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Condensing boiler, surges of HOT water

I have a one year old Navien 240e boiler which occasionally sends surges of extra hot water. The domestic hot water on the boiler is set at 115degrees and normally the temp reading (reading C on the control panel) reads around 115. But when we get the unexplained surges of hot water the boiler panel reads upwards of 130 to 135. Water temp measures the same high temp at faucets. I know there's a mixing valve inside the Navien; could that be the issue? Or maybe a failing temp sensor in the boiler?

I've searched elsewhere on this forum and on other sites but I only find mentions of drops in temp (cold water sandwich) as opposed to sudden increases in temp.

Any ideas?



  • Jamie HallJamie Hall Member Posts: 14,942
    Is your water somewhat hard? If so, I'd expect that the mixing valve is hanging up...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Larry WeingartenLarry Weingarten Member Posts: 2,040
    Hi @oceantrvlr , I agree with @Jamie Hall , but also, have you contacted Navien? They will likely want to know about something that could be a liability issue with their equipment.

    Yours, Larry
  • oceantrvlroceantrvlr Member Posts: 7
    I have slight hardness buildup (very light) on some fixtures but I wouldn't expect enough to crud up a valve. Is there a way to descale the entire Navien unit?

    I'll try to speak with Navien this week. I know they'd rather speak with contractors vs homeowners but, in the past, I was successful in having them help me diagnose an electrical issue using a multimeter. I'll try them again.
  • oceantrvlroceantrvlr Member Posts: 7
    Navien wouldn't help me over the phone. Need to hire a certified tech to come troubleshoot.
  • oceantrvlroceantrvlr Member Posts: 7
    Picked a certified plumber from Navien's website and they came and diagnosed a failing/sticking three-way diverter valve which, funny enough, has already been replaced once since installation! So, my NCB-240e is now on its third diverter valve in two years. The new plumber suggested that debris/scale/buildup in the home heating loop (not in the domestic HW) is the cause of the sticking valve. So, they recommended and installed a Navien NaviClean magnetic filter to try to capture what they think is causing the failures of the diverter valve. It all makes sense. I'm not sure why I haven't found more reference to increased domestic water temp in relation to a failing diverter valve. Hopefully this thread helps someone else avoid a headache.
    Larry Weingarten
  • oceantrvlroceantrvlr Member Posts: 7
    I guess it was too good to be true. We continue to get surges of very hot water. And after the Navies repair, replacing the diverter valve in the boiler, the surges of hot water are even hotter now. The Navies is set at 115 for DHW and we have been getting water temps as high as 165, shown both on the digital panel of the Navien and also measured at faucets.
  • hot_rodhot_rod Member Posts: 14,425
    Something is still not working correctly.
    It's always a good idea to have a 3 way thermostatic mixing valve to protect against failures like that. Some states require mixers on all DHW sources.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • oceantrvlroceantrvlr Member Posts: 7
    Yeah, the problem certainly doesn't appear where we have a mixing valve. But I think we only have mixing valves in the showers. The faucets are all separate hot and cold valves so, I don't think that would be a mixing valve, correct?
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