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Greenstar Heatslave producing very limited supply of hot water

Discussion in 'Plumbing and Central Heating' started by Marcodn, 26 Oct 2020.

  1. Marcodn

    Marcodn

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    Hi, would be extremely grateful for any pointers on this problem

    The full name of the boiler is the Worcester Greenstar Heatslave 18/25 Oil combi. The boiler has a 69L heatbank. We leave the water permanently set to on.

    When I run the hot tap nearest the boiler i get hot water as expected for the first couple of minutes. At 2 mins the temperature starts to fall and by three mins the water is luke warm.

    This is not enough to complete a shower without the wife having to rinse conditioner out of her hair in cold water

    The boiler is firing up as soon as I start drawing water From the tap. As I understand it, you are initially drawing hot water from the heatbank and the boiler keeps replenishing that with hot water until you turn the tap off.

    It seems the boiler is not able to keep up with the rate the water is required at and I’m inclined to think that the water starts getting colder after the hot water that was stored in the heatbank has been used up. If turn off the tap, the boiler keeps going for a while (topping up the heatbank, I presume) and wait for a while you can draw hot water again, but not for long.

    Thanks
     
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  3. oilhead

    oilhead

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    Is the flowrate noticeably less than it has been? You don't draw off the heatstore, you circulate that water through a plate heat exchanger which may be scaled up. How does the hot tap flow compare with the cold water tap at the same point?
    Is this a new problem, or has it always been the same?
     
  4. muggles

    muggles

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    Could be a scaled heat exchanger as Oilhead says, or it might be that the heat store isn't heating properly
     
  5. Marcodn

    Marcodn

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    Hi. Thanks for the reply and the correction on how it works

    The hot water flow hasn’t decreased and is much like the cold. It’s not a new problem in that we removed a bath as there was never enough hot water to make it usable (we used an electric shower instead) In August we removed the bath and electric shower and had a shower installed (mixer type) running off the boiler. At first all seemed well - you couldn’t turn up the shower to full heat because that would be scolding and the shower wouldn’t run out. Now as I say it drops after a couple of minutes. Even turning the heat up to full makes no difference
     
    Last edited: 26 Oct 2020
  6. Marcodn

    Marcodn

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    I had someone look at it and they said since the boiler fired up fine and put out very hot water they thought that there wasn’t a problem with it, but they didn’t leave the tap running until it went cold as I had
     
  7. Marcodn

    Marcodn

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    So is there a sense in which the hot water in the heatstore is ‘used up’ even though it’s not being drawn directly from there? I Now understand it go to the heat exchanger To heat up incoming cold water
     
  8. oilhead

    oilhead

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    There are several points to consider.
    How old is the boiler?
    Is there any scale reducer fitted to the incoming cold water?
    Does the central heating work properly?
    How much hot water are you drawing off the boiler? ( It is designed to deliver 18 lts/min at a 40oC raise for a total of 90 litres. With the incoming mains at about 50-10oC at present, this will only give you 45o-50oC at the tap. )
    If you reduce the flow at the tap does the hot water last longer?
    What setting do you have the Hot Water at?
    Does the diverter valve operate correctly?
    Maybe you should get the opinion of someone more experienced to 'look at it' and express your concerns as it is being assessed.
     
  9. Marcodn

    Marcodn

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    • The boiler was here when we arrived in 2012
    • There is a combimate
    • Central heating appears to work fine
    • A rough check with a jug and a digital stop watch is 1l/4s which is about 15l/min
    • Yes, at a rate of 1l/15s or 4l/min it was still piping hot at 5minutes
    • Both the hot water and the heating dials are set a 4
    • I can comment intelligently on the diverter valve

    Since I last posted I ran the Worcester tech support. (I didn’t know I could do that initially as the boiler is long out of warranty I would imagine)

    He told me to check, sensor, stat, diverter and plate in that order
    He said I should get 3 to 5 minutes hot water before the burner fires up, but I told him it comes on as soon as I turn the tap on. Then he suggested the heat store wasn’t being heated up properly

    The trouble is I have called an engineer out under a care plan. He did a service and said the boiler is fine, but it’s clearly not
     
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  11. oilhead

    oilhead

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    Turn up the hot water stat to just under maximum, wait for the burner to stop and then try the flow tests again. From experience, I think his 3 -5 minutes is a little optimistic before the burner fires.
    Many oil combis have a fixed hot water thermostat at 80oC
     
  12. Marcodn

    Marcodn

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    Thanks, OH.

    OK turned the hot water stat to just under max (6) and ran the tap full open. There was a slight difference in time before it lost heat, but only 20-30 secs max, which is probably within the margin of error. I waited until the burner had stopped after turning the stat up to max, but when I opened up the tap the boiler fired up again immediately. Even if the Worcester tech support was being on the generous side with his estimate of 3-5 minutes before it fires up, it shouldn’t come on immediately, should it?

    Also the CH was off and I opened up the radiator next to the boiler but there was no heat - does that rule out the diverter?
     
  13. oilhead

    oilhead

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    Did the burner run when you turned the stat up? If so, for how long? You need to let it run up to temperature before testing.
    You can watch the diverter valve. Switch on the heating and hot water and turn down the hot water stat to off. When the heating is running, turn on a hot tap and you should see the lever move.
     
  14. Marcodn

    Marcodn

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    Hi. The burner did run when the HW stat was turned up *(and I did let it run for a while until it stopped), but I didn’t note how long that was for. I can retest that and also the second routine you mention.
     
    Last edited: 28 Oct 2020
  15. Marcodn

    Marcodn

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    Ok I ran all the tests again.

    Firstly, I got a bigger container to fill and more accurate timing. 12 litres took 31 seconds so 23Lpm

    I ran the hot tap until is was look warm then it took the boiler 5m 35s to heat to ‘4’ on the stat
    Turned the stat to ‘6’ (max) and the boiler fired for a further 3mins

    So with the stat at 6 opened up the hot tap fully. At around 3 mins the water was losing heat, 3:40 it was luke warm
    Ran the same test again with stat on ‘4’ and got same results, although I have to say I kept my hand under the tap as much as possible this time and it seemed to vary a bit. I could keep it there for 30 secs before pulling away and obviously running towards 3mins I could put my hand there

    I did the routine with the diverter valve and the CH. i could hear a click and some grinding, but I couldn’t see anything move because after removing the grey (Drayton) cover I still couldn’t see any valve and I didn’t want to tamper any further

    Cheers
     
  16. Terrywookfit

    Terrywookfit

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    The spec for your boiler is a max of .... 90 litres of water @ 40° temp rise Which equates to 18 litres for 5 minutes.

    If you are drawing water at 23 litres per min then 90 /23 = 3.9minutes.

    Also check nozzle size on burner has not been de -rated.
     
  17. JohnD

    JohnD

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    part of that will be due to the incoming water supply from the main being colder in winter than in summer; so you mix less cold into the hot, and the boiler takes longer to heat it (or delivers cooler HW)
     
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