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Bosch Combi circulator stays on

With a Bosch Greenstar ZBR...3A, circ pump is always on, even when unit is not firing for heat (May-Oct). On-demand DHW is fine. Should circ pump always be on? If I unplug the pump inside the boiler, the unit won't fire for DHW. Boiler came w/house, probably 10-12 years old. Two zone valves, neither is calling. If I (homeowner) understand right, constant circ can be an aspect of letting the boiler achieve its greatest efficiency, by only requiring minor heating to maintain heating temp. The manual has cautions against changing "Constant mode." One other thing: I disconnected the (optional?) outdoor temperature sensor, as the installer had put in where sun was shining on it, and it seemed to impair the functioning of the heating mode. Now it sends a constant error code saying that the temp sensor is disconnected. I don't think the one thing has anything to do with the other, but thought it worth mentioning.


  • fenkel
    fenkel Member Posts: 162
    Do you have the fw200 controller installed?
  • Multiman
    Multiman Member Posts: 4
    YEs, I do
  • GGross
    GGross Member Posts: 706
    check in your manual if the boiler has freeze protection mode. It is possible that the boiler defaults to thinking it is freezing temp when the outdoor sensor signal is lost to protect the boiler against freezing.

    Other than that you are correct that constant circulation is sometimes an intentional design choice. Generally constant circulation is best executed while also using an outdoor sensor, this way the boiler knows when it is warm enough outside that it can shut off the pump. If it were me I would hook up the outdoor sensor and set the heating curve to correctly heat the house, sometimes outdoor sensors can be affected by location as you noticed, most of the boilers I work with have an offset in the programming to adjust for those irregularities
  • fenkel
    fenkel Member Posts: 162
    With the fw200 controller installed the boiler goes into freeze protection at a set point temp… when the temp goes below the factory setting, the internal pump runs constantly.. you can adjust it, but you can’t shut if off…