for both the Unvented Hot water Cylinder supply and the Mains water Cold supplies to various outlets.

    But increasing the Mains pipework sizes as I described does not `Guarantee` that there will be a great Flow from the outlets - nor that more than 2 Hot water outlets can be used at the same time as that is limited by the 22mm Cold Feed to the Unvented Cylinder.

    However - I feel that what I described gives the OP the best chance of being able to use Mains fed Hot and Cold water at more than one outlet at a time - although with regard to the unknown resistance of the existing pipework to various outlets what volume / flow will be available at the outlets is anyone`s guess.

    I hope that You were not offended by my message to you of last night regarding the Pipe cross sectional areas.

    By the time that I had almost finished typing my message including my Calculations I could see that your figures were based on external diameters - or possibly Copied & Pasted from a Table ? - because the message had taken me so long to type [I type with only one finger] I decided to submit it.

    I was not trying to be pedantic or `trying to be funny` towards You.


  15. justlead1

    justlead1 Trusted Plumber GSR

    Plumbings one thing reading another. Sorry Chris i seem to be lacking in the latter.
    • Like Like x 1
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