Opentherm to EMS gateway

[MadeTomatoHue]

Why not just use the already built OT to EMS interface with the Honeywell OT Bridge?
https://www.robbshop.nl/nefit-ems-op...verter?sqr=ems

I looked at them once I decided I wanted to try load compensation on the boiler, and they would have been an easy way in, but AIUI they are no longer available: "Niet op voorraad" = "Out of stock".

I'm also unsure how EvoHome OpenTherm works with DHW heating - does it call for 100% heat via the bridge, or "forget" to adjust from whatever it has set CH demand to? I was going to play with adjusting flow temp to ~10C above current stored water temperature to see if it adds to overall efficiency.

[bruce_miranda]

OT on Evohome is rather silly. DHW = 90C target flow. CH = 10C to 90C target flow. It almost expects your boiler to ignore that ridiculous 90C set point and clamp it at something sensible. We know of some Viesmann boilers that didn't allow that and were ramping up to 90C as per the instruction!

[Stevedh]

My Evohome OT bridge only ever calls for a maximum of the value I set on the boiler. My boiler is set to 65 and that is the maximum temperature it calls for (I have tried other temperatures and the behavior is consistent with that conclusion). Interestingly the display on the boiler changes to match the ot temperature its using, so not sure what would happen if I changed it I when ot has set it to something else.

I think though that is the boiler as I think it it returns the set temperature as its maximum temperature when queried by the OT Bridge. I will have to confirm that though as its been a while since I looked at the logs.

BTW mine is a 2017 atag economiser .

[bruce_miranda]

How are you measuring what the OT Bridge is requesting? Because if I check my boiler, I too get an impression that the OTB is only requesting the max as set by the boiler.When infact what is happening is that my boiler setting is clamping the max requested. It's only when I monitor things from the OT side, do I see that the OTB is requesting a 90C max, which my boiler ignores and uses its on max set.

[kevinsmart]

The OpenTherm bridge reads the boiler CH setpoint boundaries and ranges the control setpoint (flow temperature) based on the upper boundary.

Control setpoint = 20C + % demand * (Max setpoint - 20)

My boiler always reports 80C max setpoint, and it can’t be adjusted via the boiler controls, so I override this in my custom OTGW firmware.

I weather compensate the CH flow temperature (capping it), switching to 70C when I detect DHW heating.

[bruce_miranda]

The OpenTherm bridge reads the boiler CH setpoint boundaries and ranges the control setpoint (flow temperature) based on the upper boundary.

Control setpoint = 20C + % demand * (Max setpoint - 20)

My boiler always reports 80C max setpoint, and it can’t be adjusted via the boiler controls, so I override this in my custom OTGW firmware.

I weather compensate the CH flow temperature (capping it), switching to 70C when I detect DHW heating.

Which boiler is this?

[kevinsmart]

It’s a Vokera Unica 36HE. Not my choice, had an Intergas before I moved, but I worked around its limitations with my OTGW firmware

[MadeTomatoHue]

The OpenTherm bridge reads the boiler CH setpoint boundaries and ranges the control setpoint (flow temperature) based on the upper boundary.

Control setpoint = 20C + % demand * (Max setpoint - 20)

My boiler always reports 80C max setpoint, and it can’t be adjusted via the boiler controls, so I override this in my custom OTGW firmware.

I weather compensate the CH flow temperature (capping it), switching to 70C when I detect DHW heating.

That's pretty much the approach / formula I was going to use, assuming that the valve % represents demand in the same way.

I'm going to experiment with whether ramping DHW flow temp adds efficiency - may be hard to tell unless averaged over quite some time as it might be heating 150l from 15 degrees one day and 100l from 18C the next. I think the theory is that since inefficient / efficient gas consumption is only ~5% difference if it needs more than 5% more time when ramped then there is no net benefit. But with the option to explore it'll be interesting to do so .

[Stevedh]

How are you measuring what the OT Bridge is requesting? Because if I check my boiler, I too get an impression that the OTB is only requesting the max as set by the boiler.When infact what is happening is that my boiler setting is clamping the max requested. It's only when I monitor things from the OT side, do I see that the OTB is requesting a 90C max, which my boiler ignores and uses its on max set.

I have an ESP8266 OpenTherm Gateway and can see what is being requested etc. What I don't understand is why the evohome opentherm bridge bridge drops its requested heat to 10C (the minimum), for 5 minutes at a time (my logging interval) even when the room is no where near the requested heat. The boiler I would understand, short cycling (or whatever it is called again), but the opentherm bridge. And that can't just be the odd error..

[bruce_miranda]

I have seen this happen too. Any link to the ESP8266 OpenTherm Gateway you are using?

[Stevedh]

I have seen this happen too. Any link to the ESP8266 OpenTherm Gateway you are using?

https://diyless.com/product/esp8266-opentherm-gateway

and although it is called an esp8266 it should work just as well if not better with an esp32, well the script I used looked like it had last been used with an esp32 as I had to comment out a couple of esp32 specific lines to get it to build.

[kevinsmart]

I suspect you are seeing Evohome switch to OpenTherm on/off load control setpoint/10C because your boiler is exceeding setpoint+threshold and cutting the flame.

This can happen if you are heating a single zone.