Having Opentherm error issues with navien LCB700 oil combi boiler.

edited September 5 in General Questions & Topics

Hi, I just installed my Wireless Receiver, temp sensors and rad valves etc this evening, its too hot to test thoroughly as my house is so hot today the heating cant come as the rooms are all too warm.

I managed to set one room half a degree warmer than it was (set to 25deg when room was at 24.5) and the boiler came on, water pump only and circulated warm water from the internal heat store - burner didnt light (this feels correct to me as there would have been very low demand for heat) so im assuming all is working fine.


im having some intermittent (sometimes every 2 mins other times it goes half an hour or so) opentherm errors displaying on my boiler NAVIEN ERROR CODE 783 sometimes these last a few seconds and the boiler screen just flashes red then clears and the error can be viewed in the logs. other times the error will stay on screen for some time and its even cuasing the central heating to erroneously turn on which would bwe sending hot water round a short circuit as all the rad valves would be off in this hot weather.

the errors i could live with - but the boiler turning on when it shouldnt is a real worry.

how can i debug this - how can i find out what opentherm requests the tado is making when the error is displayed on the boiler. i really dont want to have to wire this up in relay mode and lose the modulation opentherm provides... i need log files so i can work out whats going on...

any help appreciated.

Edit: Temps have gone down a bit and managed to call for a 2 degree increase and the boiler started up as expected. so it does seem to be working. however the opentherm errors are happening every couple of minutes while its actually heating...

faulty, tado ? faulty boiler main board ? not compatible ? i dont know where to turn to diagnose this. neither the boiler nor tado seem to expose enough info to assertain the problem.


  • Have you asked Tado support whether your boiler is compatible? That might shed some light on it.
  • i chatted to a tech before purchase - he was confident it would work seeing as my nest controller was operating it correctly before that. I assume he knew that if nest could control it then so could tado.

    having spent multiple hundreds of pounds in 5 or so seperate purchases im going to have a nightmare returning this whole setup if it doesnt work -

  • It seems odd that it would work with nest and not with Tado. You could try adding an Opentherm gateway to see what’s going on.
  • roycruse
    edited September 6

    I have a works do tonight so will get back on this Thursday evening - if i need to invest in a opentherm gateway to debug this i will... but first id like some reassurance from tado/navien that it should be working. I'm going to try and refit the nest and see if that does indeed still work.

    Opentherm is opentherm right - it should work - tado is opentherm, my boiler has opentherm???

    how can i tell if the opentherm connection errors are physical / link layer OR data layer i don't know if there's a physical issue with the voltages / ampages being transmitted and received or if its interference or a software issue and the tado is transmitting garbage or if the tado is sending valid opentherm commands and requests and the navien just doesn't know what to do with those. etc etc etc

    It seems everyone is just using opentherm on a wing and a prayer and none of the systems give out any meaningful error codes or logging to debug it when it doesn't work.

  • roycruse
    edited September 8

    Still hoping to find another TADO=>OPENTHERM<=NAVIEN user in the community here somewhere. I switched the boiler back to my nest opentherm controller and played around increasing and decreasing thermostat temps and the boiler behaved as expected - and most importantly no errors. When i switched back to Tado opentherm i got about 10 mins of use before the errors started flooding in again.

    Support say it should be working, and are looking into it - but you cant beat someone with the same or similar setup to give you there words of wisdom.

    Ive ordered an OTGW so if support dont get there first ill hopefully be able to give them a nudge with some data

    Just really need to get this sorted before winter is here...

    Any help or ideas of things to try would be great.... Is there a way to access opentherm logs from the tado api ?

  • got myself an Opentherm gateway.

    With the opentherm gateway in between my tado and the boiler - it all works fine - all the errors are gone from the boiler (presumably blocked or corrected by the gateway)

    any suggestions as to why ??? I have 2 theories.

    1. the opentherm signal from the tado is not to spec or faulty in some physical link layer way. I have confirmed the voltages are very slightly out of spec with my oscilloscope low voltage is 7.2v and high is 15.4v but it is this .2v out of spec on the low voltage enough to cause all the errors I get on the boiler. The opentherm gateway is obviously then retransmitting the messages in spec (i havnt measured this with the scope yet)
    2. the opentherm gateway in "gateway" mode is correcting or blocking some of the opentherm messages that the boiler doesnt understand thus preventing the errors.

    anyone got an idea how to narrow it down ive attached the opentherm gateway logfile - near the end i switched it from gateway mode to monitor mode and you can see normal communication just changed to the tado transmitting the same messages over and over and getting no response from the boiler. (based on the behaviour, my gut tells me its because the out of spec voltage, but my brain asks is my boiler really going to sensitive to that .2 of a volt error.

    Any help, info or suggestions gladly received

  • Hi Roycruse,

    Did you get the Tado system to work correctly and are you happy with it now?

    We're just about to have a Navien 700 oil boiler installed and looking at having a Tado system.


  • Tado are replacing my wireless receiver as it is suspected to have a too high bitrate on the opentherm interface (see this discussion here https://www.domoticaforum.eu/viewtopic.php?t=14111 )

    I will update here if the new unit solves the problem or just suffers from the same issue.

    My Tado is transmitting opentherm at around 0.92ms per bit and opentherm is supposed to be 1ms per bit. That said the spec does allow for some leeway.

    The best diagnosis we have so far is the tado is too fast and the navien not tolerant of this. Your mileage may vary.

    Once I get the new Tado unit I'll re measure everything and see if I'm in the same boat or not.


    If your a tinkerer - putting an opentherm gateway in between the two fixes the timing issues and provides a working system that is really good. (ive got 4 room and 6 smart valves) The tado calls for lower or higher CH flow temp based on the demand from the rooms and radiators.

    DHW is also able to be switched on and off but the navien is ignoring the temperature requests from the tado and gets locked on delivering water at 50deg c but i think this is a uk law anyway.