Thank You for the posts on this site for helping me resolve some Evohome configuration questions. Once you know the answers, it seems so easy - but not before. I hope my story and experiences below will be helpful to future visitors.

Phase 1, The Legacy System!
My original installation had mechanical Thermostatic Radiator Valves (TRVs) on nearly all radiators and a single thermostat on the hall wall. When the thermostat called for heat, the Central Heating (CH) pump started and all rooms went to their set temperature, unless the hall heated first and the thermostat turned off the CH pump!

Thanks to an article in PC Pro by a certain Paul Ockenden (thank you Paul!) , I converted the system to Evohome (EH). The TRVs were replaced with EH HR92 Radiator Controllers and an EH Wireless Relay Box (BDR91) was connected to the boiler in place of the wires from the hall thermostat. The BDR91 was paired to the ‘Boiler Control’ on the EH controller. Since the BDR91 was playing the role of CH thermostat, this configuration worked perfectly, and the EH 'Boiler Control' parameters were available to fine tune the configuration.

Phase 2, A new Boiler.
When the old boiler failed, I replaced it with a Worcester Bosch 8000 Life Regular Boiler and a Greenstore unvented Hot Water tank. The installation was performed to an S plan configuration, with one pump and two motorised valves. The ‘old’ BDR91 was to be re-tasked to the Central Heating and an EH Hot Water kit (ATF500DHW) was acquired, which contained a Wireless Cylinder Thermostat Transceiver (CS92) and a second BDR91.

I think it is fair to say that the EH installation manual is a little light when it comes to understanding – and the wording in the Guided Configuration on the EH controller gives rise to further confusion! The Guided Configuration refers to ‘Stored Hot Water Configuration’, which implies that the other option must relate to heating – this is NOT the case! The Stored Hot Water configuration option will allow you to configure the Hot Water Sensor (CS92) and BOTH BDR91s controlling the motorised valves for Hot Water and Central Heating. The confusion continues when you look at ‘System Devices’ on the EH Controller and see that there is an option for Stored Hot Water and Boiler Control!

As I had previously controlled the heating in phase 1 through the ‘boiler control’ (emulating the thermostat), it took research on this forum to understand the ‘new paradigm’. The motorised valves contain an electrical connection to the boiler that activates the boiler when the valve is open. Therefore, once either valve opens (due to CH or HW heat demand), the boiler will be fired. In this ‘S’ plan model there is no need for ‘boiler control’ – it is built in via the valves. So, to convert my system to S Plan, I had to 1) Set boiler control to NONE (ie remove the phase 1 use of boiler control) 2) Reset/Initialise the pairing on the BDR91s (press and hold the button until it fast blinks red) and then 3) Run the Stored Hot Water Configuration option, which paired the Hot Water Sensor, then the HW BDR91 and then the CH BDR91. A quick test and all was working.

Beyond Phase 2 – When Worcester Bosch (WB) and EH do not play well together!
It took a few weeks to work this out, but….

As the ‘heating season’ got going, EH controlled the zones, progressively closing the HR92s in a zone as the zone reached its target temperature. From running the CH 100% of the time when there was a cold zone, once all zones were near their target setpoint (and hence all HR92s were partially closed), EH started to ‘cycle’ the CH BDR91 at regular intervals and for short periods of time. Meanwhile, the WB boiler went through its own ‘shut down cycle’ whenever the CH BDR91 turned off and so was actually still running for a couple of minutes after EH thought it was turned off. In practice there could be only 1-2 minutes between the boiler shutting down and the next cycle starting. I was seeing the boiler ‘come alive’ at the start of each cycle, but the burners did not show as lit. Out flow temperature dropped to below 30C and hence the radiators were all but cold and as a result, zone temperatures started to fall from their setpoint. It was only when the zones dropped well below their setpoints, that EH ran the CH BDR91 for longer periods of time and at that point the burners fired.

Those in the know, will recognise the EH behaviour as TPI (Time Proportional & Integral). The problem is that Honeywell confirmed that there is no way to control this behaviour – there are no ‘cycles per hour’ or ‘minimum run time’ parameters available outside of ‘boiler control’ – which was no longer in use. It seemed that there was intelligent software in the WB boiler which was not playing well with this constant short cycling!

Maybe OpenTherm was the answer? EH has an OpenTherm bridge, but WB uses EMS. WB also has an OpenTherm bridge in their EasyControl range, but this is to allow their EasyControl range to talk to an OpenTherm boiler and not the other way around. WB technical support told me:

“Worcester boilers do not support the Open-therm protocol as the Worcester controls operate using an EMS protocol. We would not be able to support an interface that converts from Open -therm to EMS as these would not have been tested with the products. Third party controls would only be supported if operating the boiler via a 230vac switched demand.”

I contacted the Evohome shop (a great and friendly company who supplied my EH solution) and their support told me that IF every radiator had an HR92 fitted (a fully zoned solution), I should not have installed a motorised valve for the heating circuit, only for the HW, and then relied on the HR92s to prevent flow around the heating circuit. In this model, the CH BDR91 would have been installed as a boiler control – giving me access to the EH parameters I needed.

Phase 3 – The Final Solutions
I needed to find a way to make the WB less sensitive to ‘short burn cycles’ and to get access to the relevant parameters within EH – and hopefully without any plumbing changes! I therefore changed the EH configuration again:

First, I cleared the EH/BDR91 config and reran the Guided Configuration as a Stored Hot Water only system – this paired only the CS92 and the HW BDR91. I then manually paired the CH BDR91 as a Boiler Controller.

Rather than change the wiring, the CH BDR91 still operates the CH motorised valve and therefore triggers the boiler through the existing wiring from that valve. This means that when there is CH demand, the CH valve opens and the boiler fires. When there is HW demand, BOTH valves open.

At first glance, this looks wrong, but in practice this is the same as if the CH valve had been removed and the CH BDR91 connected to the boiler directly! So long as all the HR92s are closed (Zone up to temperature/low setpoint/heating off), there will be no flow around the CH plumbing – hence the CH valve is effectively acting as the bridge to the boiler controller.

Now that I have the CH BDR91 defined as Boiler Control, I have access to the EH ‘cycles per hour’ and ‘minimum run time’ parameters!

The largest value for the EH ‘minimum run time’ is 5 minutes – which is now set. On the WB side, there is a parameter called ‘Standby Time’ which controls the ‘Anti Cycle Mode’ (option 3-b2 for the record). The default is 5 minutes – which explains why the burners never fired in the phase 2 scenario! By reducing this to 3 minutes (and with the EH minimum run time set to 5 minutes), the burners are now running on every cycle.

So far, phase 3 is delivering the goods and remains the current configuration!

In Closing
For anyone who has an EH solution and a need to install a modern (non Opentherm) boiler, I hope there are some ideas in here which will help the ‘clever’ software on both sides play better together! Of course, there may be a much better solution..

I am still open to a means to bridge EH to WB’s boiler control more intelligently (OpenTherm to EMS? EH to EasyControl bridge?), provided that this would not invalidate the multi-year manufacturer warranty offered by WB.

Phase 3 seems to be a fine end point, but is anyone able to offer a suggestion for Phase 4?