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Discuss System venting water in the Central Heating Forum area at UKPlumbersForums.co.uk.

  1. fixitflav

    fixitflav Member

    Hope some of the experts here can help me with this!

    I have a conventional system, heat-only boiler, F/E tank, HW cylinder, diverter valve. Upgraded an earlier system in 1998, plumbed boiler - open vent (22mm) – cold fill (15mm) - pump. Open vent and cold fill are the recommended 150mm apart. Cold fill up-and-under. Both pipes go straight up to the F/E tank.

    When I retired in 2010 I did some improvements including fitting a Magnaclean and changing the galvanised F/E tank for plastic. There was some sign of water coming out of the open vent, so I lengthened it to 750mm above the water in the F/E tank.

    I’m fairly sure it was OK after that (no more water coming out of the vent).

    Earlier this year the pump (Grundfos 15-50) was getting noisy, so I changed it for a Grundfos UPS 2. Everything seemed fine, but the other day I happened to feel the pipes and the open vent and cold fill were hot. There was a steady trickle of water from the open vent.

    The vent outlet is brought back down to just below the cover of the F/E tank, but the trickle is nowhere near enough to form a siphon (and if I thought it was I would drill a hole in the top of the inverted U). The system has Sentinel inhibitor and 2 – 3 weeks ago I took a sample when emptying the Magnaclean for a bright wire nail test, and they’re still OK.

    I’ve turned the pump down to minimum, maybe a little improvement, but it hasn’t stopped. The trickle starts immediately HW is called, doesn’t have to get hot first.

    I’m baffled, as according to my calcs the headloss in 150mm pipe length is < 0.2m, even at 5m3/h, which is something like the pump output at maximum setting and zero head. And at say 2m3/h at minimum setting I get headloss 0.03m. So I would expect my 0.75 m to have stacks in hand even in worst case. Two boiler manuals I have both say minimum height of open vent loop 450mm above F/E tank water level.

    Some pics attached. I don’t think anything outside the airing cupboard has any bearing on the case, but I can give more details – boiler model etc if necessary.

    Has anybody had anything similar or can make any suggestions?
    upload_2018-5-25_12-44-3.jpeg

    upload_2018-5-25_12-44-35.jpeg

    upload_2018-5-25_12-44-58.jpeg
    Read more: https://www.******.com/diy/threads/system-venting-water.503277/#ixzz5GVrWnepq
     
  2. Gasmk1

    Gasmk1 GSR

    check your cold feed into the system isnt blocked
     
  3. fixitflav

    fixitflav Member

    It's not easy to check, but I'm pretty sure it isn't, as when water is dribbling from the vent pipe there must be water flowing in from the cold feed, or I would expect other symptoms.
    There's a bit of a development - the symptoms I described are when it's HW only calling (either/or CH/HW, not a mid-position valve). Now the weather has cooled off I've tried it on CH and it was OK, at pump minimum. Switched pump to max and it started again. Went to pump mid and it continued. Back to min and it stopped.
    So it does seem to be flow-related. The HW circuit must be quite low resistance, 28mm pipes except the last bit in the airing cupboard, as originally it was gravity HW, but there's still the boiler. But it's still way outside what I would expect, and obviously it's undesirable.
    Thanks for the comment though.
     
  4. Stigster

    Stigster Plumber

    It is showing the symptoms of a partial blockage somewhere. You could use a magnet to try to find it, concentrating on tees and elbows. I know you have a magnaclean installed but hopefully there will be enough ferrous material in the blockage to find it with a small powerful magnet.
     
  5. fixitflav

    fixitflav Member

    I believe I'm right thinking the only place a partial blockage would cause these symptoms is in the 150mm between the open vent and cold fill connections on the pipe. I suppose that can't be ruled out, but it seems pretty unlikely. Anywhere else and blockage would reduce the flow, making the symptoms less likely.
    But I can't think of a more plausible explanation, and it shouldn't be too hard to take a section of pipe out and rod it, so I'll do that sometime. Thanks!
    PS I seem to remember reading, maybe on this forum, that using a magnet can magnetise the sludge, which then clumps together and turns a potential blockage into an actual one.
     
    Last edited: May 25, 2018
  6. jtsplumbing

    jtsplumbing Plumber GSR

    Cant see in photos but is there a bypass B4 the 3 port valve ??
     
  7. fixitflav

    fixitflav Member

    No
     
  8. deg

    deg Member

    Does it happen when heating is on also?
    Is the gate valve on the return from the coil closed down too much?
     
  9. fixitflav

    fixitflav Member

    Thanks for everybody's help with this. I've now cured it, it was a partial blockage between the open vent and cold fill. Plenty of clag in there. The next thing is a powerflush and chemical clean!
     
    • Winner Winner x 1
  10. fixitflav

    fixitflav Member

    Thanks for everybody's help with this. I've now cured it, it was a partial blockage between the open vent and cold fill. Plenty of clag in there. The next thing is a powerflush and chemical clean!
     
  11. Ric2013

    Ric2013 Plumber

    And if it were able to form a siphon, it would not be flowing uphill.
     
  12. fixitflav

    fixitflav Member

    I'm not with you. If a siphon were formed, the effective height of the loop would be the distance from the pipe outlet to the water surface in the F/E tank.
     
  13. Ric2013

    Ric2013 Plumber

    I felt you were suggesting that the loop could be siphoning water from the vent into the cistern, but if the loop were full of water and both ends open, the water would tend to flow down the vent, not UP the vent. The open end should not be dipping into the water on the cistern, so no way of keeping the loop full of water anyhow.
     
  14. fixitflav

    fixitflav Member

    If water is being vented, the pipe must be full to the top of the inverted U- bend.
    In a situation where siphon action is wanted (not on a C/H F/E tank, obviously), so you could take advantage of siphonic assistance, above a cetain flow rate a siphon would be formed.
    In my job as a process engineer, there were situations where a siphon was needed. Did this by a return bend on the bottom of the pipe, or dip the pipe end into an overflow tank, below the level of the o/f weir. Worked OK.
     
  15. Ric2013

    Ric2013 Plumber

    You seem to think that you are more highly academically qualified than I am, and, in scientific subjects, you probably are. That said, I do have a very good practical understanding of how a siphon works, gained through playing with hoses and the garden pond from the age of five years old, so I do understand your point about a certain flow rate being needed to commence siphonic action.

    Yet I still fail to see what you mean when you write:


    The vent outlet is brought back down to just below the cover of the F/E tank, but the trickle is nowhere near enough to form a siphon (and if I thought it was I would drill a hole in the top of the inverted U).

    Even if the flow rate were sufficient to commence siphonic action, how could the siphon drag water uphill, up the vent, into the cistern, as it would be working against gravity (exactly the opposite to how a siphon works)? It couldn't, so this isn't what you meant. How could the open end of the pipe, not being below the waterline drag water down by siphonic action? Again, it couldn't, due to the fact that as soon as the water level in the pipe dropped to the same level as the water level in the cistern, there would be no more inducement to flow (except a trace of inertia) and the siphonic action would stop.

    So why would you drill a hole in the pipe if you thought the flow rate were sufficient to have filled it to full bore? I hope you can understand why I thought you did not properly understand how a siphon works, and perhaps explain what you intended when you wrote the above-quoted sentence.
     
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