The Efficient Home Forum

Fr3eWar wrote: (just because of opentherm i choose viessmann, here in Hungary you can not by another opentherm boiler).

There must be other brands as well, for example buderus GB012. That is not a recommendation, I only know my viesmann.

Fr3eWar wrote:That would be a big problem with my only floorheating system. Water temp should be between 30-40 °C.
What will i loose, if i connect to boiler with BDR?
The domestic hot water is made in an 200l indirect puffer by the boiler with factory viessmann NTC temperature sensor

As described by Bart, the issue here is the Viessmann locking out control of the Max Flow Temperature of the boiler and defaulting to 80 deg.

There is away around this which some in this thread including myself have successful managed to implement.

You can connect a Opentherm gateway between the boiler and controller to manually set the Max Flow Temperature to a value which you wish.

If you read further back in this thread You can get further info.

Stuart

But even then (depending on the plumming) the occasional overshoot may happen, at least on the Viesmann. Mine at least, if 1 rad heat demands results in a temp request on opentherm for let's say 40 degrees, but all the others radiators are closed, will allow water temperature to go over 50 before it shuts off. Probably it can't modulate the flame flame power small enough for these kind of small power demands. I don't know how other boiler handle these kind of situations, but maybe, if there is a very strict requirement in a heating system for the water temperature to not exceed a certain value, that should be handled by an underfloor heating thermostatic mixing group, not by the boiler.

Bart wrote:I can give you a copy of the firmware but I think that I need to contact the original author first. He might request for a specific version signature.

Could you possibly send me a copy of the firmware (and the modified asm source code file too)?

Yes, but understand that it is experimental and may not work as expected for you.

I've had some time to dig in to home assistant, I had to add the SR command support to pyotgw lib and to the opentherm gateway component, but now I can control max CHW from home assistant.

WOW! This thread took on a life of its own

I think it should be the Evohome controller's job and not the dial on the front of the boiler. The controller would be able to use additional information such as the outside temperature if required. But, also, when you have an underfloor controller and it is the only thing calling for heat the Evohome just says boiler, give me 100%. Which again is ridiculous. There should be a CH (radiator) max temperature and a UFH max temperature on the controller so it could be much more intelligent. There's no need to supply the UFH with 80C to the mixing valve, it could save more energy just ticking it over at a lower temperature.

As a software developer, these implementations would take 30 minutes with access to the source code. It seems a major problem with traditional product companies that they just don't understand the nature of software. They employ a team to write some firmware then wind down it down. No continuous development. We were told there would SmartThings integration but that never happened.

After years of using the product, it's not really that intelligent or "learning". It's got some pretty simple logic to decide on what heat to request from the boiler and adding a couple of limits wouldn't have much affect on how it all operates.

Bart wrote:Yes, but understand that it is experimental and may not work as expected for you.

I've had some time to dig in to home assistant, I had to add the SR command support to pyotgw lib and to the opentherm gateway component, but now I can control max CHW from home assistant.

It would be great if you could post the files and lines modified to save time re-inventing the wheel.

simonh wrote:

Bart wrote:Yes, but understand that it is experimental and may not work as expected for you.

I've had some time to dig in to home assistant, I had to add the SR command support to pyotgw lib and to the opentherm gateway component, but now I can control max CHW from home assistant.

It would be great if you could post the files and lines modified to save time re-inventing the wheel.

Sorry to bump this thread, stumbled across it as my plumber has given up on my boiler haha.
I have a viessmann 200-W, I’m using evohome and the viessmann cylinder control box (as not using viessmann cylinder) connected to a BDR91.

I’m super keen to get opentherm, but no idea what module I need to buy let alone what needs configuring.

Does anyone have a link to the module I need?

Seen this but no idea if it would help:
https://www.tindie.com/products/jiripra ... BAVwpVYcR0

Hi Bart,

I've finally got my setup fully up and running with HA running on a RPi 4 and the Nodo Shop OTGW (through MQTT) passing OT messages between the boiler (Vitodens 111-W) and my Evohome controller. My next step is to limit the Max CH flow temp (when I started everything up again having removed my BDR91 the temp immediately shot to 80 deg).

As you've already achieved this would you mind sharing the code with me? I spend enough hours of my day hitting my head against brick walls as it is so any help greatly appreciated!!

After a number of years of ignoring the problem of my radiators being set to max / off by the boiler and opentherm, I’m wondering if going back to the simple on/off boiler control via a BDR91 would be better? This was my original plan as the boiler was installed by the previous owner and has an external temperature sensor connected.

The only reason I chose opentherm was that it was easier to wire up. I did physics at school so understand basic wiring diagrams, but couldn’t work out where to wire in the direct on/off wiring from the BDR91.

Can anyone give me some advice on this?

Thanks

Jon