TADO works as a Relay System or Modulating?

edited November 2022 in General Questions & Topics


I have a Logic combi esp1 35 boiler which was controlled by a Danfoss tp5001 wired thermostat. The thermostat had only two wires, COM and NO. Its manual states it is an "On/Off load compensation thermostat". The boiler has a "Fully modulating operation to low outputs" (from the manual). The manuals state that both boiler and Tado support OpenTherm.

I replaced Danfoss with a Tado v3+ wired thermostat.

Is Tado, based on the heat request, able to modulate the heat the boiler supplies in order to reach and maintain a specific temperature or works as a relay system, where the heating system turns on (100%) or off (0%) completely for specific time cycles based on the heat request?

Best Answers

  • johnnyp78
    johnnyp78 ✭✭✭
    edited December 2022 Answer ✓
    Yes, as are the instructions next to the picture. As it says if you’re not gas safe you should get a professional to open the boiler.
  • johnnyp78
    johnnyp78 ✭✭✭
    Answer ✓
    A weather compensation kit would be the way to go. You probably wouldn’t even need to install Tado unless you wanted to use it for geofencing.


  • If your boiler supports it - some logic boilers need an opentherm harness - you can use the digital connections (+ and -)
  • The old thermostat did not have +/- wires. Therefore, I am stuck with a wired relay system or we have to install an opentherm harness and connect the wireless Tado receiver to it?

  • Do you have a Tado thermostat? That has digital connections. If you want to use a wireless receiver you need the eu version.
  • If you currently use relay, by default, when installing the tado thermostat, the app will be giving you instructions how to do an 1-on-1 replacement, so you'll still be using relay.

    It is possible to change to opentherm, but you need to more or less know what you are doing, or get help from here/tado support/a professional/proverbial nephew/...
  • iKarampa
    edited December 2022
    Therefore, since the wired thermostat has only COM and NO outputs, a professional needs to run new wires from the boiler to bring in the +/- ones ?

    Or get the wireless EU version of the tado which supports OpenTherm?
  • Again, the wired thermostat has digital connections. Are you seeing all the posts? Connect to + and - on the thermostat and the digital connections on your boiler. No new wires needed.
  • Sorry, very new to this thermostat business!

    I made a diagram from what you said earlier, I assume that this is what you are suggesting?

  • In general: The wires might already be wired for OpenTherm, as that should fall back to old-school 'flame-on!' behavior when the two bus wires are connected directly.

    It might just be needed to move a small connector block over on the boiler from relay to opentherm input, or flip a switch/change a menu setting, or you need to move the wires over to another set of connectors and bridge the pre-existing connection, a special conversion box might even be needed to translate opentherm to your system's dialect of boilerese and vice versa, or maybe you need to really open up the boiler proper instead of just some wiring center, and therefore need someone with an official letter from the archbishop or whatever that qualifies them for that type of work (gas safe engineer)...

    Specifically, the User Manual I found, https://idealheating.com/uploads/documents/logic-combi-esp1-user-guide.pdf , doesn't really mention the rewiring of controls, probably they think this should be left to a pro...

    https://idealheating.com/uploads/documents/logic-combi-esp1-installation-and-servicing.pdf has some more info in section 2.18 on page 32 and 2.19 on page 33.

    Seems to me one actually can do the electrical (re)wiring without messing with the actual gassy parts, so ought to be legal to it on your own. I'm not familiar with the laws in the UK & EI, or even a heating engineer, just some bloke on the internet, so you'll have to use your own judgement here, but Diagram D is probably your desired end state, and you might have any of the other ABC's currently in place.
  • Thanks, this makes sense now (to me!).

    I did not mention that I have 2 zones in the house, controlled by identical Danfoss thermostats and wiring. I assume the professional should be able to sort out re-wiring for both.

    Next step would be to get a trained boiler installer who understands the above. Would this be a good start?

  • luteijn
    luteijn ✭✭✭
    edited December 2022
    The issue with a multi zone system is that the zones usually have some sort of valve to switch water off for the zone. Basically what tado would do at the radiator level with their smart knobs, creating 'micro-zones', but at a more macro level, e.g. upstairs and downstairs, or living-area and bedrooms.

    You'll need some way to open and close the relevant zone-valves when something in the zone requests heat. With a on/off relay type thermostat in each zone (and perhaps letting the boiler decide to modulate based on its own sense, e.g outdoor T-sensor, or return T of the water), this would be relatively easy, just connect the thermostats to their zone valve, and logical-OR all the valves together (put their signals in parallel, using some sort of diode or separation to avoid feedback from one valve opening the other).

    With opentherm, you'll likely need some extra kit to operate the valves, and it should be slaved to the zone's heat-calling devices (if you make it independent, it might not yet open/already close the valve while there is demand for heat and the boiler is running already. Or divert heat meant for another zone into their zone unnecesarily.

    I think there's quite a few other threads covering this sort of setup, but it might be more prudent to first just try a one to one replacement of the on/of thermostat by a tado thermostat in relay mode. Tado is 'intelligent' compared to one of those 20th century mercury-drop thermostats, but there is likely already quite a bit of intelligence built into your boiler, the zoning, and even your existing thermostats. A tado thermostat might not add much more than remote control via the app, and different (not necessarily better) scheduling.

    You might even want to just hook up the tado in addition to the existing system, often there is a provision for some sort of external input that you could use to have some form of remote control. I'd have to see if there's anything in the thermostat manual...

    https://www.gebruikershandleiding.com/Danfoss-TP5001/preview-handleiding-802515.html (the link is in Klingon, but the actual installers manual it links to in proper English), talks about remote sensor contacts on page 5. You'd need to have a specific version of the thermostat. I suppose one could just mount tado in parallel (OR-circuit) or series (AND-circuit) with the existing thermostat too, depending on what to achieve..
  • johnnyp78
    johnnyp78 ✭✭✭
    edited December 2022
    @iKarampa you should probably have mentioned that! Can’t use modulating connections with zone valves. You need two Tado wired thermostats, connected to each valve with a relay connection. One will be the zone controller for your upstairs rooms, the other will be zone controller for your downstairs rooms.
  • Another (perhaps unwise...) option might be to just make sure both zone valves are always open, turning the whole thing into one big(ger) zone. Then use TRVs (possibly 'smart' ones) to create microzones, and just have one thermostat (possibly smart) control the boiler via opentherm.

    This may not make economic or even common sense, I guess it depends on how large the house is, how variable your heating needs are, and if you already have TRVs on your radiators, comfort vs costs etc. I don't know enough about typical heating systems used in UK/EI to make any recommendations, and 'winter' may not be the best of times to experiment with your CH even if gas were still cheap. Incremental changes and having an exit strategy/easy way to revert the changes to the status quo ante are probably the way to go.--Which makes it all the more irritating to have to rely on Tado-support and Qualified Engineers to make these changes for you.
  • This boiler came with a new build property, I was expecting it would be configured to use the most efficient mode. I did not know zoning is complete different playfield. However, I should have mentioned it earlier (sorry!).

    Maybe installing a weather compensation kit would help the boiler operate more efficiently?

    The boiler seems to be have an economy setting on the central heating thermostat knob (I think it is the flow temperature above which it won't condense?). According to the manual:


    The boiler is a high efficiency, condensing appliance which will automatically adjust its output to match the demand for heat. Therefore gas consumption is reduced as the heat demand is reduced. The boiler condenses water from the flue gases when operating most efficiently. To operate your boiler efficiently (using less gas) turn the central heating temperature knob (B) to the ‘e ‘ position or lower. In winter periods it may be necessary to turn the knob towards the ‘MAX’ position to meet heating requirements. This will depend on the house and radiators used. Reducing the room thermostat setting by 1ºC can reduce gas consumption by up to 10%.

    @luteijn, I do not see any remote sensor contacts in the existing thermostat:

  • Looking at was discussed so far, Tado's real benefit seems to be when working together its smart thermostatic valves, to start the boiler when needed. Of course, added smart benefits of remote control, geofencing, etc are very welcome!

    I will speak with the original installer about adding a weather compensation kit. I will report back with results once its done!

    @johnnyp78 , @luteijn: My regards and thanks for taking the time to help.

  • You probably just have the basic version of the tp5001 then, not the 'A' version. There's some spots on the PCB that haven't been filled, some of those are probably for the wireless variant, and maybe some are just missing headers for the remotes, but likely the software is also hobbled so even if we knew where the contacts were, probably no use..
  • What setup did you do for in the end. I have just moved into a new build and have the same setup and im looking to do the same.

    From what I have read Opentherm, so best bet is going to be to have wired stats. Did you go for the whether compensator in the end?

  • I'm in identical situation, I guess your new house is by Anwyl? They seem to like TP5001B thermostats.

    Could you let us know what did you go for in the end, and most importantly, was it worth the expense & effort?

    Many thanks!

  • I have Tado installed in a new build but be cautious because they install tiny radiators to pass the EPC rating. In doing so they are rated at 70c which OpenTherm never seems to get to do..my experience on an Ideal Logic espi 38 is that it gets to 60c max. Which means the radiators underperform and heating isn't sufficient enough. Back when I was on the danfoss thermostat, the house was nice and warm (toasty).