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Discuss Baxi combi zoned is additional bypass needed? in the Central Heating Forum area at

  1. HoodedClaw

    HoodedClaw Plumber


    Just installed U/F heating in my extension so put S plan in with 2 valves on the return.

    When the valves close the boiler carries on for a little while (orange connected to boiler heat demand).

    The manual says 'The boiler utilises the primary side of the DHW plate heat exchanger as an automatic integral bypass' but to my ears it sounds pressured when the valves close.

    Is it worth fitting an external bypass to be on the safe side or let the internals deal with it?

  2. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    I think it boils down to system capacity. Maybe give manu a ring and double check
  3. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    I would always on an splan or put a delayed shut on one of the two ports
    • Agree Agree x 2
  4. HoodedClaw

    HoodedClaw Plumber

    Thanks, think I'll have to, the 3 min overrun will sit on my mind otherwise.

    Real shame I put 2 2 ports in, just realised (now I've got it running) just a 2 port on the Rads would have done, as the U/F needs to start up first and I can't think of time that I wouldn't want the U/F on when the Rads are. Might change it to just the 1 zone valve, leaving the U/F for the overrun. Still 2 timers and thermostats and as the U/F has its own pump the heating only time shouldn't bleed far with it not pumping.
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