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Discuss Rads coming on when hot water is on in the Central Heating Forum area at

  1. jaydebruyne

    jaydebruyne Plumber GSR

    ive just changed the 2 port on the heating side as I thought it was letting by as all rads started to get hot when only water on demand.

    Fired up boiler on demand for water once changed 2 port, flow pipe to rads underneath 2 port stone cold but rads still warming up at a slow rate.

    Checked bypass and it's fine. I put 10 replacement rads a couple months ago and when heating on demand all heat up nicely.

    Heat only boiler, unvented cylinder.

    I've just noticed it's the return pipes from rads heating up when hot water is on

    Any suggestions? Blockage somewhere?
    Last edited: Jun 30, 2017
  2. Stigster

    Stigster Plumber

    Is the pipework following the three tee rule?
  3. jaydebruyne

    jaydebruyne Plumber GSR

    Not sure as return paths are concealed. "Apparently" this only started happening when I changed the rads! Pffft
  4. Stigster

    Stigster Plumber

    I wonder if the old rads had unidirectional TRV's (if they had TRV's) which prevented this reverse circulation? Now you have new rads in with bi-directional valves the problem has been able to show itself?
  5. jaydebruyne

    jaydebruyne Plumber GSR

    Good point!! I'm gonna try pop back today so I'll see if I can get a butchers at the return path and order of the heating, water and bypass.
  6. jaydebruyne

    jaydebruyne Plumber GSR

    So the order after the pump is:
    DHW, Heating, Bypass
    Should be:
    Bypass, DHW, Heating. Same order on return. But return pipework is concealed.
    That right?
  7. KingDoey

    KingDoey Plumber GSR

    DHW should be the final connection before the boiler
    • Agree Agree x 1
  8. Stigster

    Stigster Plumber

    Yes, KingDoey has it right. The original installer didn't follow the 3 tee rule. DHW last connection before boiler. This diagram shows you what happens when you tee in the DHW return before the Heating circuit. I reckon that's what you've got going on and it has only shown as a problem with your new bi-directional TRV's.
  9. GH77

    GH77 GSR Top Contributor!!

    Put a check valve on CH return
    • Agree Agree x 1
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