Main Combi 24 HE Hot water runs hot cold hot

Discussion in 'Plumbing and Central Heating' started by Dik-hurricane, 20 Mar 2016.

  1. Dik-hurricane

    Dik-hurricane

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    Hi all

    After some suggestions on a problem I'm having with my combi boiler, its a main combi 24 he.

    The problems occurs mainly when running the kitchen tap or the shower, the hot water runs for a few minutes and then goes cold for a little while and then back to hot again, and does this repeatedly.
    some times more frequently than others.

    I have noticed that the problem does not occur when the central heating is turned on.

    I have had a boiler repair company come out and they replaced the diverter valve kit, which seemed to fix the issue whilst he was here but the fault returned soon after.

    When running the hot water tap in the kitchen I have noticed that when the hot water stops the boiler loses pressure and the burner light goes out, then the pressure returns and the boiler light comes back and the hot water returns.

    Any help or advise will be very much appreciated.
     
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  3. Sim777

    Sim777

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    obvious fault to any RGI. Get a local engineer in to fix. not a diy job.
     
  4. Dik-hurricane

    Dik-hurricane

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    Hi Sim777 Thanks for your response and I'm happy to hear that the problem should be obvious, I'm not going to be attempting to fix this myself but would like to be informed when I speak to the engineer who "Fixed the problem" initially. The chap who investigated the boiler is a registered gas safe engineer and based on the problem I described above changed the diverter valve kit he has revisited his work and still believes what he has done is correct although the problem still exists. Any clues pointers would be appreciated.
     
  5. Agile

    Agile

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    Well the clue he needs to address is that the hot water still runs hot/cold.

    How can he think he did the right thing if the fault is still there.

    Please keep us updated.

    Tony
     
  6. Dik-hurricane

    Dik-hurricane

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    That is true, However he accuses my tenant of running the taps too slow, Today I visited the house myself just to get first hand and I ran the tap at what I consider a reasonable speed and the fault occurs.
     
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  8. Agile

    Agile

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    That is totally unscientific.

    To turn on the boiler needs about 2.5 litres per min.

    To keep the burner running IF IT IS SET CORRECTLY needs perhaps 3-4 litres per min.

    To make any meaningful assessment you have to operate the boiler within its normal operating parameters.

    Tony
     
  9. steve32

    steve32

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    Normally a service will correct this fault min and max gas pressures need to be checked in hot water mode.
    If this doesn't sort it the gas service engineer will know what parts to replace.
     
  10. adlplumbing

    adlplumbing

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    ask the tenant to check bottom of the radiators, and see if if they have cold spots.
    If so relay that to your GSR
     
  11. wildmatch

    wildmatch

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    Came across this thread accidentally but do know the ins and out of the Main 24HE combi boiler. So I am replying in case anyone else has the same problem. The Diverter Valve (DV) is only part of a sequence to get Domestic Hot Water (DHW) and needs to be controlled. so the DV may be fine but you still get no DHW. This is because it is not seeing the demand for DHW via the sensing pipes connected to it and in that case it will be a faulty or clogged Venturi assembly! For those who are interested I will elaborate. Although I am not saying you should do this as a DIYer.


    When DHW is demanded, water from the Cold water inlet on/off valve and filter flows into the inlet/return manifold, where there is filter and Venturi, and continues to flow through the DHW plate heat exchanger, then onto the running DHW tap. There are two sensing pipes from the inlet/return manifold where the Venturi is housed to the DV. under pressure one pipe will charge the DV and the other pipe returns to the inlet/return manifold. If working correctly you can see the DV operate by the valve rod protruding out and hitting the mirco switch. If it is not coming out fully then the Venturi is not sending enough pressure to it. this could be either being faulty or the assembly is clogged and needs to be cleaned. (This is of course only if the DHW pressure is normal).
    The Venturi can be found in the filter cartridge screwed into the left side of the inlet/return manifold. Within the cartridge body is a filter gauze, flow regulator and the Venturi itself. These usually come as an assembled replacement part but can be separated for examination/cleaning. If checking the above sequence the boiler power supply may be isolated since it is all on water pressure.

    If there is no hot water at all then it is usually a faulty DV or Pressure Differential Valve (PDV). NOTE: this is for the Main Combi 24HE water, there are some that have another venturi type for air/gas pressure and are not related. Main Combi 24HE Venturi assembly part number is 248051 available from Plumbase.

    Sequence of events on boiler when demanding DHW (regardless if Central Heating (CH) is on or off)
    1.Demand for DHW is sensed by the Venturi and pressurizes the sensing pipes in turn operating the DV diaphragm. Cold water runs through the DHW plate heat exchanger where it will be heated by CH water.
    2. DV diaphragm operates the valve rod in the front to actuate the microswitch and rear to actuate the PDV.
    3. PDV diverts the pumped CH hot water from the CH system through the DHW plate heat exchanger and back to the primary heat exchanger and repeats the process. A By-Pass pipe at rear of PDV returns any excess back to the inlet/return manifold.
    4. CH hot water heats up the cold running water in the DHW plate heat exchanger making DHW to the tap.

    The following can be done by a DIY'er.
    If the DHW flow is diminished then there is a problem with the cold water inlet valve that incorporates a filter which may be clogged, this is easy to remove and clean.
    Under the boiler there should be 5 pipes, from left to right these USUALLY are 1. CH heating Flow. 2. DHW outlet. 3. Gas. 4. Cold water inlet. 5 CH return.
    1. Turn switch to off (middle) position.
    2.Turn off the cold water inlet on/off valve NOTE: the filter is just above in a blanking cap.
    3. At a hot tap run the DHW till flow stops and leave open.
    4. at the cold water inlet valve, Undo the blanking cap and remove the thread bush
    3. Extract the filter and rinse thoroughly in clean water.
    4. Reassemble
    5 Turn off the DHW tap and turn on the cold water inlet on/off valve
    6. Check the flow of DHW water.
    7. Turn switch to reset for a couple of seconds then to the position required.

    If still diminished it may be required to clean/check the Filter Cartridge from the Inlet/return manifold but this should be done professionally

    Hope you enjoyed reading and feel free to pick up any faults in the above.

    :)
     
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