Boiler modulation on Worcester Bosch 38cdi classic erp

Hello to the community. I’m hoping someone more knowledgeable than myself can shed some light on this. I have had the above combi boiler for 5 years now and it has been controlled using the supplied Worcester comfort 11 RF wireless thermostat/programmer and the associated wireless receiver unit which was fitted into the front of the boiler. In an effort to gain more control of how and when different rooms in the house were heated I opted to fit the Tado heating control system. I duly purchased the Tado starter kit V3+ which consisted of the internet bridge, a smart wireless thermostat with stand and a programmer with hot water control. I also purchased 6 smart radiator thermostats. The system was installed by me, replacing the Worcester controls and has been working great for a week now.

However, I have now discovered that my boiler is a modulating boiler, and I believe that is how it was operating when the old controls were fitted. Now that the Tado controls are running the show, I’m not sure that is still the case. The only connection I have between the boiler and the new Tado programmer is a 4 core flex of which only 3 cores are used, and they are connected to the neutral, live and ch NO terminals in the programmer, with a link fitted between the live and ch COM terminal. I may well be wrong here, but I don’t see how that could be controlling the boiler using modulation?

So in essence what I am asking is, have I now lost modulation control for the boiler by fitting the Tado system? If so, is it possible to connect the Tado controls I currently have in a different way to restore modulation? If not is there some way of restoring modulation control by adding additional Tado controls? Or am I just talking rubbish? Any help with this would be greatly appreciated.


  • Hi there,

    I had a similar problem when I purchased a Tado system recently. I too have a very similar setup to you - a Worcester combi which supports modulation with the Worcester provided thermostat.

    Unfortunately, your suspecions are correct - you're not controlling the boiler with modulation with your current setup. To support modulation you need to connect via a digital bus. For Worcester boilers this is EMS, other brands (Ideal, Viessmann etc) use OpenTherm. If you look in your boiler there should be two terminals marked EMS.

    The UK model of Tado Wireless Receiver only supports a basic "call to heat", (aka heating on/off), which is how you've connected it. Frustratingly the non-UK version and the previous UK version, the Extension Kit do support these digital control interfaces. I contacted Tado support and they confirmed this.

    So your options are to replace your Wireless Receiver with an Extension Kit or stick with your current, non-modulating setup. Tado don't sell the Extension Kit anymore, but I picked one up one eBay for around £70. I coupled this with the cheaper wired starter kit for a wireless, modulating setup.

    Any more questions, let me know, sounds like we have similar problems/questions 😃

  • Hi #tomtomtomtoms

    Thank you for responding, it sounds like i may have a way forward then. Could I trouble you to elaborate a little on which model of boiler you have this working on, and what was the model number of the extension kit you used?

    There is one for sale on ebay with the model number EK01-TC-UK-03 Extension kit. Is this what you are referring to?

    Unfortunately I know nothing about central heating systems and was relient on the specific instructions that Tado sent me to set up the current installation. I assume if Tado no longer sell this extension kit in the UK, getting fitting instructions for my system wont be possible?

    Did you have the same Tado set up that I have at the moment? ie the wireless receiver? if so, did you have to make any further alterations to your wiring when you fitted the Extension kit? Sorry to ask you all these questions but I would need to know that this is something I could actuall do with my lack of knowlege about heating systems.

    Many thanks for your help thus far.

  • tomtomtomtoms
    edited November 19

    No worries about the questions, it's all quite confusing! I had to a fair amount of research before I attempted the install. It seems to be working well now though.

    I have a Greenstar CDi Compact, and yes the EK01-TC-UK-03 is the model I purchased.

    Fitting instructions are still available here: - you'll want "Extension Kit Manual - Low Voltage Digital Installation", in particular "Wiring: Worcester HT-Bus". EMS/HT-Bus seem to be used interchangeably.

    I purchased the non-wireless version (without the new Wireless Receiver) and the Extension Kit separately. I had a similar setup with my previous Worcester thermostat, a plug-in wireless receiver, which I removed. I then wired the extension kit into the EMS terminals on my boiler, as per the instructions above. Those terminals also provide power for the extension kit, so wiring wise, it's actually simpler.

    However, there is other wiring you might need to do as you've already wired yours in the non-digital, 240v relay way.

    Not my picture, but it should look like this:

    You can see the two wires on the left connected the EMS terminals. You'll also need two jump wires for that go from the L terminal to the CH and HW LR terminals. That in effect keeps the central heating/hot water "on", but allows the Tado Extension Kit to control via the EMS connection. These jump wires were already there in my setup, so it really was the case of the two EMS wires.

    My understanding is that you don't technically need the HW wire, unless you want to control it via Tado. Which for a combi doesn't make much sense, unless you're preheating hot water.

  • Thanks for that Tomtomtomtoms. Unfortunately that picture isn't the same as the internal layout on my boiler. This is the layout I have.

    This was before I had connected the currently installed Wireless Receiver, so as you can see, there is a small ceramic link between the LR & Ls terminals. I removed this when connecting the Wireless receiver. Also I don't appear to have the EMS connector strip that you refer to, however I do see the BB connections over on the RHS of the board which I think are the same thing?

    Currently the Wireless receiver has a 230v supply so am I correct in assuming that this will NOT be required for the Extension Kit as it will be connected to the BB terminals which I think provide a 36v supply?

    I'm not sure what to do about the ceramic link I removed...Should it be reinstalled between LR & Ls, as for all intents and purposes the Extension kit is going to be doing the same job as the original Worcester Comfort 11 receiver?

    Did you have to go into the "Installer menu" on the smart thermostat to change any of the settings?

    This kind of reminds me of a line from an old song that went somthing along the lines of "the more I find out, the less I know"

    sorry to be bothering you again!

  • Hey @Notascoobie

    Not a bother at all. I spend probably way too long trying to understand all this myself, if so I manage to help someone else that's a bonus!

    As you say, your layout is quite different. I'm pretty sure you're right though - restore the link from LR and Ls and connect over the BB terminals. As you correctly say, the extension kit will be performing a similar function to your old Worcester receiver. From my understanding BB is the labeling used on older boilers, EMS is for newer models. I think they use same protocol, just maybe slightly different versions depending on the age of your boiler.

    Power-wise, yup the BB terminals will supply 36v which is sufficient to power the extension kit. So it'll just be two cables from the boiler to the extension kit.

    You're also correct about using your thermostat to access the installer menu. You'll need to change the connection type to D17. There should be instructions on how to this in the installer manual I linked to previously.

    It's such a shame Tado removed the ability for us in the UK to connect to a boiler this way (digitally that is) from the newer models. I feel overall it's much simpler. The system can talk to the boiler in a much more intelligent way, which seems almost essential for a "smart thermostat"! 😀

  • Well, the Extension Kit to replace the wireless receiver arrived today, earlier than expected. I decided to tell the Tado app that I was actually replacing the original Worcester receiver, so it gave me the instructions to complete that task, which has now been completed….I think!

    I don’t know if it because I told the app that the extension kit was replacing the original Worcester receiver rather than the Tado wireless receiver, but the instructions ended without me having to go into the installers menu to change any settings. Hot water is working as expected, as indeed is the central heating.

    Yesterday I turned down the knob that controls the boiler flow temperature from its setting of 5 to just below 4. This reduced the indicated flow temperature when the boiler was running from between 65c ish down to between 50c and 55c and had quite a marked effect on the comfort levels in our rooms, an improvement on the comfort levels I should add. After fitting the extension kit today, I have been watching the boiler temperature and if anything it has dropped a bit more, dependent on demand obviously, and the pump appears to carry on running for much longer than before when the burners have shut down. At one point I observed the flow temperature get down to 29 to 31c, burners off, pump running. I don’t think I’ve ever seen that before.

    Anyway it seems to be working as it should as far as I can tell, and other than not accessing the installers menu to change any settings from the previous set up I think I’m sorted.

  • Nice one! 👍

    Interesting it switching the connection type for you - saves you a job. I guess you could validate it yourself by accessing the installer menu, but it sounds like it's working well so probably redundant. Sometimes if it ain't broke...

  • Ah, I may have spoken too soon! I was looking at the home screen on the app, when the hot water tile, which I'm not sure was there before, suddenly switched from off to on. And regardless of whether it was on or off, the hot water was only luke warm. Even turning up the water knob on the boiler wouldn't get it any hotter. So I went into the installer menu, and hot water was selected, so I deselected it. Everything else seemed OK, HC01, D17, TS. This seems to have sorted it. Not sure why if these settings are important, the instructions via the app don't mention them.
  • And now I'm wondering how you adjust the domestic hot water temperature as the knob on the boiler doesn't appear to have any effect.
  • Right, now that I've deselected hot water in the installer menu, the hot water tile has disappeared again off the app. So I'm thinking I'm all good again.
  • I have the same exact boiler but I'm using the wired thermostat kit ( - well, 2, I have underfloor heating in my extension) - am I doing it wrong? I'm very new to all this, am I missing out on features by not having the wireless extension kit?

  • I don't know about the underfloor heating, but if you have radiators with smart TRVs on them, then you should be able to connect the +/- 36v terminals in the wired thermostat to the BB terminals in your boiler and that will give modulation control of the burners and hence the flow temp. This will be in addition to the modulation of the pump that the boiler does as a matter of course.
    Hope that helps.
  • Ps. I don't think your missing out by not having a wireless extension kit. The only reason I got one was because the wireless receiver I got with my starter kit, doesn't support modulation as it doesn't have the 36v terminals. You however have the wired thermostat and that does have the 36v terminals.