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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