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Renewable Energy: Twin cylinder problem. Trapped air , lots of gurgling

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    • CommentAuthorjamesingram
    • CommentTimeMay 15th 2016 edited
     
    Hi.
    I've fitted a cylinder up in the loft next to my open vented header tank , outlet of cylinder is about same level as outlet from header , a bit tight but I figured as long as header top level stays above this it'll be ok.
    It's heated by a pv immersion dump. 120 litres its main pressure but I've used as open vent (freebie) all 22mm pipe outlets inlets etc.
    Output goes to cold feed in original gas boiler heated 120l cylinder on 1st floor airing cupboard.
    See drawing , vented as shown. Ignore thermal bit.
    It's sucking air somehow and getting trapped so flows a bit effected when running the bath hot tap. (Back filled with main to clear out any air locks)
    Any ideas why?
    Water is probably hotter in top cylinder so wonder if it causing problems as it rushes in and up to top of lower cylinder.
    Sorry for poor grammar wrote this on phone
    Planning to create 2nd circuit on pv dump to do lower then upper cylinder
      2016-05-15_19.26.51.jpg
    • CommentAuthorEd Davies
    • CommentTimeMay 15th 2016 edited
     
    When the bath tap is fully open the pressure at the top of the conventional cylinder will drop quite a bit. The pressure at the top of the solar cylinder will drop by roughly half that amount. Could that be enough to drop the water level there below the feed from the solar cylinder so that air is sucked in via the solar vent pipe?

    If that's happening then I think air will be sucked into the conventional cylinder and could ultimately stop the flow out of the top of that until the bath tap is shut off and the air escapes via the conventional cylinder's vent and the whole lot refills.
    • CommentAuthortony
    • CommentTimeMay 15th 2016
     
    I would loop both vent pipes a meter above the header tank and down again.
    • CommentAuthorEd Davies
    • CommentTimeMay 15th 2016
     
    Tony's suggestion would help if the pressures in the system were a bit on the high side and one or both cylinders were splashing a bit of water back into the header tank but if the system's sucking air in then the pressures are likely low so I can't see how extra loop height would help. It wouldn't inhibit air from flowing in in any meaningful way.

    A non-return valve in the vent pipe might help for this specific problem but a) is scarily un-failsafe and b) would result in collapsing the cylinders if somebody tried to drain them with the valve in place.
  5.  
    I suspect the system is sucking air in the overflow pipes (as said above). I would loop the overflow pipes down to the base of the cylinders and then back up to the header tank. This will give a much longer path (the height of the cylinders) for the air to get sucked in and in my experience has solved the problem for me in the past. Looping the vent pipes above the header tank won't alter the length of the column of water and so won't alter the problem.
  6.  
    Thanks for the input chaps. Peter that sounds like a good solution, I'll try it after work tomorrow and update.
    • CommentAuthorEd Davies
    • CommentTimeMay 15th 2016
     
    Dunno if looping the vent pipe downwards is legal in Hungary but…
    • CommentAuthormike7
    • CommentTimeMay 16th 2016 edited
     
    Posted By: Ed DaviesWhen the bath tap is fully open the pressure at the top of the conventional cylinder will drop quite a bit. The pressure at the top of the solar cylinder will drop by roughly half that amount.


    I agree. One way to go would be to reduce the resistance of the feed pipework from the header to the upper cylinder and from the top of that cylinder to the T with the vent pipe. It's the pressure drop along this path that's causing the problem. Larger bore pipe,large radius bends and larger bore flanges if possible. Limiting the maximum flow rate with a valve downstream of that T would also help if that was acceptable. I presume its not possible to raise the header tank?
    • CommentAuthorjamesingram
    • CommentTimeMay 16th 2016 edited
     
    The top cylinders got 22mm flanges so a bit restricted there, but could go 28mm up to it though that may not make much difference as it'll bottle knock at the flange.
    Got room to raise header 4" and drop the tank into loft floor a bit so might gain 10" overall.
    Looping the vent pipe , as long as it's open and I pull the bends is that such a potential problem regarding airlocking restricting expansion?
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016 edited
     
    Approved Document G only says a “suitable” pipe of at least 19 mm inside diameter so maybe it's not actually illegal to loop down but a lot of other sites say you must go upwards continuously. But, yep, I can't see how a short loop would stop it sucking or blowing as required under normal conditions. Not sure it even needs to go down as far as the base of the tank.

    The failure would be if the water in the vent froze then somebody turned on a tap downstairs. With a straight vertical pipe I guess there's some hope of pulling the ice plug down to the T and blocking off the flow whereas with a loop that wouldn't happen and the tank would likely collapse. Also, freezing itself is much more likely with a loop than with a vertical pipe which would have at least a bit of convection transferring heat upwards (at least down to 4 °C).

    Edit to add: on second thoughts, with a loop how would any air which comes out of solution in the hot water (remembering that hot water can generally hold less gas in solution) escape?
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 16th 2016
     
    Don't you just want a non return valve, but one that work with air rather than water. So something like an old bike valve, the ones that had a small rubber tube over a hole.
    Those 'farty' cushions have one.
    • CommentAuthorDantenz
    • CommentTimeMay 16th 2016
     
    I wonder whether it's emptying the water out of the vent pipe faster than it can re-fill due to the low static head at the conventional cylinder and then sucking air down this vent.
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016
     
    Posted By: SteamyTeaDon't you just want a non return valve
    That's a fairly bad idea. If the cold feed to the tank is blocked for any reason (freezing, limescale bung, valve closed for draining or whatever) and you turn on a hot tap anywhere you'll create a low pressure in the cylinder which will likely collapse it.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 16th 2016
     
    Yes, all that is valid, but aren't these things designed to fail in a safe manner. Just a flap with asymmetric springs after all.
    • CommentAuthorjamesingram
    • CommentTimeMay 16th 2016 edited
     
    Pretty sure it's sucking at the top cylinder vent.

    Ed , how about doing a loop on the top cylinder vent giving a column of water long enough to handle the pressure difference but stick an air admittance valve on the top before the loop goes down then back up to the vent to header. This would release and air build up in that loop.
  16.  
    Steamy yep a low pressure flapper valve I think you mean.
    Would prefer to find another solution if possible than anything that may block it's ability to expand and vent.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 16th 2016
     
    Could you just 'pop a punctured jonny' on the end of the vent pipe to see if that cures the problem?
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016
     
    You'd want an air *admittance* valve to let in air to stop cylinder collapse if the vent pipe loop was frozen or otherwise blocked. But that would just give you back the problem you're trying to solve.

    Plus you'd want one of those other valves which let air out but not water (also, I think confusing called “AAV” but not air admittance valve - I've forgetten the name) to scavenge air coming out of solution. I suspect that in PiH's use the extra gas here was just carried away with the flow of water to the taps. You might get lucky like that, too.

    This is getting awfully complicated and Heath-Robinson. If you can raise the cold tank and drop the cylinder then that would be best, I think.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 16th 2016
     
    Posted By: Ed DaviesThis is getting awfully complicated and Heath-Robinson
    Draw a diagram, I always liked playing Moustrap :bigsmile:
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016
     
    Posted By: SteamyTeaYes, all that is valid, but aren't these things designed to fail in a safe manner. Just a flap with asymmetric springs after all.
    The problem is that if the NRV works as designed (preventing air flowing in via the vent pipe) other failures (deliberate or accidental blockage of the cold feed) could result cylinder collapse.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 16th 2016
     
    Seems to me though that it is worth trying to find out the answer to the problem before applying a solution.
    I think that it is the height difference, but a bit of rubber may show that to be the case. Be a shame to move everything and find it has not sorted the problem out.
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016
     
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016
     
    Agree that a *very* temporary bit of tape over the vent to check would be a good idea.
    • CommentAuthorringi
    • CommentTimeMay 16th 2016
     
    his is not advice, get it checked by a qualified person (with insurance cover) as I am not paying to rebuild your home if it goes wrong!

    Why not put both cylinders next to each other at the same height? Or even the conventional cylinder above the solar cylinder.

    Personally I would put the conventional cylinder above the solar cylinder. Connect the top of the solar cylinder into the bottom of the conventional cylinder. Expansion pipe ONLY connect to the top of the conventional cylinder, freed from header tank connected to the bottom of the solar cylinder. E.g. think of it as one cylinder that gets rather thin between the two halves. (No values between the cylinders!)

    PS also think about the risk of legionella disease as you are storing warm water in the solar cylinder that may not get hot enough to kill legionela for many months. A electric element (60c) in the solar cylinder, on a time switch to come one once a week for a few hours, would short this out. (The best option is to have the solar cylinder as a thermal store, with inhibitor/disinfectant in it.)
    • CommentAuthorringi
    • CommentTimeMay 16th 2016
     
    This is yet another thread that reinforces my view that thermal solar water heating is not worth it in the UK! (Unless you use a LOT more hot water summer that most people do, e.g. large B&B)
    •  
      CommentAuthordjh
    • CommentTimeMay 16th 2016
     
    Posted By: ringiThis is yet another thread that reinforces my view that thermal solar water heating is not worth it in the UK! (Unless you use a LOT more hot water summer that most people do, e.g. large B&B)

    This is a PV system, not a solar thermal one, I believe.
    • CommentAuthorringi
    • CommentTimeMay 16th 2016
     
    Posted By: djh
    This is a PV system, not a solar thermal one, I believe.


    Then way is there two tanks, and a pump fixed to the solar tank..... (I think the confusion comes from the pump running from a small PV panel.)
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 16th 2016
     
    Posted By: ringihis is not advice, get it checked by a qualified person (with insurance cover) as I am not paying to rebuild your home if it goes wrong!
    I think James knows what he is doing.

    http://www.youtube.com/watch?v=mViO9mnCTBo
    •  
      CommentAuthordjh
    • CommentTimeMay 16th 2016
     
    Posted By: ringi
    Posted By: djh
    This is a PV system, not a solar thermal one, I believe.


    Then way is there two tanks, and a pump fixed to the solar tank..... (I think the confusion comes from the pump running from a small PV panel.)

    Because he said:

    Posted By: jamesingramIt's heated by a pv immersion dump

    See drawing , vented as shown. Ignore thermal bit.
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2016
     
    Posted By: ringiWhy not put both cylinders next to each other at the same height?
    Guessing wildly: in most houses the normal DHW cylinder is in an airing cupboard with nowhere obvious to put another cylinder nearby without losing significant floor space.

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