DIY OpenTherm Weather Compensator for Honeywell evohome

[Otto-Mate]

Kevin Smart has created his own DIY Weather Compensator for evohome…

https://www.automatedhome.co.uk/tuto...l-evohome.html

M.

[DBMandrake]

Nice one!

I ordered an OpenTherm Gateway, Soldering Service, Enclosure and FTDI cable from nodo-shop.nl out of curiosity, to monitor communication between the Evohome R8810A OpenTherm bridge and boiler. It turns out that the bridge does read the outside temperature from the boiler. Also, I learned that the boiler does not report the Max CH water setpoint set by the front panel but overriding this does influence the Control setpoint temperatures sent by Evohome.

I guess this quote answers the long debated question - is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control, with each blaming each other - turns out it's the Intergas boilers fault. If the bridge sends a modified max flow temperature figure to Evohome it is respected by the Evohome in the temperature range it requests...so it's the boiler not reporting the max allowed flow temperature which is at fault.

[bruce_miranda]

On the topic of Opentherm. I have managed to decode all the OT messages between the Evohome Controller and the OT Bridge. If you are using an HGI80 or similar, in Domoticz, you will now see new devices appearing that allow you to track various OT parameters like the Control Setpoint, Boiler Flow and Return Temperatures, the Boiler's Relative Modulation etc. Until my PR gets merged in with the rest of Domoticz, you can download the code from https://github.com/brucemiranda/domoticz and let me know how you get on.
This now means that boiler parameters can be monitored without needing a separate OT Gateway, eBUS monitor or any boiler software.

[kevinsmart]

Nice one!


I guess this quote answers the long debated question - is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control, with each blaming each other - turns out it's the Intergas boilers fault. If the bridge sends a modified max flow temperature figure to Evohome it is respected by the Evohome in the temperature range it requests...so it's the boiler not reporting the max allowed flow temperature which is at fault.

Thanks! Indeed as I found out, at least for me, the system works better with proportional temperatures calculated from a 90C maximum, rather than the 60C I have set at the boiler.

So I ended up limiting requested flow temperatures to the weather compensation calculated maximum, rather than returning this temperature to Evohome.

It may be that given time Evohome would adapt but I wasn’t prepared to wait or risk this during peak heating season.


I look forward to seeing how Resideo support weather compensation with their Q2 firmware update, but I’ll probably still use my solution to optimize it based on my local temperature sensor rather than weather station readings.

I have my weather station registered with Wunderground but Evohome must be using a different service as temperatures vary.

[blowlamp]

Nice one!


I guess this quote answers the long debated question - is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control, with each blaming each other - turns out it's the Intergas boilers fault. If the bridge sends a modified max flow temperature figure to Evohome it is respected by the Evohome in the temperature range it requests...so it's the boiler not reporting the max allowed flow temperature which is at fault.

This is the wrong conclusion as far as I'm concerned.

As I've pointed out on a few occasions, connecting a more fully featured OpenTherm thermostat to a weather compensated Intergas boiler means that the boiler expects the thermostat to use its own compensation curve as well as its own maximum central heating temperature setting. The boiler just relays the outside temperature reading over to the thermostat so it can calculate a water temperature to send back to the boiler. It's then down to the boiler how it maintains that temperature.

My OpenTherm/weather compensated Intergas boiler & Remeha iSense thermostat combo works perfectly as above, as does the inlaws Intergas/Nest v3 setup configured as an OpenTherm installation.

[bruce_miranda]

@kevinsmart given that you have studied the OTGW firmware in some detail, could you explain why the OTGW clamps the Control Setpoint to 60C even when it's in Monitor mode. Without the OTGW, my OT Bridge instructs my boiler to ramp up to 90C, ofcourse my boiler doesn't do that. However just the presence of the OTGW means that the Control Setpoint is rarely more than a few C over 60C, 66C is the max I saw. Why is the OTGW modifying messages even in Monitor mode? Is there some failsafe in the firmware that does that?

[kevinsmart]

@kevinsmart given that you have studied the OTGW firmware in some detail, could you explain why the OTGW clamps the Control Setpoint to 60C even when it's in Monitor mode. Without the OTGW, my OT Bridge instructs my boiler to ramp up to 90C, ofcourse my boiler doesn't do that. However just the presence of the OTGW means that the Control Setpoint is rarely more than a few C over 60C, 66C is the max I saw. Why is the OTGW modifying messages even in Monitor mode? Is there some failsafe in the firmware that does that?

Bruce, when you have switched the OTGW to Monitor mode do you see any flashing LEDs?

In Monitor mode you should see no LED activity and the OTGW should not be modifying data. If you are definitely in Monitor mode and seeing this behaviour I’ll try to work out why.

[bruce_miranda]

I was definitely in Monitor mode and confirmed that using the OT Monitor and querying PR=M. Hence my surprise that just by adding the OTGW in monitor mode it was changing what the Control Setpoint max could be. The thing is that when you have it set to Monitor mode, you cannot even change the Control Setpoint max on the OT Monitor software.
The weird thing is that the highest the Control setpoint got to was 66.00 but never higher than that. But without the OTGW in the chain the Control setpoint is set quite routinely to 90C.

[kevinsmart]

I was definitely in Monitor mode and confirmed that using the OT Monitor and querying PR=M. Hence my surprise that just by adding the OTGW in monitor mode it was changing what the Control Setpoint max could be. The thing is that when you have it set to Monitor mode, you cannot even change the Control Setpoint max on the OT Monitor software.
The weird thing is that the highest the Control setpoint got to was 66.00 but never higher than that. But without the OTGW in the chain the Control setpoint is set quite routinely to 90C.

That’s very strange because in Monitor mode there is no firmware intervention to transfer data across the gateway. If you can just double check that no LEDs are lit, I’ll take another look at the firmware.

[bruce_miranda]

Yes confirmed, no LEDs are lit.