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Alpha CB28, cold water

Discussion in 'Plumbing and Central Heating' started by stevemcn, 6 Dec 2008.

  1. stevemcn

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    Hi folks, I am having a problem with my Alpha CB28 Combi.
    When I run the shower, a manual mixer, the boiler fires up fine. It will stay alight for varying amounts of time and will turn off. It then fires back up again within about 10-15 secs.
    Obviously during this interval the shower is then running cold until the boiler fires up again.
    Sometimes if you leave the shower running for say 15 mins, the boiler will stay on and all is well.
    The same thing happened last year, I changed thr Primary Temperature Sensor (PTS) & this sorted it out. So the first thing I changed was the PTS, but this time it did not work, unless I bought a faulty PTS. Any ideas, before I put a hammer to it.....
    I also changed the DHW.
    If anyone from Alpha reads this, can I just say this is the worst boiler I have ever bought, nothing but trouble from the start & Alpha as a company no help whats so ever. At least we have heating........
     
  2. ChrisR

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    The clip-on switch may be intermittent, or the hot water heat exchanger blocked, or the diverter valve stuck .
     
  3. Paul Barker

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    1/ Isolate the cold entry to the boiler. Run a single hot tap (not a mixer) if water comes out you have a problem external to the alpha which you have to fix anyway. Probably caused by your shower. The solution is single check valves at the point of use of all mixer taps and showers manual or automatic.

    2/ Your boilers behaviour is more associated with blocked plate heat exchanger than flow switch. But the flow switch is diabolical. Diaphragm isn't my first thought but a feint differential diagnosis.

    A person who fixes alphas could sort it.

    You think the Alpha cb28 is a bad boiler? You haven't lived.

    It's only fault is the dhw flow switch. I wish other boilers were so predictable. Alphas are the easiest boilers to fix on the planet.
     
  4. methaneman75

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    i would check the flow switch first, cheap and easy to change.
    These boilers are quite pants, every one ive worked on needed expansion vessel recharged
     
  5. stevemcn

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    Update.... this morning I put the old PTS back on, still no good, put new one back on and no hot water or heating. Both A & B (green/red) lights come on together & flash. Fan & pump working but no ignition.
     
  6. stevemcn

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    .
    Hi Chris, I also changd the clip on switch,,,, how easy is it to unblock heat exchanger??, if it is blocked that is, is it easy enough to get off.
    Thanks for your ideas and help.
     
  7. Paul Barker

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    If you are still suffering only from the original fault then your lights would be A illuminated B flashing and the fault is blocked plate heat exchanger if it only does it on hot water.

    though you have bought many parts you are floundering in the dark and some of the help you have been given hasn't been illuminated. I did warn you that the behaviour you described does not point me straight at the flow switch. Though it is a useless part it is not cheap and not indicated primarily in this instance.

    As i said you need someone who knows what they are doing with your boiler to see to it. The money you have wasted so far would probably have paid for it.

    It is not an easy job repairing a modern combi. Any Alpha service agent would have nailed your fault in the first visit and taken about an hour.

    the information you leave out or give incorrectly makes it impossible to fault find it for you here and anyway you choose different peoples' advice which leaves your head in conflict. The thermisters on that model of Alpha are absolutely bullet proof. I changed one or two but was wrong to do so. The guy who covers the whole North of England also found the same to be true. Regarding the comment about expansion vessels. I very much doubt any boiler manufacturer makes their own, I also doubt that there are very many suppliers of expansion vessel. I have encountered many Alphas and only fitted 3 expansion vessels, though i have fitted a similar sample to all other makes of boiler. The design of your boiler apart from the flow switch is as good as it gets for that type.
     
  8. stevemcn

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    Thanks for your advice Paul. The part I bought was only £10., worked last time not this and the DHW switch was purchased in error a coulple of years ago. The chap who fitted the boiler has been trying to trouble shoot over the phone to help me & is coming to see it soon (but not soon enough)...when I say soon it could be a few days......I have a feeling a phone call to an Alpha engineer will be next on the cards. I`ll let you all know what the problem was.
    Fan heaters are bloody great when it`s all you have.
     
  9. Paul Barker

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    My email is in my profile, put yours in yours and I'll send you a pdf of the fault codes, if you email me I will give you a phone number. Can't talk tomorrow as will be on hospital trolleys in corridors all day.

    Trouble with manufacturers help lines is you can't talk to a field engineer (well unless they are rubbish ones, because my mate who phones them a lot always comes off the phone I say what did they tell you, "it needs a pcb") of course it doesn't, he isn't knowledgeable enough to feed them the right information and they are just wanting him off the line. To get proper help you have to know plenty already to be able to get over the get you off the line hurdle and search thm deeply, this often reveals a numpty on the end of the phone who has never been out in the field. But sometimes you get a deep person who has the keys to unlock your fault.
     
  10. stevemcn

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    Hi Paul,
    The chap who installed the boiler came over yesterday (he is a friend of mine), he took the pipe off the fan, blew or sucked it & the thing fired up. So at least we have heating back on, but still no hot water. He thinks the problem may be with the diverter valve assembly. Can you tell me how it works ?. I would like to know what triggers the DHW switch, the switch as you know sits on the valve, on the valve is a small hole where the switch sits on. I assume that in the hole is some sort of pin that moves up & down that in turn operates the DHW switch??. Could it be possible the valve has got gunked up. If I put a thin screw driver in the hole should I be able to see whether if the pin is working by seeing if it goes up & down. Thanks for all your help with this matter Paul….by the end of it I`ll know every moving part (not). My e-mail address : mcneicesteve@hotmail.com.
     
  11. Paul Barker

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    I'll draws you a picture and put it on photobucket.

    [​IMG]
     
  12. Paul Barker

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    A tear in the diaphragm prevents movement of the pushrod, or maybe you get sufficient movement on the initial surge of water demand but it gives up and drops back under the powerful spring force.

    however this is not indicated in your case.

    If you get steady A light and flashing B light the boiler is sensing rapid rise in primary thermister. This tells it that the pump isn't working properly or the thermister is faulty but both scenarios are very rare. The actual thing which is most likely to be happening is that the primary water flow is impeded during hot water operation by a clogged plate heat exchanger.

    You don't even need temperature sensors to tell you this is the problem the flow pipe to the plate heat exchanger to gets steaming hot rapidly you can't touch it. the boiler cuts out and gives you this lockout. At the same time the hot water is very poorly heated.

    In a way it is a blessing in disguise that the Alpha utilises the thermister to prove the pump, because basically when telling you the pump has failed, what it is really discerning is that the plate heat exchanger is blocked.

    To clean which there have been many similar threads lately.

    I am not saying your flow switch or your diaphragm are in great shape. But the plate heat exchanger is indicated primarily. You probably have white thermisters. These are absolutely bullet proof.
     
  13. Paul Barker

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    Don't forget to run the hot water test i mentioned previously this is crucial to good hot water performance especially on a shower.

    Only boiler manufacturers' repair men seem to be converscent with this issue, but it is crucial to fixing hot water problems and much more common than many realise.

    if more plumbers were aware of the issue they would fit single check valves to every mixer tap or shower, but they never are fitted so they are clearly ignorant of this combi nightmare.
     
  14. stevemcn

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    Hi Paul, my friend has spoken to a guy who knows everything there is to know about Alpha boilers, he works on them all the time. Everything points to a dodgy diverter valve, I am hoping to take it off today & will check heat exchanger also diaphragm. I`ll get you informed of progress.
     
  15. stevemcn

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    Meant to say keep you informed......
     

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