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Low pressure hot water from Baxi combi boiler

Discussion in 'Plumbing and Central Heating' started by Paul Shapland, 27 Feb 2018.

  1. Paul Shapland

    Paul Shapland

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    Hi,

    I have a 3 year old Baxi Duotec combi boiler and over the last few days the pressure/flow of the hot water to the taps has decreased considerably.
    There is a noticeable sound of the water passing through the boiler that was not present before.

    I have checked/cleaned both the water inlet and restrictor valve filters according to the manual, but this made no improvement. Have also checked that the condensate pipe is not blocked.

    Their are no error codes showing and the central heating side works fine.

    Can anyone suggest a possible cause please.

    Thanks for any help you could give.

    Paul
     
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  3. terryplumb

    terryplumb

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    If All hot taps are suffering from the same problem of reduced flow ,I would suspect the plate hex may be partially blocked with limescale ,do you live in a hard water area Paul ? Also check the cold water isolator on boiler is fully open .The second filter is located at the flow switch / regulator ,is that the one you mean ?
     
    Last edited: 28 Feb 2018
  4. Paul Shapland

    Paul Shapland

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    Hi,
    Yes all taps have reduced flow and we do live in a hard water area.
    I have just measured the cold water flow through the inlet tap on the base of the boiler and this seems very similar to the measured flow from all of the hot taps.
    When compared to the flow from cold taps it seems about half the flow.
    This makes me think that there may be a problem with the cold feed pipe to the boiler and not a problem with the boiler itself.

    Any comments on this would be appreciated.

    Thanks
     
  5. terryplumb

    terryplumb

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    If the filling loop can be disconnected from boiler ,see what flow rate this gives into a bucket.this is usually branched off the cold mains feed and will give an indication of cold delivered to inlet valve. Inlet valve maybe restricting ( unlikely but possible ). Is there any isolation valve upstream on the cold supply line to boiler branch ? Did you measure flow before Inlet valve or immediately after it and before the regulator ?
     
  6. Paul Shapland

    Paul Shapland

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    Sorry, but I don't know the correct names/terms to describe all the various parts of the boiler so please bear with me.
    I did actually measure the cold water flow through the inlet tap by disconnecting the filling loop and filling a bucket.
    I filled the bucket for 30 seconds and marked a reference line on it. I then filled the same bucket from from each of the hot water taps around the house and found that it filled to approx the same mark in the same amount of time.
    The cold water feed that goes to the boiler has a tee off to a toilet cistern before it enters the inlet tap on the boiler. I measured the flow into the bucket at the cistern and this filled to the same line in about 15 seconds (half the time).
    The pipe between the tee and the boiler inlet tap is 15mm copper (same as the feed to the cistern) so I expected to get the same flow rate, but maybe there is some restriction in the actual tap. This tap appears to be open as far as I can tell, but can't be absolutely sure unless I disconnect it to examine. I guess there is some amount of restriction through this tap as it is like an isolated valve and the bore is smaller than the 15mm pipe.
    Maybe the next move is to take this tap out and check it????
     
  7. terryplumb

    terryplumb

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    Before you go down the route of removing the isolator valve on the boiler ,can you find out how many litres per minute you get from hot taps. Time 1 minute of water running into bucket ,then measure how many litres are in the bucket. If you don't have a measuring jug in your kitchen use ,something like a 1 litre empty milk bottle ,if you get my drift. The test you have done using filling loop / WC comparison may not be like for like.the filling loop internal hose bore is much less than 15 mm copper tube ,around half. And still does not give a true picture of what is passing thru boiler isolating valve / into boiler. You said earlier that the low flow of water from all hot taps is a recent problem that has arisen a few days ago , was this sudden , or slowly getting less and less flow over a period of days ?
     
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  9. Paul Shapland

    Paul Shapland

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    Sorry for the late reply.
    Yes the flow has slowly got less and less over the last few days.
    Here are the flow test results from each hot tap.
    Sink Tap nearest to boiler 5.25 lt/min
    Sink Tap furthest away 5.50 lt/min
    Basin tap upstairs 5.25 lt/min
    Bath tap upstairs 5.50 lt/min

    For reference
    Cold Inlet tap at base of boiler 6.00 lt/min
    Cold sink tap nearest to boiler 11.00 lt/min
     
  10. terryplumb

    terryplumb

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    Cold Inlet at boiler , I assume is taken from the filling loop ? The valve on the boiler is very unlikely to be your problem. Its either the filters ,something broken off inside boiler and restricting the waterway ,or the plate HEX is partially blocked with limescale.and its the latter that I would investigate next .
     
  11. Paul Shapland

    Paul Shapland

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    Yes cold fill at boiler was from filling loop.
    The flow at the hot taps is 5.25 to 5.50 lt/min and the flow into the boiler is 6.00 lt/min which is not much different.
    I thought that this drop of max 0.75 lt/min may just be due to the normal restrictions through the boiler and that's why I thought that the blockage might actually be in the cold feed.
    I don't have any measurements for the flow at the hot taps before this problem started, but it always seemed to be about the same flow as the cold taps.
    I will check the flow at the toilet cistern again and compare it to the flow at the cold fill at boiler and see what this is.
    The length of pipe between where it enters the cistern and the cold fill on the boiler is only about half a metre.
    Do you think the cold water pressure is about normal for mains pressure?
     
  12. terryplumb

    terryplumb

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    Colds OK ,your boiler probably can't supply a decent temperature at that rate. I would expect around 9 LTS pm at hot taps ,but I don't know the kW rating etc of your boiler ,anyway that's not important. Don't waste your time on WC etc. The rate you get from filling loop will be a lot less than the 15 mm pipe going into the boiler.
     
  13. Paul Shapland

    Paul Shapland

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    OK, thanks for the advice and all the help you have given.
    I am a bit reluctant to take the boiler apart just now as we need it running with central heating all the time at the moment, so might wait a couple of days.
    The boiler spec is Baxi Duotec 40he

    Thanks again
     
  14. terryplumb

    terryplumb

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    In this weather as long as you have heat and some hot water I think your right Paul ,leave it for time being . cheers regards terry.
     
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