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Thread: Baxi Luna issue

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  1. #1

    Default Baxi Luna issue


    Baxi Luna 310 Fi combi boiler

    I generally don't get into extensive service on gas appliances.
    Had a customer call with "no hot water at shower" but has very hot water at lav and kitchen sink. I suspected moen cartridge so I replaced it.
    After nosing into it more, there is hot water when using the shower head, but not when using tub spout, unless moentrol valve is open to allow only a small amount of flow then it gets hot.
    this tells me the boiler is not keeping up with the higher flowrate of the tubspout vs showerhead or sink faucets.
    This boiler has been in for a few years I believe and this problem only started after the original installer performed a service on it and changed a diverter inside the boiler that controls the flow for heating/domestic water load.
    There shouldn't be any control required for heat vs domestic flow as there is no call for heat, just to be sure I turned the boiler to only domestic hot water mode and the problem remains, making me suspect its not the diverter itself.
    Any ideas what else could cause this?

  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    The thing can deliver 100,000BTU/hr, which is enough for a shower flows, but pretty marginal for tub fill rates. It's possible that the heat exchanger is liming/scaling up if this is a hard-water area, but whatever the service guy hacked inside the boiler is the more likely culprit, and you may not be fully backing out whatever tweaks he made. Did changing modes make ANY difference? (Measure the water temps at the faucet in both modes, running at least 10 gallons in each mode.l)

  3. #3


    thanks for the quick reply.

    the previous guy simply removed the diverter valve assembly as a whole and replaced it with a new replacement - I wouldn't say he "hacked" anything in.

    secondly changing the mode made no difference at all.

    I got the service manual online and it doesn't show any procedure to adjust the diverter assembly.

  4. #4
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    If it screwed up the DHW output, intentional or not, it was hacked. Was it really the right part? Was it installed correctly? If the change in performance was concurrent with the swap out, it's a flag. Why was it changed out?

  5. #5
    In the Trades Tom Sawyer's Avatar
    Join Date
    Nov 2010


    I'm betting in a limed up exchanger
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    SOme of these things use a flow restrictor to help ensure you can get enough heat rise. If he changed that, allowing more flow, it will never be able to get as big of a delta-T as it did before.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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