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Loss of heat to radiators, but still hot water - usual suspects seem ok...

Discussion in 'Plumbing and Central Heating' started by monkeyfinger2, 6 Dec 2016.

  1. monkeyfinger2

    monkeyfinger2

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    Glow worm Hideaway boiler, Honeywell 4073 3-port valve, Grundfos pump, microbore pipework. System has had no recent physical changes – heating back on after summer and running for some weeks since then. Recent sudden loss of heating but still hot water present in tank. Turning CH on and off with the programmer, the 3-way valve cycles back and forth and the boiler fires up. The CH pump can be heard running, very smoothly. It takes 20 mins before any heat can be felt in pipe to valve (port AB). Then it takes similar time for heat into CH pipe from port A. It is a good hour before the first rad in system starts to feel remotely warm. Eventually this rad gets fairly hot but all others on system stay cold, with only vague warmth to the feed pipes. No air can be bled from the radiators (water looks very clear flowing from these), and the pump has a small amount of blackish water but no air when drained. Needless to say, this is not how the system used to be! Any clues here, or am I looking at a blockage somewhere?
     
  2. ianmcd

    ianmcd

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    sounds like the pump to me,just because the motor is spinning doesnt mean that the impellor hasnt sheared
     
  3. monkeyfinger2

    monkeyfinger2

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    Ok...how would I check though? The shaft certainly turns easily with a screwdriver when the drain plug is out, but I guess it still would if impellor had sheared. If it had sheared off, surely I would never get hot water up to the pump from the boiler and I have a tank of hot water...?
     
  4. ianmcd

    ianmcd

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    which pump do you have ?
     
  5. monkeyfinger2

    monkeyfinger2

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    Grundfos Selectric
     
  6. ianmcd

    ianmcd

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    good make , close the isolation valves either side of the pump and use something to catch a small amount of water that will come out, remove the 4 Allen Bolts that hold the head onto the body and gently split the pump and examine the pump head and impellor, I have seen them sheared or the blades totally blocked you will see what I mean when you take it out , obviously isolate the electricity supply before starting, a very easy job
     
  7. monkeyfinger2

    monkeyfinger2

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    Ok, I'm with you. I assume there is a seal between the 2 halves, will this need replacing or ok to re-use? Saw another suggestion that when switched off, if the shaft rotation stops slowly, this could be another sign that there is no resistance, but sound simple enough to split the body.
     
  8. ianmcd

    ianmcd

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    there is an O ring seal and they can be reused, and yes the shaft rotation is also a good sign that something is amiss
     
  9. monkeyfinger2

    monkeyfinger2

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    Thanks for your help Ian - I must admit that the pump sounded and felt super smooth whilst running. This may be a good or bad sign! I will carry out the checks and re-post.
     
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  10. monkeyfinger2

    monkeyfinger2

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    Just a thought though...how would hot water get to the tank if the pump is not functioning?
     
  11. I Like Sleeping

    I Like Sleeping

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    Gravity circulation.
     
  12. monkeyfinger2

    monkeyfinger2

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    Checked and found the pump to be fully operational. Still getting hot water to the storage tank. Pipe at top of tank is hot. Pipe in centre and at the bottom are cold (or vaguely warm, I guess from conduction). Any other suggestions? Certainly, the feed to pump and port AB remains cold over 20 to 30 mins of boiler firing.
     
  13. jackthom

    jackthom

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    Does the feed to the motorised valve also take a long time to get warm when hot water is being heated?
     
  14. monkeyfinger2

    monkeyfinger2

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    Yes...everything is seriously slow. Radiators really not getting hot at all, but I can't understand how I'm still getting hot water. Checked the 3 way valve again and it's definitely working - I can watch it moving between positions with the cover off and switching demand. With the actuator removed the valve turns freely through its small arc of operation.
     
  15. jackthom

    jackthom

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    So is the boiler repeatedly coming on for a short period, quickly getting up to temperature then switching itself off for a while?
     
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