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Discuss Loss of flow rate - normal or too much in the Plumbing Forum area at

  1. ANDY557

    ANDY557 New Member


    I'm getting quite a big loss in flow rate upstairs compared to downstairs. Here are the details:

    I'm not getting a great litre per minute of hot water from my combi, I'm getting around 12 lpm (which outputs 18 lpm at maximum). When I looked into it, I'm only getting about 12 lpm of cold water into the boiler from the mains, so the boiler seems to be working as it should.

    BUT, I get 40 litres per minute of cold water at the property boundary (according to Yorkshire Water). At the first tap (an outside tap), I get about 20 lpm which I'm happy with. I recently had new alkathene pipe laid from the boundary to the house so there can't be any problems there, hopefully.

    A little further along the plumbing, still downstairs, I tested the flow into my washing machine and it is also about 20 lpm. However, the pipes (all 15mm plastic push-fit) then go behind the plaster and up the wall to the next floor. I exposed some pipes and tested the flow rate on upstairs, directly from the 15mm pipe, and I am only getting 15lpm. My questions are as follows:

    1. Does it sound normal that I am losing 5lpm from downstairs to upstairs?
    2. I also don't understand why I'm losing a further 3lpm by the time the cold main reaches the boiler (boiler input is 12lpm) which is only about 5 metres from the pipes giving 15lpm, and on the same floor.

    Thanks in advance.

  2. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    What make and model of boiler is it?
  3. ANDY557

    ANDY557 New Member

    It's a Viessmann Vitodens 111w.

    So, not exactly a combi, but a combi for the purposes of this discussion I think?
  4. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    I am not a Veissmann expert but just looked at the manual and it boasts that it should giving you around 18ltrs/m. Are all valves open fully into/out of the boiler?
  5. ANDY557

    ANDY557 New Member

    Yeah, the Veissmann guys told me I should get 18litres/m too but pointed out that I won't get that with 12litres/m of cold going in!

    Yes, all valves open fully into and out of the boiler.

    What do you recon about the drop in litres/m from downstairs to upstairs that I mentioned. Is this normal?
  6. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Yes the 111 is a storage combi so can do much more than your 15lpm

    I would put it frankly down to your plastic pipes 15mm plastic ends up around 10mm bore by the time you talk of inserts etc

    My personal opinion/experience I would install 22mm copper from your new stop tap to the boiler you should be golden then
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  7. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    As above or can you trace back where the supply to the boiler comes from and upgrade the part that reduces? May be a 15mm isolation valve somewhere which reduces the flow further
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  8. ANDY557

    ANDY557 New Member

    Thanks for the replies, really appreciate it.

    The difficulty is (as I'm sure is often the case) that a lot of the pipes are now very hidden. We can take up the carpets and floorboards without too many problems but there is quite a long stretch on the ground floor and 1st floor that is buried under tiled floors (with electric underfloor heating) which I I really want to avoid taking up!

    Maybe the 'nuclear option' is best - leave the existing piping to retain cold water supply for taps, toilets, washing machine etc and run a new dedicated 22mm copper pipe directly to the boiler from the stop-tap. I'll have to go through the ceiling to avoid taking up the tiled floor upstairs.

    That's what I'll do, unless anyone thinks otherwise?
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    • Agree Agree x 1
  9. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    That's a sensible option run a supply just for the boiler
  10. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    That is what i would do, you will get a drop off in pressure and flow on the higher floors due to the weight of water exerting pressure dowards towards the stoptap the mains water supply has to over come this so obviously it will effect pressure and flow the higher you go , there are ways to help overcome this employ the services of a good plumber and ask about accumulators.cheers kop

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