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Baxi Luna 310 fi DHW cycling

This unit runs DHW and Radiant heat. When there's a demand for DHW the burner will kick on and bring the water output to about 125 F but then the burner will shut down. The water temp will get down to around 95 F and then the burner comes back on and the temp climbs back to 125 F and the cycle starts again. Each cycle takes almost 2 minutes. If the demand doesn't have much flow it will not happen, the output temp stays at about 125 F. I found that turning the output temp control down keeps this cycle from happening unless more than 2 sinks are calling for hot water.

I've tested the emergency thermostat. It has continuity (zero resistance) when the system is cold. I opened a hot water tap at a low flow rate so that the boiler wouldn't cycle and then unplugged the thermostat which should make it believe that the temp is too high and the thermostat has opened. The burner did shut off but it also caused an error that had to be cleared by turning the main switch to the R (Reset) position. It didn't just start making hot water again when the thermostat was plugged back in so I don't believe that is what is causing this cycling

The Baxi Tech Support won't help me because I'm not a contractor but I'm competent enough to swap out a part and smart enough to know not to mess with any of the fuel or exhaust components. Any help will be greatly appreciated.



  • superdave
    superdave Member Posts: 155
    Is this a 310FI?? the way to tell is the front has a small door that flips down. Send me photos If you are running one faucet and unit don't turn over to DHW you might need diaphragm. If the unit is turning on but not modulating the gas valve needs to be adjusted and or cleaned. Did you put on a strap on T stat to read the piping temp It also could need new NTC sensor on DHW side. How is the water clean PH.
  • Alan (California Radiant) Forbes
    Like Dave said, it might be the diaphragm (Part #5405330). Most of the DHW problems that I've seen on these boilers have to do with a bad diaphragm.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Bryan96
    Bryan96 Member Posts: 5
    Thank you for responses.

    Yes, it's a Baxi Luna 310FI

    It does cut to DHW and the faucet icon lights up when a faucet is on, even when it's a very low flow, so I don't believe the diaphragm is faulty.

    "Did you put on a strap on T stat to read the piping temp"
    No, I only took temp readings of the water at the faucet.

    "It also could need new NTC sensor on DHW side"
    Is there a way to test the DHW temp sensor?

    When the DHW temp dial is set to a high setting or when there is a high flow of hot water (like 3 faucets on full blast) the burner will do the cycle I described in the original post. As long as the burner isn't having to burn hard it maintains the DHW temp.
  • superdave
    superdave Member Posts: 155
    IT could be the NTC sensor. I still would check the gas PSI in inches of water. After you check that then put a test on the outlet side turn on DWH up all the way with high flow see what you are getting (5.62" on high fire then pull off 1 wire off gas valve this brings it down to low fire and this setting should read .72"