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Combi Boiler Hot water issue - Pressure vs Flow?

Discussion in 'Plumbing and Central Heating' started by flibberflops, 12 May 2020.

  1. flibberflops

    flibberflops

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    Help! Can you please help me understand pressure vs flow how to get a really hot shower fed from a combi boiler?

    Our Worcester Bosch combi Boiler gives us “warm” water when any sink hot tap is fully open but you have to turn down the tap flow to get scorching hot water (Small Bungalow, so only one floor involved)

    We now have a new Grohe shower installed and we are struggling to get the water really hot. It’s hot, but not really hot…. Just about acceptable with the heat dial at max. The shower has separate flow and heat deals and I have noticed that you can get it a lot hotter if you turn the flow down to nearly off. Therefore I presume this is some kind of flow issue (like the taps). The standard Grohe flow restrictors don’t seem to help the situation.

    So what’s the solution can I resolve the issue by just trying different flow restrictors on the shower or should I be looking at the cold water pressure?

    Secondary, as the hot taps can’t be used fully open (and retain high heat) should I be looking at the cold flow / pressure feeding the combi boiler?

    Overall, is this a pressure or flow issue, or both?

    Any pointers would be great. Thanks
     
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  3. Chris_W

    Chris_W

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    Pressure is the amount of force behind the water, flow is how much it flow without restrictions/resistance. Imaging driving a car normally and then with the doors open (restriction). If you’re getting poor hot water delivery from your boiler, it could be that the pressure is too great for it to deal with, or needs a part and or service.
     
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  4. durhamplumber

    durhamplumber

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    Look in your instructions and they will tell you what to expect...Ball park...28kw combi delivers 13litres/minute temp rise of 35c...Basically...less flow equals hotter water..
     
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  5. picasso

    picasso

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    Diverter valve stuck midway, undersized gas pipe, partially blocked plate heat exchanger ,there are lots of reasons why your hot water is poor, you could start with the boiler model and size.
     
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  6. muggles

    muggles

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    It could simply be that your boiler isn't powerful enough to keep up with your demand. If that's the case, you'll need a new boiler to fix it
     
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  7. flibberflops

    flibberflops

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    Thanks for all the replies so far. Apologies I never considered that I could have a boiler issue. I have only lived here a year and the tap issue has been like it from day one.

    To answer the questions...

    Its a Worcester bosch Greenstar 30si (25kw)

    "Hot Water Temp range" = 40-60
    "Hot Water 30 degree rise" = 14.3
    "Hot Water 40 degree rise +/- 15%" = 10.8

    Gas input pipe = 22mm (as per manual) + 22mm pipe comming out of the gas meter (assume it's 22m all the way)

    I don't know if this helps but a basic bucket test on the shower was showing...
    Cold supply only = 12 LPM (or 8LPM with flow restrictor)
    Hot Supply only = 3.5 LPM (no flow restrictor)

    Also in the old shower, we basically got good heat but would randomly fluctuate between cool and too hot to stand in without adjusting the shower (when no other water was in use in the building). I assumed it was the shower, but could it have been the boilier?

    I am guessing from your answers so far that this has nothing to do with flow restrictors and probably a boiler issue?
     
  8. flibberflops

    flibberflops

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    by "Demand" do you mean heat? Otherwise we are talking about a situation where no other water is in use ( hot or cold) and the 25kw boiler is located 4 meters away on the same floor.
     
  9. picasso

    picasso

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    A 30si is 30kw to hot water so not sure why you have put 25kw in brackets, what type of gas meter do you have ? if you don't know post a pic.
     
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  11. flibberflops

    flibberflops

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    25 kw = Sorry, I missed read the specifications (has 25si in the column next to it). Yes you are right, its 30KW (Doh!)

    The gas meter is a fairly new Liberty EG4V 10 smart meter (nothing else is connected to the gas other than the kitchen cooker... and these issues happen regardless of cooker use)
     
  12. muggles

    muggles

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    No, by demand I mean the flow rate of hot water you're trying to draw
     
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  13. Chris_W

    Chris_W

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    Try the same test at the kitchen sink. Also check the cold inlet valve underneath the boiler to see it it’s open fully, and check the stopcock is fully open.
     
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  14. MrBenchmark

    MrBenchmark

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    Everyone is missing the obvious the new shower is probably limited to 43c. You will need to adjust the temperature stops to get it it go hotter!! Normally by removing knob or control lever.
     
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  15. flibberflops

    flibberflops

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    Spooky... I was doing the sink test as you posted....

    Sink cold water tap = 15 LPM
    Sink hot water tap (fully open and waiting 60 seconds to warm up) = 10 LPM - water was circa 40 degree's
    Sink hot water tap (3rd open and waiting 60 seconds to warm up) = 4.5 LPM - water was circa 55 degree's - painful to pour onto skin (I am not getting that in the shower and sink is much further away from the boilier)


    Valve and stopcock confirmed fully open
     
  16. flibberflops

    flibberflops

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    Oh FFS, you are right.... Grohe manual "The tempreture of the outflowing water is set to 43c.... etc".

    I feel like such a knob.

    Thankyou all so much for your help, I really appriciate it and I'm sorry I wasted your time.

    Spotters badge for MrBenchmark!
     
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