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Discuss going from 22mm pipe to 28mm? in the Fittings & Pipes area at

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  1. Graeme

    Graeme New Member

    We have a fairly long loop of secondary return pipe from our h/w cylinder.
    The pipe is 22mm for about 3metres then increases to 28mm for the loop,
    then returns to 22mm for the last 3metres to the tank. We have to re-route
    this run and I'm wondering if there is any reason why it cannot all be 22mm.
    Please can someone advise me?
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Does it have a pump on it ?

    Also any chance of a few pictures?

    CHRISX Active Member

    Hello Graeme,

    These comments are obviously without having any knowledge about your property or your Hot Water Supply requirements:

    Usually the only reason for 28mm pipework to be used on a Domestic Hot Water Secondary Return would be where there are multiple Hot Water draw offs connected into the 28mm section or where a Shower Pump or similar is installed - but to be installed correctly the 28mm [Hot Water Supply section] should be connected directly to the Hot Water Cylinder.

    The 28mm should be the Hot Water supply pipe to Bath, Shower, Taps etc. and there is usually no need to have the return section any larger than 15mm as this just allows for circulation of the Hot Water in the `Loop` back to the Cylinder.

    Sometimes pipework is upsized to overcome `internal resistance` - friction slowing down the water flow - for example where there are a lot of elbow fittings on a pipe run - that can be slightly beneficial - but that would not be huge improvement unless the 28mm Hot Water supply pipe was connected directly to the Hot Water Cylinder 28mm outlet.

    Sometimes Plumbers also upsize pipework if there is a Shower Pump being installed - again probably trying to overcome internal resistance - again ideally the upsized pipework should be connected directly to the Hot Water Cylinder or Cold Water Storage Tank.

    So there can be reasons to upsize a section of pipework which has to have multiple Elbow fittings - in some instances upsizing a section of 22mm to 28mm will cause the water flow through the 28mm section to have a resistance [because of multiple elbow fittings] which roughly equals the water flow through the previous section of 22mm pipe.

    Whereas because of the multiple Elbow fittings on that section if it was installed in 22mm pipe the internal resistance [friction] would negate the water flow to an unacceptable level.

    That would usually be done to `solve` a problem with resistance because if a Plumber could see that there was no choice but to have to use multiple Elbow fittings which would cause unacceptable resistance he should have upsized the whole run of pipework at the installation stage.

    Unless the layout of your property and the Hot Water draw offs dictate that the 28mm Hot Water supply pipe has to run around the property and almost back to the Hot Water Cylinder you would only need it to run to the last Hot Water draw off - then reduce down to 15mm to return to the Cylinder.

    In answer to your question as to whether the Hot Water Secondary Return can be re-run in 22mm pipe - check whether you think that any of the situations that I described above were the reason why your property`s pipework was installed in 28mm & 22mm ?

    Something to keep in mind is that unless the person who installed the pipework simply did not know what they were doing - a Plumber would not go to the expense of installing 28mm Copper tube if it was not necessary.

    IF your property is an average sized Home - with just one Bathroom / Bath / Shower / Kitchen / Washing Machine etc. - does not have a powerful Pump on the Plumbing pipework and the Hot Water supply leaves the Cylinder in 22mm pipe you should be able to re-run the Hot Water Supply pipework in 22mm and continue the Secondary Return pipework back to the Cylinder in 15mm.

    However - because of my comment that a Plumber would not go to the expense of installing 28mm Copper tube where it was not necessary I must advise you to have a Plumber check out why they think this was done.

    Whether you do re-run the pipework or not it needs to be very well Insulated.

    Because the Hot Water Supply / Secondary Return pipework is losing heat / removing heat from the Cylinder water all of the time that the Secondary Return Pump is running you MUST ensure that all of the pipework is Insulated to a very high standard - not using the cheap foam pipe insulation that is readily available.

    To do the job correctly the Pipework Insulation should be done to a `Professional Standard` - good quality pipe insulation = Tubolit / Armaflex - Mitred joints - Glued seams etc. - otherwise you will be paying much more money for your Hot Water than you need to.

    Unfortunately - often the way that underfloor pipework has been installed does not allow the retrofitting of pipe insulation to be done correctly - regarding the depth of joist notches having to either not exceed the maximum allowable depth - or having been cut to just about accommodate the pipes.

    I hope that some of this is helpful - You will probably get other replies from our Members.

    Last edited: May 29, 2017
  4. Graeme

    Graeme New Member


    CHRISX Active Member

    Hello Graeme,

    I don`t know whether I have seen all of your message - did you finish it with - `I suppose there would be less "drag" with 28 mm` ?

    From your description of the work that you will be having done in future - I must advise You that a property that has `4 Showers and 2 Kitchens` would almost certainly need a complete upgrade / redesign of the Hot Water system [inc. more Hot Water storage ?] and much larger pipework even if you are expecting to be able to run even 2 Showers simultaneously and that is without any other Hot Water draw- offs / any Kitchen use.

    Also - even with larger Hot Water Cylinders - the Cylinders either have to contain enough Hot water to satisfy the Showering demands and draw offs from the Kitchens or have to be able to be re-heated to keep up with the Hot water demand.

    Even with a couple of Water Mains fed Electric showers to alleviate the demand on the Hot Water Supply pipework the Hot water supply would probably still have to be upgraded - and there would have to be very good Volume of Mains water [not pressure] coming into your property - ideally measured at the furthest point from the pipework entry and also measured with other Mains water outlets running to see how they will negate the Flow rate.

    As I mentioned before reading your plans for more Shower rooms and an extra Kitchen - You definitely need to engage a very experienced Plumber to investigate and design the Hot Water and Cold Water Service to see what needs to be done to achieve a suitable upgrade to your Plumbing to suit your future plans.

    You must try to find a Plumber who has experience with properties that have multiple Bathrooms / Shower rooms OR engage a well established Plumbing Company who have the ability to design a Plumbing system to suit your future plans.

    Even without knowing the Plumbing system in your Home I would be fairly sure that you will need a complete `re-plumb` throughout to achieve what you have planned.

    Making any errors in the Plumbing design for a Home that will be having 4 Shower rooms and 2 Kitchens would be `disastrous` and very costly to rectify.

    Last edited: May 30, 2017
  6. Graeme

    Graeme New Member

    Thank you Chris for your time & replies. (Yes that's how I finished)
    We do have a good system designed by Nu-heat of Honiton. We have an air sourced
    heat pump with an enormous hot tank (about 6' high and 2' dia. Can't remember how
    many litres!!) but they put 22mm pipes thro' the wall to connect to the old 28 mm
    existing flow and return. There is good flow thro' taps/showers even when 2 are on
    call at the same time. I wondered, if there are no 90 degree elbows, just the tees off
    to various units, and there has been a reduction in the first 3', would there be any
    point in enlarging afterwards? I seem to be taking alot of your time!

    CHRISX Active Member

    Hello again Graeme,

    When You describe this - `but they put 22mm pipes thro' the wall to connect to the old 28 mm existing flow and return` - from your previous messages I take it that you mean that before the large Hot Water Cylinder was installed there was a 28mm `Loop` of Hot Water pipework from the previous Cylinder - a 28mm section of Hot Water Supply pipework from which the Hot water outlets were teed off and for some reason the remainder of the `loop` was / is a 28mm section of pipework which was the `Secondary Return` .

    The available volume of water from the Hot Water Supply pipework has been significantly lessened by the fact that the 28mm pipe has not been connected directly to a 28mm connection on the Hot Water Cylinder.

    The 22mm connection to the Cylinder determines the available volume of Hot Water into the 28mm pipe and it is much less than if it was a 28mm connection.

    To see what I mean just do the simple area calculation for both Pipe Diameters [Radius squared x 3.142].

    For the purposes of simple calculations [without getting pedantic about decimal points] the internal diameters are roughly 19mm for the 22mm Copper [9.5mm radius] and 25mm for the 28mm Copper [12.5mm radius].

    You will see that the calculations show that the 28mm Copper tube is approximately 73% larger in cross section and that also means that the volume of water available from the 28mm would be approximately that amount greater than from the 22mm connection to your Cylinder.

    The Secondary Return pipework would not need to be increased to 28mm to connect to the Cylinder.

    I hope that this is helpful for you to decide whether to have the Hot Water Supply end of the Hot Water `Loop` connected directly to the Hot Water Cylinder.

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