Gravity Hot Water circulation

Discussion in 'Plumbing and Central Heating' started by Glanmor, 10 Oct 2018.

  1. Glanmor

    Glanmor

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    Hi
    I have an old Gravity based open vent hot water system. Boiler on ground level.
    Boiler fires but hot water does not reach indirect cylinder upstairs.
    have tried flushing system few times but no joy.
    is there any tricks to help the initial flow of hot water?
    Thanks
     
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  3. ianmcd

    ianmcd

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    only a few things can stop it circulating, air lock , blockage, blocked cold feed
     
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  4. Glanmor

    Glanmor

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    ianmcd, thanks for looking into this.
    Flow pipe from Boiler clear all the way upto arching vent in the F&E tank
    F&E Feed clear as its able to re-fill after flushes carraied out already.
    I cut the 28mm Flow pipe that connects Boiler Flow to the Cylinder and flushed the coil.
    Trouble is Boiler fires up for a minute and cuts out due to overboil detected by boiler thermostat. Allowed to run like this for 12 hours, but hot water never reaches cylinder.
    Im 100% ceratin all pipe are clear but how can i know here the arir lock is. Even tried re-fill with high pressure mains water in the hope to shift any air lock but no joy.
     
  5. fixitflav

    fixitflav

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    Is the central heating circuit pumped? Does that work OK? It might give a clue.
    Where have you tried bleeding it from?
     
  6. Glanmor

    Glanmor

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    Have central heating from same boiler, it is pumped and it works perfect. All radiators full.
    I tried bleeding/re-filling entire circuit from lowest radiator draincock using mains water but no joy.
    Tried bleed/re-fill from draincock on hot water side return pipe fitted close to boiler using mains water but also no joy.

    Its as if it needs refill with boiling water to help it kick into circulation.

    Thanks for looking into this.
     
  7. ianmcd

    ianmcd

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    then it is most likely an air lock they can be a real pain to get out, if you can connect a mains pressure hose to the drain point try that and see if you can force the air out through the expansion pipe, usually slackening the upper connection on the coil is enough but you might have to do it a few times , make sure everything is off when you do it though
     
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  8. fixitflav

    fixitflav

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    Is there a bleed point at a high point near the cylinder primary? That's the obvious place to release air from the gravity circuit.
    Is there an iso valve on the boiler return, after the rads and cylinder returns are commoned? If so you might be able to use the pump to force water through the gravity circuit. Close the valve and block the open vent above the header tank - a rubber bung should do it. Turn the boiler stat down so it doesn't fire, and run the pump. If you do yourself a sketch you'll see what I mean. That might force air from the gravity circuit into the rads, where you can bleed it off.
     
  9. Glanmor

    Glanmor

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    There is no bleed point on or near the cylinder primary. Infact, in an attempt to get circulation going I cut cylinder upper pipe last week in order to facilitate a propper/sure flush of the cylinder coil, thinking a block in the coild was preventing it. I wish I fitted a bleed point then.
    No isolation valve present, after the rads and cylinder returns are on their own and not commoned. Its an very old Glow-worm BBU, yet first fault in 20 years!
    I see your point about trying to move airlock elsewhere where it would be easy to shift.
    Now that you mention this, would adding water into arched vent pipe on the F&E tank fill the Boiler Flow pipe and could this mass of water in this pipe be heavey enough (given the height) to disslodge an airlock? May be its not that simple.

    What puzzles me is that I can understand boiler over-boiling and stat cutting out on WATER ONLY setting, then why on WATER & HEATING setting, where boiler is on for much much longer yet unable to start the circulation in the water side loop.

    Thanks for your thoughts, much appreciated.
     
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  11. Nige F

    Nige F

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    Try and trace the pipes from the BBU to cylinder - they should be 28mm and rise all the way - no flat areas.
     
  12. Servotech

    Servotech

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    How long is the run of the primary flow and return circulation pipes? 28mm? What's the circulation hight like (this is the distance between the center of the flow and return pipes on the cylinder and boiler? servotech
     
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  13. Glanmor

    Glanmor

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    Sorry ianmcd, just saw your last response. I considered slackening upper pipe on the coil but gum on that union is as hard as solder now. As mentioned to fixitflav I cut that pipe last week to enable propper flush of the coil, now wish I fitted bleed point there whilst I was at it.
    I will try refill with mains again as doing this is within easy reach. Thanks for your time.
     
  14. fixitflav

    fixitflav

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    It might be worth repeating it and fitting one next time! But you might be able to bleed by slackening a compression fitting.
    OK, that rules it out.
    It might be worth a try. The water will end up filling the F/E tank via the outlet, so keep an eye on level. It would be a big help if you close a valve somewhere in the cylinder return, doesn't have to be local to the boiler. That would ensure the water goes through the coil.
    When the pump is running, the pressure at boiler return is higher than at flow, due to the boiler headloss. This tends to cause flow in the "wrong" direction through the coil. But on a big old boiler headloss won't be much, maybe only ~ 0.2m, clearly not enough to cure the problem.
     
  15. Glanmor

    Glanmor

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    Nige F & Servotech - Flow pipe from BBU to Cylinder is indeed 28mm, so is the Return, but I think there is a 'flat' area.
    BBU is on ground floor. Flow pipe goes up one wall (say 2.5m high) into the ceiling, then crosses (say 4m) under floor of the upper Rooms to the opposite wall and into the Bathroom where it is vertical again for attchment to Cylinder. Cylinder is probably 1m high. Comming back to the boiler 28mm Return same from there.
    Without stripping out the floor boards I cant say if the cross-room horizontal run is inclined in the wrong direction. Assuming fitted correctly as it has worked for decades.
    It too was worried about that part but I resorted to re-fills using mains water via draincok, rather than trusting low pressure F&E Tank feed, in the hope of shifting any airlock. Im ceratin there is no blockage in any of these pipes. Thank you both for your input on this.
     
  16. Servotech

    Servotech

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    I was checking for a long run of primaries with very little circulating height.
    However from what you've said there seems to be plenty of circulating height on those primaries and a 4m horizontal run is not that excessive either.....in theory, if there's no back-fall, it should all work.
    Have you tried removing the thermostat from the phial in the BBU and give it a good old boil up to see if anything happens circulation wise (the boiler will make loud bangs and cracks in doing this; so 20mins only).

    You seem to have done all the right things Glanmor like back filling the system under mains pressure to blow out any air locks.

    I do know of an old dodge that will get it all working if all else fails.
     
  17. Glanmor

    Glanmor

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    Servotech, I had considered the 'good boild up' method but scared that it might introduce another serious issue and possibly loose the precious Central Heating which is working perfectly at present.
    In trying to understand physics behind over-boil method - I can see that on the WATER-ONLY setting, the boiler kettles off as soon as boiler thermostat detects stationary water overheating. About 60 seconds its lasts. Boiler remains off for slightly longer than usual until water in it cools down again. Now heres the bit I dont understand... if i now flick controls to WATER & HEATING, the same boiler fires up, stays on much longer (say 15 mins) whilst all radiators get hot and room temp reached. I cant understand why this 15 min duration isn't long enough for the water circuit to over-boil into a 'good boil up', afterall its comming from the same boiler.
    Sigh!!!!! I guess gravity systems and theory never got on with each other.
     
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