Interfacing with an Interegrated Worcester Bosch CH Controller

Hi there,

As the title says, I'm trying to figure out the signal from my boiler's controller that indicates hot water demand. Unlike ancient systems that operate the timer and thermostats at mains voltage, this one has 7 pins of an 8 pin connector connecting the controller to the boiler's main board.

I thought at first I could piggy back the connections to the integrated diverter valve but have established that this is in fact a stepper motor controller by what seems like a L293D chip (the upshot being that the wiring to the diverter is doing fancy things to precisely control the rotation of the motor and is not giving me the simple binary signal I am looking for).

Having also drawn a blank searching for all variations on "DW-20 pinout" I can think of, I've learned that Bosch commonly use a proprietary bus called EMS and that others interface to this through a 3 pole jack plug. Trouble is, there is no such plug or socket in my system, so if the internet is to be believed the EMS bus might utilise some of the pins on the simple header style connector. Or it could just set various pins high or low to indicate central heating demand or hot water demand.

Problem is that any detection skills I may have had in my teens are long gone and I'm embarrassingly incompetent with my multimeter (I can figure out if something is live at mains voltage, what my car battery is doing or if there is a break in a wire but not a great deal else. I only get Arduino circuits working by following instructions to the letter).

Please can someone help me point me in the right direction to deduce which pin (if any) on the controller indicates HW demand? Ideally, I'd like to do this without frying the boiler main board.

Many thanks in advance.


Who do you call when the boiler stops working? Have you asked them for documentation?


Which controller is it?

Can you detect the boiler coming on by other means?

Who do you call when the boiler stops working? Have you asked them for documentation?


Hi Paul.
It's a complicated story, but it's not a working/not working problem. My CH engineer is happy with servicing and replacing components but not with getting into the nitty gritty of proprietary control mechanisms. Whilst the design has provision for external wiring centres, these are only used when NOT using the Worcester Bosch integrated controllers. I want to use the signal from the DHW demand to control a relay that operates an additional diverter valve, potentially with some delay logic in there. I'm happy with that part of it just need to know whether I need a 5, 12 or 24V relay. The boiler main board has a transformer which puts out 25V and 9V so I'm guessing that all the logic stuff operates at one of these (excuse me if this is a naïve deduction).

Which controller is it?

Can you detect the boiler coming on by other means?

Hi Wildbill.
I’m specifically interested in the DHW circuit. The boiler comes on for either CH (radiators) or DHW (indirect cylinder) demand in something pretty similar to an “S-Plan” configuration (independent control valves for CH and DHW) so just detecting that it is on doesn’t help too much with my situation. I’m led to believe that as it is a modulating boiler, controlling it with binary external controls loses the ability of the boiler to modulate its output based on demand (i.e. in certain circumstances it will run at low power for a long period rather than full power for a short period). The only other thing I could think of is to put some kind of independent flow sensor in the DHW circuit but this means draining the system down and running additional power supplies etc which I’d rather avoid.

My hope is that I can piggy back the DHW demand signal to control a relay; I just don’t know how to safely detect that signal and don’t want to go sticking meter probes at every permutation of 7 wires I can think of if there is a smarter way.

Oh - the controller is a Worcester Bosch DT20 RF and the boiler a Junior 24 System.

Thanks in advance!

Not sure I’m interpreting you correctly but ......I wondered if you can get the “signal “!you want from the hot water tank thermostat and the programmer calling for hot water at that time . Those are external to the boiler ? ( ie if the tank stat AND the controller ask for heat , assume the boiler is running to satisfy that )

Also the modulation is usually on outlet flow temperature to control it to a preset value . You could easily strap an external sensor to the pipe for that.

You mention the water control valve - these are not ( usually) stepper motors but simple ac motors that drive to a stop and then stall to hold that position; spring being used to return. So the valve motor supply will give you a signal.

Thanks Hammy,

I read another post on this forum talking about XY questions where the OP did not explain enough of the problem and instead tried to solve the wrong one, so perhaps I need to supply even more information.

I have a CH system that was "almost totally" installed quite a while ago. A few days before the original CH engineer was supposed to finish and sign off his job, he split up with his girlfriend. The next day (distraught, seemingly) he packed in all the jobs he had on the go (including mine) and left the country to find himself.

Roll on a few years and I have a system that I don't really use (as, of course, it is still to be signed off). As I mentioned earlier, the system is basically an S-Plan, built around a Bosch 24i System boiler and an Albion unvented cylinder (the installation of the latter is all hunky dory). The boiler has an optional integral diverter valve (to heat the cylinder via an "indirect" heat exchanger coil). From what Building Control tell me, the cylinder MUST have a 2 port valve on the inlet controlled by an (adjustable) temperature control thermostat and a fixed high limit stat, both of which OPEN when their limit is reached. These stats are installed to the cylinder but not wired in; they all expect to run at mains voltage, supplied from a "wiring centre". The boiler, however, only expects a "volt free" thermostat on the hot water circuit and as mentioned has its own diverter valve which shuts off when the set temperature is reached. Its own control circuits shut the boiler off if the system temperature exceeds the high limit.

So, how to incorporate the external 2 port valve for the cylinder? My plan is to use the DHW demand signal from the controller to close a relay to switch mains voltage (from the boiler supply circuit) through the stats to the pump via a 240V coil relay. When this last relay is powered, it will close the circuit to the main board cylinder stat connection. Since there may be some timing issues, I am contemplating introducing an Arduino board to delay the closing of the valve and/or stat sensor as required to not have the valves playing ping pong.

Upshot? I need to figure out how the integral (plugged in to the boiler fascia) controller tells the main board to open the diverter valve - which it can physically only do through the 7 pin connector between them. I seek help in doing this in a save way so that I don't inadvertently end up shorting out the controller/main board. I'm hoping that there will be a ground connector and some pin that is switch high (9V? 25V?) when the timer tells it to switch on the hot water.

Apologies for all the words but that's the whole problem.

Managed to find a DT20RF manual online that warns of 230V and 24V inside the boiler casing.


So given the controller connects via a bit of ribbon cable, it seems most probable that it runs at 24V. (It has a little display panel, a bunch of buttons and an RF receiver with which to communicate with the remote room thermostat.)

8 wires on the ribbon, 7 pins on the plug.

Pin 1 = Grey.

How dumb would it be to assume that pin 1 is ground and then hunt for 24V that appears on another pin when the water is switched on? I suppose that at least one other pin will be permanently at 24V to provide power to the controller's daughter board and some other pin will indicate that heating is required. No idea what the 3 other pins might be for...

Is this being over simplistic? :o