Bypass valve heating rads

Discussion in 'Plumbers' Talk' started by Iain MacLeod, Jun 23, 2014.

  1. Iain MacLeod

    Iain MacLeod New Member

    Hi, I'm looking at my partner's vented CH system and I'm a bit confused as to why the bypass valve is heating the rads when there's HW demand. There are two motorised valves and the HW one is definitely closed, the pipe is cold 30cm from the valve. Looking at schematics (figure 12 here http://idealboilers.com/installer/wp-content/uploads/sites/2/2013/08/116797-4.pdf) it looks like the bypass flow shouldn't go anywhere near the rads. Any suggestions as to what's wrong? I can only imagine a screwup in the piping...
    Last edited: Jun 24, 2014
  2. sam spade

    sam spade Member

    I assume you mean CH one is definitely closed.

    I doubt if it is the bypass valve causing the problem.

    Are all radiators getting hot, or just a few?

    If just a few, it is probably reverse circulation caused by mis-connection of the return from the hot water cylinder. This connection must always be the last one before the boiler, i.e. no radiator returns should be between cylinder return and the boiler. If there is a radiator connection, heat can travel the wrong way up the rad return and the rad(s) will warm up. You can tell if this is happening by feeling which of the two pipes connected to the rad is hotter. It will be the opposite way round to normal, i.e. when the heating is on.
  3. Iain MacLeod

    Iain MacLeod New Member

    Hi Sam, thanks for spotting the mistake; yes, it's the CH one that's closed. All of the rads, across both floors of the flat, are hot - not just warm - and the only post-pump pipes that are flowing are the cylinder and bypass.

    I'll check and see which end of the rad is hotter and if that doesn’t shed light on it I guess I'll have to lift some boards and see where the connections tee in. The cylinder's in an upstairs cupboard - behind a washing machine - there's not much to see above the floor.
  4. tom.plum

    tom.plum Screwfix Select

    best way to check is, from cold turn on hot water only, then go to the nearest rad and see which side is coming hot first, as soon as you feel a temp change, turn on the heating, if the hot side changes over to t'other, its reverse circulation as sam suggests,
    but if you are right about central heating pipes being cool before the valve, then it will be reverse circulation anyway due to wrongly piped system,
  5. Iain MacLeod

    Iain MacLeod New Member

    Cheers guys. The schematic in the boiler manual put the cylinder return on the far side of the CH load; is there a good good reference source - or even an idiot's guide - on line anywhere for this kind of stuff?

    If it was just the rad return(s) on the wrong side of the cylinder return I'm guessing there wouldn't be a significant flow through the rads, i.e. how can it flow if the main feed valve is shut?

    Looks like this will go some way to explaining why her heating bills were high through the last summer:) God knows how long it's been set up like this, not less than two years...
  6. tom.plum

    tom.plum Screwfix Select

    here's a kwik diagram, the return from the cylinder can get into a rad return because its been teed in after the cyl return, this alows the water to enter the rad circuit even thou its shut,
    SAM_0404.JPG
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  7. Iain MacLeod

    Iain MacLeod New Member

    Last edited: Jun 24, 2014
  8. sam spade

    sam spade Member

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  9. Iain MacLeod

    Iain MacLeod New Member

    The other half's favoured plumber seemed out of his depth/a bit clueless, despite being shown Tom's diagram above. If the penny does not drop when he "has a look at it" when fitting some new rads, is the only way to resolve this to re-plumb the heating system, or would check valves in the offending central heating leg(s) work/be a good idea?
  10. plumberboy

    plumberboy Active Member

    If you have reverse circulation I would correct the pipe work,check valves in the leg will at some point fail and become a restriction.;)
  11. Iain MacLeod

    Iain MacLeod New Member

    I was afraid that might be the general conclusion, though I'm guessing correcting the pipework will not be a trivial exercise given the cylinder is upstairs and some distance away from the boiler...even tracing the pipes is going to be quite disruptive.
  12. Is boiler & pump running with no HW or HTG demand
  13. Iain MacLeod

    Iain MacLeod New Member

    No, only seen/detectable when HW is on and HTG is off.
  14. Is it an auto bypass or a gate valve, is it a new system
  15. Iain MacLeod

    Iain MacLeod New Member

    Auto bypass, it's not a new system, it's 8+ years old. Though I've yet to look for/find where the heating tees in, or check the rad ends on HW vs CH for that matter - sorry, busy-busy, must try to do it this weekend - I can see that the cylinder is at the end of the line. Tom's explanation seems very likely; do you think it could be something else?
  16. How many radiators get hot when HW demand only, has it always done it? Do you have one HTG zone only? Do you have balancing valve (gate valve) on RTN out of cyl? Is it open enough,
  17. Iain MacLeod

    Iain MacLeod New Member

    One heating zone, over two floors, and they all get hot when it should be HW only. No balancing valve - as far as I can see. It's not a great pic, but here's one that might help: IMAG0009.jpg
  18. Are you sure the HTG zone valve is not letting by? Has this system always done this? Have you been there 8 years & it's been fine?
  19. Iain MacLeod

    Iain MacLeod New Member

    The valve is fine, the pipe is cold a short distance downstream from it. She moved in there two years ago and only noticed that there was an issue a couple of months ago...
    Last edited: Aug 30, 2014
  20. It must if been piped up wrong originally, all the heating RTN's need to come together to make a common RTN. This needs to be tee'd into the primary RTN to the boiler with the t in the correct orientation. You may have RTN's teed in all over the place? If you can't trace it all you may have to consider drastic action! Re pipe it all!!

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