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Baxi luna 310fi pump issues

Hello, we have a baxi Luna 310 fi combination heat and DHW unit installed in 2007. many many issues over the years, TODAY we have no hot water. when calling for DHW pump is engaging and icon showing pump fault or low system pressure is lit. Unit has been reset and bled several times today with no successful results. Standalone pressure guage shows 20 psi or greater. Could pump be bad or any other ideas/thoughts are welcome.



  • Check the diaphragm. Replace it if it has any holes in it.

    Part #5405330 is a new diaphragm.

    Call Marathon Sales at (800)461-4651 for further help.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hourTwo btu/ per sq ft for degree difference for a slab
  • frustratEDed
    frustratEDed Member Posts: 3
    diaphragm has been removed, examined and replaced at least once maybe twice in past few months. The last thing that was replaced was the venturi assembly....which seemed to fix all issues. That was early July. Working great ever since. Now this afternoon no hot water again. Indicates calling for hot water flow wise but no flame and then the pump fault code. Pressure has been relieved and repressurized and pump bled. Pump is coming on....you can feel it.... but still the fault code. Not sure if any water is being moved by pump but it is running. Could it need pump or pump rebuild?
  • frustratEDed
    frustratEDed Member Posts: 3
    hello again everyone. phone number that was posted above is some kind of medical hospitalist or something or another.

    Anyone have any other thoughts about any of it.

    Pump was removed last night. All appears good.
    Pump is coming on when calling for hot water and/or heat.....even though our daytime temp are in the 100's lately. Verified visually, with stethoscope and by touch. Still getting low pressure/ pump fault code.

    Any ideas welcome at this point.... other than bigger hammer. B)

    Just because somebody hid my bigger hammer. :# jk
  • Sorry about the wrong number. Try this one:

    1 844 422 9462

    I got it off their website.

    You can also email them in England here:


    When there is DHW flow, it pushes on the diaphragm which drives a pin that extends out to close the flow switch. You can see this happen in front of the brass diaphragm housing. I've replaced defective diaphragms and flow switches before, but never beyond these.

    When the flow switch closes, it's supposed to actuate the 3-way valve and turn on the pump and eventually the gas valve. It could be a problem with the 3-way valve not operating properly; either not opening or not sending a signal to the burner to come on.

    Let us know how you do.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hourTwo btu/ per sq ft for degree difference for a slab