1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Megaflo - temperature at tap is only hot when other cold taps on

Discussion in 'Plumbing and Central Heating' started by chrisd1967, 27 Sep 2021.

  1. chrisd1967

    chrisd1967

    Joined:
    9 Mar 2011
    Messages:
    9
    Thanks Received:
    0
    Location:
    Gloucestershire
    Country:
    United Kingdom
    I have a peculiar problem and I wonder if anybody has any ideas.

    I've experimented thus:

    The megaflo tank is full of hot water.

    Put the kitchen tap on hot, with no other taps on. Temperature at tap is 38ºC (i.e. poor).

    Now, with the kitchen tap still running, turn another random *cold* tap on full elsewhere in the house.

    Kitchen tap instantly rises to 53ºC (i.e. good - it's quite a way from the cylinder).

    Turn the cold tap off elsewhere. Kitchen tap drops like a stone to 38ºC.

    I have isolated the hot supply and have confirmed that you don't get a single drip out of the kitchen tap on hot when the hot is isolated - confirming that cold isn't mixing with the hot.

    It isn't specific to the kitchen tap - it happens with any hot tap.

    Any ideas what may be causing it or how I can rectify the issue?! My cold taps are all fed from the mains cold and the hot is fed from the megaflo, with the standard PRV on the input. Many thanks for any insight.
     
  2. Sponsored Links
  3. picasso

    picasso

    Joined:
    8 Apr 2008
    Messages:
    10,631
    Thanks Received:
    2,667
    Location:
    Essex
    Country:
    United Kingdom
    Your supplies are unbalanced, all the hot and cold outlets should be taken from the pressure reducing valve.
     
  4. chrisd1967

    chrisd1967

    Joined:
    9 Mar 2011
    Messages:
    9
    Thanks Received:
    0
    Location:
    Gloucestershire
    Country:
    United Kingdom
    Thanks for your reply.

    The bit I don't understand is how this causes the water temperature to be low when only the hot is on. Surely the pressure of the cold at that point should be irrelevant?

    Would you say that fitting a 3 bar (i.e. the same as on the megaflo) limiter in the main cold incomer would be a good idea?
     
  5. jeff the gasman

    jeff the gasman

    Joined:
    8 Dec 2007
    Messages:
    13,755
    Thanks Received:
    5,072
    Location:
    Cheshire
    Country:
    United Kingdom
    You are getting mixed water probably through a shower mixer or blending valve.
     
    • Thanks Thanks x 1
  6. ianmcd

    ianmcd

    Joined:
    15 Aug 2010
    Messages:
    23,521
    Thanks Received:
    9,721
    Location:
    Lanarkshire
    Country:
    United Kingdom
    As above a thermostatic mixing valve is blending somewhere
     
  7. dilalio

    dilalio

    Joined:
    20 Mar 2009
    Messages:
    7,937
    Thanks Received:
    1,293
    Location:
    Potters Bar
    Country:
    United Kingdom
    +1 for crossover.
     
  8. chrisd1967

    chrisd1967

    Joined:
    9 Mar 2011
    Messages:
    9
    Thanks Received:
    0
    Location:
    Gloucestershire
    Country:
    United Kingdom
    Thanks all.

    Annoyingly there are no signs of any isolators for my thermostatic showers, almost certainly buried somewhere, so I can't isolate the showers to try and work out where the culprit is.

    My mains water incomer immediately splits into two 15mm arms which service various parts of the house. I can fairly easily put a PRV in each of these arms, limited to 3 bar, thereby limiting the pressure going to the megaflo (although that already has its own PRV anyway) and the taps in house to no more than 3 bar.

    Would this in all likelyhood fix the issue?
     
  9. jeff the gasman

    jeff the gasman

    Joined:
    8 Dec 2007
    Messages:
    13,755
    Thanks Received:
    5,072
    Location:
    Cheshire
    Country:
    United Kingdom
    Run all the showers until hot.
    See which one goes cold quickly when you run your sink.
     
  10. Sponsored Links
  11. fezster

    fezster

    Joined:
    30 Jan 2013
    Messages:
    544
    Thanks Received:
    63
    Location:
    London
    Country:
    United Kingdom
    Are you able to isolate your HW supply? Try a hot tap after doing so and you'll probably find cold water coming through it. As already said above, this means you have a thermostatic mixer somewhere where the NRV is stuck open and allowing cold water to pass into the HW pipe.

    The only way to identify which one would be to isolate them. Otherwise, you'd have to strip each one to try and clear the blockage.

    When I had a similar problem, the culprit shower was not getting hot at all. In your case, it looks like you are having this problem on other outlets, so may not help you.
     
  12. ianmcd

    ianmcd

    Joined:
    15 Aug 2010
    Messages:
    23,521
    Thanks Received:
    9,721
    Location:
    Lanarkshire
    Country:
    United Kingdom
    turn the inlet valve to the megaflow off, then turn the showers to max hot and see which one gives cold water out
     
  13. chrisd1967

    chrisd1967

    Joined:
    9 Mar 2011
    Messages:
    9
    Thanks Received:
    0
    Location:
    Gloucestershire
    Country:
    United Kingdom
    I did isolate the hot and ran my mixer taps and didn't get a drop out of any mixer taps at all on the 'hot' setting (I didn't try showers though, I'll try them too). I took this to mean that there isn't any backfeeding going on, as surely I'd have seen cold water come out of the tap when I did that test? Unless the backfeeding only happens when there is also a flow of hot water? Puzzling.
     
  14. fezster

    fezster

    Joined:
    30 Jan 2013
    Messages:
    544
    Thanks Received:
    63
    Location:
    London
    Country:
    United Kingdom
    In that case, it could be unbalanced supplies, as said by a previous poster. Do you know what your incoming cold pressure is? Putting a PRedV on the incoming cold is one solution.
     
  15. chrisd1967

    chrisd1967

    Joined:
    9 Mar 2011
    Messages:
    9
    Thanks Received:
    0
    Location:
    Gloucestershire
    Country:
    United Kingdom
    I don't know for sure, but I've ordered a pressure tester so I'll have the answer soon hopefully.

    There is a t piece butted right up to my mains incomer stopcock that would require a lot of fiddling to move, so if it transpires that my mains water is a mega high pressure, then is my theory of putting two PRedVs in, one on each leg of the t piece from my mains incomer, OK? It sounds OK in my head, but you never know!
     
  16. ianmcd

    ianmcd

    Joined:
    15 Aug 2010
    Messages:
    23,521
    Thanks Received:
    9,721
    Location:
    Lanarkshire
    Country:
    United Kingdom
    If this is something that has just recently happened you are on the wrong track, you have crossover
     
    • Like Like x 1
  17. chrisd1967

    chrisd1967

    Joined:
    9 Mar 2011
    Messages:
    9
    Thanks Received:
    0
    Location:
    Gloucestershire
    Country:
    United Kingdom
    There has always been a bit of an issue with tepid water in the hot taps, which we have always just put down to water taking ages to arrive at the tap. Recently it appears to be worse, so perhaps the mains pressure has increased or something like that.

    My only hesitation with the crossover theory is that when the hot is isolated, we don't get any water at all out of the hot tap. If it were crossover then you'd expect some cold to come out?
     
Loading...

Share This Page