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Baxi direct hot water only when TWO faucets are running?

jmb_USAjmb_USA Posts: 3Member
I have a Baxi Luna DuoTec 40GA. Here's what I've discovered: When I run the kitchen tap alone, there's no call for hot water. Nothing registers on the boiler. It's as if I haven't turned on the water. But if i run the kitchen tap AND the bathroom tap simultaneously, boiler fires right up and runs hot water to both no problem. Similarly, running only the shower [head] will not trigger a call for HW. But the shower head AND a faucet will immediately fire up the boiler and produce HW continuously. Running the bathtub spout alone is enough (volume?) to trigger the DHW. Any theories? It seems clear that the system now needs a minimum to fire up.


  • HVACNUTHVACNUT Posts: 2,272Member
    Flow restrictors in the faucet and shower head?
  • If it's like the older Baxi Luna's, there's a rubber diaphragm that responds to the flow of water, pushing a micro-switch that turns on the burner. The diaphragm can start to tear and no longer respond.

    Call Marathon at (800)461-4657 for troubleshooting.
    Often wrong, never in doubt.

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  • JellisJellis Posts: 121Member
    I am assuming this is a new problem and the heater has worked properly in the past.

    there is a minimum flow required for the heater to start, often there is a filter on the cold inlet that can be cleaned and this may solve your problem and be a easy fix.

    Could be a bad flow sensor as well.
    Try cleaning the filter and see what happens.
  • jmb_USAjmb_USA Posts: 3Member
    Thanks for the guidance. Any chance you could point to that diaphragm and that cold inlet filter on my parts diagram/list: 240012356 DuoTec 40GA 1.33GA Spare Parts Catalog 7-15-18.pdf
  • JellisJellis Posts: 121Member
    Looks like the blown up section B on the first diagram page has the assembly for the filter. follow your cold water supply to the unit and you should find it.
  • edited June 4
    Jellis is correct.

    There's no diaphragm on these newer models. Look for part #624 on the first diagram, about 3/4 of the way down, close to the center of the page. It should have a solid red light when there's a hot water demand. If it flickers or is not on, order part 632130 (filter assembly) and then call that number that I gave you above for instructions. Under part 624, there's a nut you have to remove (18 mm or a small crescent) to replace the filter and if it breaks, you will have to replace the whole dang thing.

    Often wrong, never in doubt.

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  • jmb_USAjmb_USA Posts: 3Member
    Thanks for the guidance. The 632130 recommendation was spot on. Just swapped it out and all is well again. Cheers!
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