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How should hot water perform with a combi boiler?

Discussion in 'Plumbing and Central Heating' started by bdw, 4 May 2017.

  1. bdw

    bdw

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    We just changed to a combi boiler a couple of months ago. I am finding that we have to run the hot water for up to thirty seconds before it gets hot. Sometimes it runs hot then cold then hot again. It's a brand new boiler.

    Is this typical of combi boilers (I have never used one before)?
     
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  3. DP

    DP

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    What make and model of boiler
    Boiler running hot and cold for hot water but fine for central heating, is usually system much being the culprit
    When boilers are replaced, system cleaning needs to be effective else boiler starts to malfunction like this

    Luckily your should still be under warranty, Perhaps not so lucky, if above is the case, boiler may not be fixed under warranty.

    Call your installer and have a word with him.
     
  4. bdw

    bdw

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    The boiler is a Baxi DUO-TEC 33 Combi ERP.
     
  5. DP

    DP

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    Might be talking through a hole in the head......... when the burner light goes out, close the hw tap at that instant. Suspect you will see digital display indicating near 90 degree temperature.

    If that is the case, system dirt is the issue. Baxi might attend and flag the issue

    Am sure I will be correct if I am wrong
     
  6. Taking 30 seconds to get the hot water is fairly normal, but you can fit a combi save to save on wasting the water though. But the water shouldn't be going hot them cold, so as it's a new boiler (or have you moved into a new house) then get the installers back to check it. Do you know what the flow rate at the tap is; how long does it take to fill a 2 litre container.
     
  7. imiller9

    imiller9

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    If the water is getting hot, then running cold for 10/20 seconds, then hot again and staying hot then that can be normal especially if the boiler is already on for heating. The water within the boiler gets hot because the boiler is already on or it has been pre-heated and that is delivered to your tap. Then the cold water from the street follows it. The boiler recognises the flow of water and turns to hot water mode and then heats the water and it should then remain hot. If it constantly goes hot/cold/hot/cold, that is a fault.
    30/40 secs for initial hot water delivery is normal depending on pipe run lengths from boiler to tap
     
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  8. bdw

    bdw

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    By coincidence I met the guy who installed it this evening. I explained that was happening and he assures me that is it is normal so I will monitor it for now. Thank you for your inputs.
     
  9. Imiller9 gives a good description, so we need to check the patern from yourself first.
     
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  11. kev25v6

    kev25v6

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    Run a full bath and see if it keeps going hot then cold, after a few mins of running it should be a constant hot stream of water.
     
  12. wundaboy

    wundaboy

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    I had one of these installed last year and it's exactly the same as you describe - have to say I was a bit disappointed with how much water had to run before the hot came through on only a 4 metre pipe run - particularly having had a Worcester combi in a previous house with far less delay.

    Apart from that though it seems to be a good boiler with a decent hot water flow rate. Also, the heating has worked very well over the winter and it's been cheap to run.
     
  13. bernardgreen

    bernardgreen

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    Not only the length of the pipe run but also the thermal mass of the heat exchanger(s) in the boiler. Before the boiler can output any hot water it has to heat up the metal of the heat exchanger(s). These have to be hot before heat can get to the water flowing out of the boiler to the taps..

    Some combis have a large mass of aluminium and copper to heat up before heat can reach the water, other combi boilers have thin walled tubes with only a small mass of metal and in these boilers heat gets to the water much quicker. Most combi boilers have two heat exchangers, the first heat exchanger transfers heat from gas flames to the central heating water, the second heat exchanger transfers heat from the central heating water to the hot water to the taps. The pump circulates the central heating water between the two heat exchangers and it does not go to the radiators when heating tap water.

    It would be interesting to data log water flow and temperature to find out how much water has to be run off ( and probably wasted ) before the water leaving the boiler is hot. This might be a significant amount when it is water on a metered supply and wasted water has to be paid for. Also how much gas is used before heat reaches the water.
     
  14. d000hg

    d000hg

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    That sounds similar to what we get. It makes sense there might be residual warm water in the pipes. I get up after my wife and experience this, but I don't think I do at other times of the day.
     
  15. Agile

    Agile

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    If the boiler is hot because heating is on then hot should get to the tap within a short time.

    If the boiler is cold then it will take longer to heat itself up.

    When cold it might come faster if you only turn on the tap slightly for 10-15 seconds before opening it fully.

    I am not impressed by what the installer told you.

    Tony
     
  16. DP

    DP

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    OP your boiler, I am almost certain, has preheat function. This function heats a quantity of water ready to be used when tap is run. While this function cannot overcome cooling of water in the pipe between boiler and tap that is running, boiler gets a chance to ramp up to maintain hot water flow once the 'cooled' water has duscharged
     
  17. AGAS

    AGAS

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    no pre heat on this boilers chaps
     
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