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Pumping over, only on CH

Discussion in 'Plumbing and Central Heating' started by Schmill, 4 May 2011.

  1. Schmill

    Schmill

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    Ok, so my system;

    Pumped primary, vented system with indirect cylinder for DHW.
    The system is configured with the pump in the airing cupboard on the flow from the boiler, then to 2 motorised valves, one feeding the DHW coil, and one feeding the rads.

    I have recently cleaned my system, so I don't know if this has caused / (re-caused?) the problem due to the water now moving more freely, but anyhoo...

    If I switch on the circuit for just the DHW all is fine, the valve opens, the switch triggers, and the pump and boiler start. Switch it off and the valve starts to shut, the switch is released, and the boiler and pump shutdown and no pumping over.

    If I switch on the circuit for the CH the valve for that opens fine, the switch triggers, pump and boiler start.
    However when I then switch OFF the CH the valve starts to close and the boiler and pump switch off, the valve continues to close and as it finishes I can here the system pump over into the F/E tank in the loft (presuming I have the access hatch open).

    The pump over happens irrespective of pump speed (1/2/3), and irrespective of whether the DHW is set to come on / off at the same time as the heating or not.
    It seems to be the involvement of the CH circuit that then causes it to pump over at switch off, but I don't know why this would be?

    Is it simply that there is a greater mass of water moving around the system when the CH is on, and that then has more momentum when the pump and valves shut off and so forces water out the vent pipe?
    I have wondered if I need to close down my radiator lockshields if this is the problem, but I have no idea if this would help or not.

    Any suggestions please?
     
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  3. commercialman

    commercialman

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    Sort of , bit more involved but you have the right idea.

    Have you any height above F&E to enable open safety vent to be increased in height? , possibly just been 'hooked' over at install?
     
  4. Schmill

    Schmill

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    Nope no more space. In the loft the vent pipe goes all the way up to the roof as it is before then travelling a short horizontal distance and then dropping back down to the F/E tank.

    Presumably this water should becoming back up the fill line, rather than the vent if it was to have to go anywhere?

    Will closing down the lockshields stop the over pumping, and / or will I then get other problems?

    Actually on that note... there are a fair few notes here about how to balance the CH system, but how do you then balance in the DHW circuit? I notice that the return from the cylinder coil has a valve inline, (but no stem or handle), so i'm guessing some sort of balancing needs to happen there in order to stop the primary pumping to just the DHW cylinder coil, or to just the CH if / when both motorised valves are open?

    Thanks!
     
  5. Agile

    Agile

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    Pumping over is usually caused by sludge build up in the system. Treatment with cleaner will sometimes cure it but in some cases its necessary to cut out sections of pipe where the vent/feed pipes meet.

    Conversion to a combined feed vent will also cure it.

    Tony
     
  6. Schmill

    Schmill

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    Bizarrely I don't believe it used to pump over when it was sludged up, (perhaps because the water in the rads was moving slower?), but since putting the cleaner through there is now an issue. The vent and the fill lines are clear (as clear as pipes in service are), as the water manages to shoot out of the vent pipe, (not just a little trickle), and the system fills pretty fast through the fill pipe too.

    The fill pipe was blocked at one point in the past, and also used to airlock as it had a horizontal section that was slightly 'off' horizontal in the wrong direction. I have already cut this out and replaced with a 45 degree section instead.

    Previous:

    New:

    I am loath to combine the vent and feed, purely because it will mean altering all the pipework again, but if I do NEED to take this route how do I go about combining them? Presumably they still have to be seperate at some point?
     
  7. lightning

    lightning

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    ignore wrong post :oops:
     
  8. mysteryman

    mysteryman

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    If you do get to alter the pipework again, you should swap it around so that the vent comes off the horizontal section of pipe. Take out the vent tee from where it is, and replace it with an elbow with the flow going up into it and turning right. Then fit the vent tee in as close as possible to the cold feed, with the vent going up from the horizontal pipe. The vent tee as installed is a common error.
     
  9. commercialman

    commercialman

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    Interesting thoughts.

    I would be inclined to leave the open safety vent exactly where it is , any air collected in the system pipework will discharge itself via the 'unobstucted' route , placing the open safety vent on the horizontal pipework can cause issues whereby any air accumulated will be taken for a ride & 'miss' the opportunity to vent itself via OSV , unless you are referring to 'one pipe circulation'? note i am reffering to this particular installation as some of the time pipework has to be installed the way you suggested.
     
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  11. Schmill

    Schmill

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    I can see that having the vent on the vertical 'straight-ahead' route might encourage the water to pump over, but also would think that having it in that arrangement would help with air seperation as air would have a tendancy to travel straight up, rather than turn the 90 degree bend into the horizontal section?

    Either way... any advice on how to stop the pumping over?
     
  12. commercialman

    commercialman

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    As long as the F/E & OSV are within 150mm it makes little difference where the OSV is 'teed' off.

    Maybe you could try lowering water level in the F&E or simply increase the size of OSV above F&E waterline , other than knowing the rest of your system it is pretty difficult to advise.
     
  13. Schmill

    Schmill

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    Well they are definitly within 150mm (as the photo shows, they are very close).
    Not going to be able to do either of the other suggestions unfortunately. The water level in the F/E tank isn't exactly high as it is, only a couple of inches over the fill pipe, and the Vent pipework already goes all the way to the rafters, so no additional height available there either :(

    Is there anything else I could tell you about the system, or provide pictures off that would enable you to advise any further?

    Thanks
     
  14. mysteryman

    mysteryman

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    It is water which has a tendency to carry straight on up the vent pipe, not air. The existing arrangement is not good, and from your pics there seems to be plenty of room to do it properly. The 45 degree slope is irrelevant.
     
  15. Schmill

    Schmill

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    So are you suggesting that something like this mock-up would be better?
    Vent on the left, fill on the right
     
  16. MrGassafe2011

    MrGassafe2011

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    Use a magnet to locate the blockage - It should NOT stick to copper - If it does and you loacte it, this is the reason for the Pumping Over !!

    Obviously a history of Bloakces so recommend a Clean & P/Work Re-config !

    Good Luck.........
     
  17. Schmill

    Schmill

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    Ironically the only place there is a slight attraction to the magnet is on the actually vent pipe a couple of feet above where it T's off the main flow. I've not worried about that though as I would think if it was anything to be concerned about it would actually be stopping the pumping over (i.e. by restricting the vent pipe) not causing it :)

    All the main flow pipes have been recently replaced (due to blockages & general mankiness), so they are all nice and clear.
     
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