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NCB-240E Heat Turning on randomly

ecrespo175 Member Posts: 1
Hello All,

I have a Navien NCB-240E combi boiler that was installed back in May by my contractor, 2 Honeywell Smart Thermostats were installed as well. Two weeks ago I noticed that the unit starting sending heat to zone 1(Main floor). My contractor sent a tech in yesterday and said it was the thermostat calling for heat, even though it was set to off. Luckily my electric company was running a promo and I had purchased 2 google nest thermostats for $1 each. So I handed the tech the google nest to swap it out, also told him to swap out the one in my main bedroom(zone 2). After swapping both he also changed the wiring from my first floor, said it also might be the cables touching each other. 15 minutes after he left, I noticed the main floor google nest had a "no power error" seems the common wire was not installed. I went upstairs and realized it was very hot, sure enough the baseboard was throwing heat. The google nest upstairs was set to off as well. Called back my contractor, tech came in today again. He called Navien and they are continue to blame the thermostats. He showed me that once one of the google nests faceplates are mounted to the wall, the combi unit starts dishing out heat. He then thinks that it's the thermostats again. I'm at a lost here, I can't believe 4 thermostats are somehow calling for heat even though they are in the "off" position. Has anyone ran into this issue, if so what was the cause?


  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    You -- or your tech -- are not troubleshooting. Which you need to do.

    Go find where the thermostat wires connect to the Navien or zone panel or zone valves -- whatever. There are variations. Disconnect them. Still getting heat? Problem in zone control -- whatever system you have down there. No heat? Now jumper the two terminals to which they were connected. Got heat in that zone only? If so, good. If not, problem in the zone controls...

    No heat? Great. Now go back upstairs. Disconnect both thermostats and put them somewhere safe. There should be a red and white wire. Both disconnected from anything up there? Good. Go back downstairs and reconnect the wires. You should have no heating call. If you do, there is a short in the wiring. Now connect them to each other. You should get a heating call on that zone, and only that zone. If you don't, you have an open somewhere.

    If all is well at this point, get a nice simple thermostat for each zone and connect it up and use it until you can figure out how to get power to the Nest -- which is not as easy as it sounds, and done incorrectly will result in frying either the Nest or the boiler controls or both.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jaythepoomer
    Jaythepoomer Member Posts: 2
    Most nest thermostats do not work with the Navien Combi boiler at all. The nest learn i was able to get to work but you cant use the common from the boiler.

    i was able to wire one nest thermostat of mine using the power from my central air to keep the nest working for one zone after a lot of trial and error. When i set up the other zone with a regular nest thermostat, not the more expensive nest learn, it calls for heat the minute i snap it into place. i believe some of the other smart thermostats work right away. i have a honeywell home wifi for my other zone now. having two different apps for two different zones isnt ideal but i liked my nest so i didnt want to get rid of it.