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Accessing the EMS/BB terminal on a Junkers or Worcester boiler (and some Bosch as well).

To connect an EMS Gateway to a Heatronic 3 boiler like most Junkers, Worcester and some Bosch boilers you need to gain access to the EMS/BB screw terminal inside the boiler.

The guide is for all boilers that look like this:

General fascia appearance of a Heatronic 3 style boiler

Step one: disconnect the boiler from power

A good first step is to unplug the boiler from mains power for safety.

Step two: unscrew and unhinge the front cover

There are small variations between these models but in general they are held by 4 screws. Two are located at the bottom near the front. One on the left and one on the right.

On the top of the boiler there are usually also two screws, although some covers are held by a clip.

You can unscrew all screws that are holding the metal front cover. Don’t worry it won’t fall off because it is still held by clamping.

Next you need to pull the underside forward and then slightly up and the whole cover will come off pretty easily. It only weighs a few kilo so it’s a simple one-person operation.
If it does not come off easily, there may still be a hidden screw somewhere so don’t use force and see if you missed a screw somewhere.

Step three: flip down the control panel

Flipping down the control panel on a Heatronic 3 boiler

The control panel is held by a single screw on the top (nr. 1). Unscrew it and then you can flip down the control board.

Step four: Remove the connection cover

Removing the connection cover on a Heatronic 3 boiler

The connection cover is held by three screws. Unscrew them (nr. 1) and remove the cover.

Step five: Accessing the screw terminals on the control board

Now that the cover is off, you can access the EMS screw terminal marked ‘BB’ on the control board.

The control board of a Heatronic 3 boiler

As mentioned above the EMS bus data lines are located on ST19 on the ‘BB’ terminal pins.

Finding the BB Screw terminal block on a Heatronic 3 control board

See if there is a grey screw terminal block mounted as above. If not, there is no EMS bus on your boiler. If there is a terminal block, please check if there is about 15V DC between the BB pins. If so, you can go ahead and attach an EMS Gateway to these pins. If there is nothing or only a few mV present, there is no EMS bus active on this port. You can look for it on another part of the circuit board. On Junkers boilers there is always an EMS bus present on the BB terminal.

For Worcester availability of the EMS bus on the BB pins depends on the specific boiler model and whether an integral diverter valve kit has been installed or not. For some boilers that don’t have an active EMS bus it’s possible to swap out the code plug so it becomes active.

If you do not have an active EMS bus on the BB pins it may still be active on the other side of the circuit board. See the image below. Also see the wiki HERE.

Alternative EMS bus location on Worcester circuit board
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Powering the E32 V2 EMS Gateway via passive POE

Sometimes there is no easy way to make a 230V wall outlet or power socket near the boiler or heat pump. The E32 V2 has Ethernet, but is does not support active POE (Power Over Ethernet), which is usually 48V or 24V.

What you can do instead is use a so called ‘passive POE cable kit’ like the Digitus DN-95001 to inject the 12V DC of the BBQKees Electronics power supply to the E32 V2 over the same Ethernet cable as the LAN signal.

Each Ethernet cable has 8 leads, 4 of which are used for data and the other 4 are used for power.

These cable kits will also work for longer distances of Ethernet cable where the 12V voltage may drop a bit, because the E32 V2 will still work with f.i. 9V DC. And the bitrate of the connection is relatively low at just 100Mbit.

Below an image of such a cable kit. The female Ethernet connectors are for connecting to the Ethernet cable from the router location to the Gateway location. And then one side has a Ethernet plug which you plug into your router or switch and a female receptacle for the 5,5mm DC barrel jack plug of the 12V DC power supply.
The other cable in the kit has a male Ethernet plug and a 5,5mm male barrel jack plug for connecting to the Gateway.

Below an example setup. On the left you connect the combiner cable to your router or switch, and you plug in the 12V DC power supply to the connector on that cable.
On the receiving end you connect the incoming Ethernet cable to the splitter cable, and then the Ethernet plug of the splitter cable and the barrel jack plug into the E32 V2.

Please pay attention if your switch or router has 24V or 48V POE enabled on that specific port and you are using this passive cable set at the same time, you may blow up the Gateway. Of course I do not give any warranty if you fry the circuit board. Furthermore this page is just for reference. Always know what you are doing when messing with voltages and cables. The above example may or may not work in your specific situation.

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New version of the EMS screw terminal cable

Since the end of April 2024 I have a new version of the EMS screw terminal cable.

It’s still a 100cm cable with two stranded copper cores of 2×0,5mm2 and the wire ends terminated with a wire ferrule, but now the cable is black and a bit thinner and more flexible than the previous grey cable.

You can find the cable here or you can include it as an option on the Gateway and Interface board product pages.

Below on the left the ‘old’ version of the cable and on the right the new version.

Some more product images:

You use this cable to connect the orange or green screw connector of the Gateway to the EMS screw terminal inside the boiler or heat pump.

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Gateway model E32 V2 now available

It took a bit longer than expected but the first batches of the new Gateway model E32 V2 with WiFi and LAN are now available.

It has all the awesome features of the S3 Gateway with the same EMS-ESP32 firmware but aside from WiFi it has an additional RJ45 Ethernet port for wired networking.
Because the ESP32-S3 chip of the S3 Gateway does not support hardware Ethernet we used a big ESP32 module instead but with a beefy 8MB PSRAM and 16 MB Flash. This solves all the little memory issues of the previous E32 some users might be experiencing with the latest 3.6.x firmware when used with larger EMS systems.

Although the E32 V2 hardware has 8MB of PSRAM, currently the EMS-ESP32 firmware only supports 4MB PSRAM on the ESP32. 4MB is more than enough for now, but if needed this can be increased to 8MB in the future.
Furthermore EMS-ESP32 currently only needs 4MB of Flash storage, so with 16MB of Flash storage in the E32 V2 there is lots of room for more features.

Because the circuit board did not fit anymore into the grey enclosure of the E32 V1.0/V1.5 and the S3 a new enclosure was created. It also has mounting holes on the back. Furthermore it is UL 94 V level fire retardant.

The E32 V2 went through several rounds of prototyping and is of course CE certified and manufactured according to the RoHS directive.
The LAN port provides a very stable interface. POE is not included though, non-isolated 48V is not a good match with the 16V EMS bus.

Currently there may be limited availability of this model, from March onward continuous stock is expected.

You can find it in the webshop HERE.

If you don’t need LAN the cheaper WiFi-only S3 and S3-LR are still a great choice as well.

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Improved wall mount bracket

The wall mount bracket for the S3, S3-LR, S32, E32 and older models is 3D printed.

We are now printing the brackets on a Bambu Lab X1 Carbon printer. The quality has been improved dramatically.
The left image shows the ‘old’ bracket on the left versus the ‘new’ bracket on the right. both are printed with the same filament. You can now hardly see it’s 3D printed.

EMS Gateway new bracket
EMS Gateway bracket comparison
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New Gateway models S3 and S3-LR available

As time progresses so does the EMS-ESP firmware. We were now at a point were EMS-ESP would benefit from some more RAM memory.

That’s why BBQKees Electronics is introducing the new EMS Gateway S3 models. One with the default on-chip antenna and an identical Gateway model with an external high gain antenna for those area’s with low WiFi reception. These are the replacement for the very popular S32 Gateway.

S3-LR front
S3-LR back

Hardware changes

The S3 models looks and feel identical to the EMS Gateway S32 V2.0 they are replacing but they now come with an ESP32-S3 module with 16MB Flash and a hefty 8MB PSRAM (in addition to the on-chip 512KB RAM), which makes it more suitable for large EMS systems with a heat pump or multiple zones/heating circuits.

The new hardware continues using the existing third-gen BBQKees EMS detecting and sending circuitry design as in the S32 Gateway and the EMS Interface board V3’s, which has proven extremely reliable and robust over the years for all EMS types across all Bosch brands.

Aside from the more powerful chip, this model now also features an external USB-C connector for manual firmware updates (if ever needed). The USB-C connection makes use of the internal USB OTG controller of the ES32-S3 so we don’t need the CH340 chip and driver anymore.

What has remained unchanged are the positions and pinout of the external connectors so the S3 is a direct replacement for S32 setups. Only the LED lightguide was changed a bit.

The power options via the service jack or 12V DC power supply also remained the same.


The S3 models will be loaded with firmware EMS-ESP32 v3.6. This is exactly the same firmware with the exact same features as for the S32, just built for the different chip architecture of the S3.
The 3.6 firmware contains a number of new features over 3.5.1, most notably further memory optimizations.

Upgrade discount

If you already own an S32 Gateway it’s probably not really worthwhile to upgrade to the new S3, but if you want too or if you still have an old ESP8266 Gateway like the Premium II you can get a 15% discount on a new S3 Gateway of your choosing. Just use the contact form to contact me with your previous order ID and I’ll give you a discount code.

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New upgrade module for Premium II and older Gateways available

The ESP8266 module in older Gateways like the Premium II can be upgraded to another module in the same format.

The MH-ET Live ESP32 module was a good choice to upgrade the ESP8266 to an ESP32 chip.

Now available in the webshop a soldered and LilyGo T7-S3 board with an ESP32-S3 chip and 16MB Flash and 8MB PSRAM. It’s programmed with EMS-ESP for your convenience.
This new upgrade module is so powerful you probably never need any other upgrade again.

LilyGO T7-S3 on old ESP8266 Gateway

For the ESP32-S3 a different bin file is available on the EMS-ESP repository for EMS-ESP 3.6 and later.

For upgrade instructions see the wiki.

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New revision of the interface board available

The EMS interface board V3 has allowed many people to connect their boiler to a microcontroller.
All connectors on the V3 are on the left side. However, some people want to make a 3D printed enclosure for this board and then it is more convenient to have the UART connector on the other side.

I now made a new revision of this board (V3.1) which has the UART connector on the right side.
It still has the holes on the left side as well, but the connector is only soldered on the right side.

If you need the connector on the left side you can solder one yourself.

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Massive firmware update now available: EMS-ESP 3.5.0

Firmware EMS-ESP 3.5.0 has been available as a beta for quite some time and it had already been loaded on new Gateways since October 2022, the release version of 3.5.0 (stable) is now available as well.
Thanks to a lot of work by Proddy and MichaelDvP and assisted by a number of other contributors, this is one of the most feature rich updates yet.

Remember the firmware is completely open source

The the most noticeable new feature is going to be the addition of localization. This means you can now change the language from English to other languages. The release of 3.5.0 will contain English, German, Dutch, Polish, French, Swedish and Norwegian language.

This localization is in all major parts of EMS-ESP: the web interface and MQTT.

There are however some breaking changes. The most important one is that there is now support for multiple Gateways on a single MQTT broker. When enabled, which is now the default, all MQTT Discovery Entity IDs will include the MQTT base name and the shortname of the EMS-ESP device entity. For example what was previously sensor.boiler_actual_boiler_temperature will now become sensor.ems_esp_boiler_boiltemp. This means these entities will have to be replaced inside HA. See below. If you still want to use the old format and retain the history and script compatibility in Home Assistant then set this back to the old format.

New features and updates

Summary of new stuff:

  • Localization: addition of German, Dutch, Polish, Swedish, French and Norwegian language options.
  • Support for multiple Gateways on the same broker (for hybrid systems and for multiple boilers/cascade)
  • 100+ bug fixes and new features
  • Added initial support for ESP32 C3 and S2 architectures
  • Several new board profiles
  • Improvements in HA Discovery and MQTT handling
  • Addition of new EMS boilers, thermostats and other devices (Greenstar 30, GBH192iT, FW500, BC30)
  • Addition of additional parameters of known EMS devices (f.i. RC30 internal temp, 20+ heat pump parameters)
  • Improved handling of logic/telegrams of some EMS devices
  • Setting the 4 multi-purpose relays/inputs of heatpumps (via API)

Full release notes see:

How to update to 3.5.0

To update to the release version of 3.5.0 depends on which firmware you are on now, and when you purchased a Gateway product. In most cases you should first try out an update via the web interface. If it succeeds, you are on 3.5.0. If it fails you need to flash the Gateway via the internal USB connection on the Gateway board.

Read the instructions below for the correct update procedure for your situation.

If you are currently on any 3.3 or older version you need to use the USB method anyway.
If you are currently on any 3.4 or 3.5 version first try the update via the web interface and if it fails update via the USB port.

Method one: Update via the web interface

Download the 3.5.0 bin file from the repository. Then open the web interface of the Gateway, go to System->Upload and select the bin file you just downloaded.

The upload will take about a minute. If the Gateway has installed the firmware and rebooted itself, you should see the login screen again after a while. Press F5 to clear the browser cache. Then login.
Settings should be pertained.

In case you get an error 500 during the upload, you need to move on to method two below.

In case you have an E32 Gateway and it became unreachable after the update, the board profile might have been reset to S32 or Custom. Please login to the ems-esp access point, set the board profile back to E32, save, and reboot. Ethernet will be activated again.

Method two: Update via the USB port

If a lot of cases you need to upload this firmware via the USB port on the inside of the Gateway. It is the only time this is needed, if 3.5.0 is installed once, all future updates can be done via the web interface.

You need to open the Gateway, plug in the USB to a computer and load the firmware via the EMS-ESP flash tool. DO NOT use another flash tool as it will not work.
All settings will be lost after this update, so make a note of all important ones.
(You can also backup your settings and customizations before the update and load them once the update is done).

For the detailed update procedure look here:

If you have questions about the update you can send me an email via the contact form.

Breaking change: MQTT entity ID’s

For a number of entities in Home Assistant the unique ID has changed in the auto discovery config . This means these entities will have to be replaced inside HA.
This is because there is now support for multiple Gateways on a single MQTT broker. When enabled, which is now the default, all MQTT Discovery Entity IDs will include the MQTT base name and the shortname of the EMS-ESP device entity. For example what was previously sensor.boiler_actual_boiler_temperature will now become sensor.ems_esp_boiler_boiltemp. This means these entities will have to be replaced inside HA. See below. If you still want to use the old format and retain the history and script compatibility in Home Assistant then set this back to the old format.
There is a new checkbox in the Settings to keep the ID as in firmware 3.4 for those who do not want to switch now.

Another way is to rename the entity ID’s in the HA database with a script. See:

Possible bugs

After the update to 3.5.0 it is possible that sometimes during loading or viewing of the web interface you’ll get an Error 507.
This is because the new web interface needs a fairly large chunk of memory, which is not always available. You get this error 507 then. All other services on the Gateway will continue to work. See:

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Loxone Miniserver

Lately I’m getting more questions about the integration of the EMS Gateway with Loxone.
Currently you need to use the Rest API with HTTP GET and POST to communicate between the two.

I decided to register as a Loxone installer and purchased a nice Loxone Miniserver Go myself to learn about the system. The Miniserver has arrived this week.

In the coming time I’m going to set it up, see what the fuzz is all about and then I’ll create a good integration guide for Loxone.
This thing really is Mini