Usage scenario's Opentherm Gateway

[Simba82]

Hi all,

I am going to replace my old Nefit central heater relic with a new and modern system, as such I was looking for ways to control the heater. I understand that OpenTherm is the most generic standard, even though there seem to be quite a few discrepancies among manufacturers.

Thus I stumbled on the OTGW. It's one of only a few solutions I could find to control a OT heater. However, there are some things I can't quite get my head around. As I read it the gateway still requires a thermostat, or is this optional? Is it not a complete replacement for the thermostat, or am I not understanding the usage of the gateway correctly? My second question, and its answer may possibly solve my previous question .. The gateway has no (header for) a internal temperature sensor? Finally, Does the PC has to be connected at all times, or can you just upload the rules to the gateway?

Attached is a quick draw of how I now understand the OTGW. I would like to hear what it can do in the average household, as I'm understanding it now, all functions can also be provided by a good thermostat.

best,
Sjoerd

[hvxl]

You understand correctly. The gateway isn't a thermostat. It was designed to be used together with a thermostat. Controlling a central heating system in such a way that you get a comfortable temperature while still being as energy efficient as possible is much more complicated than it may look at first glance. Many years of research have been invested into getting that right by thermostat manufacturers. I didn't see the need to try to replicate that. In addition, a thermostat provides a nice interface that the non-geek members of the household are comfortable with to manually intervene with the standard program.

The gateway can be used to:

• integrate your central heating with a domotica system.
• remotely control your thermostat.
• inform you via email or text message when there's a problem with your heating system.
• provide smart power for a thermostat when the boiler doesn't support it.
• clean up the opentherm signal if either of the devices goes too far outside the specs for the other device to understand.
• connect an outside temperature sensor or provide the outside temperature to the thermostat from some other source.
• automatically lower the thermostat setpoint when you arm the security system upon leaving the house.

If you don't need any of these features, or have a thermostat that can provide all the relevant features, then there's no reason to use the gateway.

Not all of these features require a PC to be connected. Also the outside temperature sensor is optional.

The 4.0b0 firmare will allow you to use the gateway without a thermostat. You then need a computer to instruct the gateway what to tell the boiler. This is done at a very low level. The computer needs to somehow determine how hot the water in the system should be. It can't just say it wants the room temperature to be 20 degrees (which you can with a thermostat connected). Note that I only added this feature because several people asked for it, not because I think it's a good idea.

[Simba82]

Thanks a lot Schelte, that was very useful

Somehow, deep down, I'm not fully convinced yet that writing thermostat software is that complicated, but for now I'll put the idea of my own software thermost out of my head

[denis.bredikhin]

I'm also interested in such usage of Opentherm Gateway.

Can't you please explain, how thermostat can decide temperature of hot water in boiler, if it doesn't know outside temperature?
I thought that temperature of water is controlled by boiler itself, taken into account inside temp, outside temp and setpoint. Is it not true?

[hvxl]

Most central heating systems don't have an outside temperature sensor. A thermostat therefor generally determines the control setpoint (target water temperature) based on the difference between the current room temperature and the desired room temperature and how fast it wants to get there.

A bigger temperature difference translates into a higher control setpoint. On program changes, the thermostat can start early to reach the desired room temperature. It can then use a more economical, lower control setpoint than when the thermostat setpoint is manually raised (in which case it tries to reach the desired temperature within a shorter amount of time).

The boiler controls its modulation level based on the control setpoint it received from the thermostat, along with the max relative modulation level allowed by the thermostat.

[denis.bredikhin]

I have a boiler Viessmann Vitodens 200-w with outside sensor, and no thermostat now. So now boiler calculates water temperature only on the base of target room temperature and outside temperature.
How will it work if I add OpenTherm thermostat to the system? I thought that thermostat will just control target room temperature (just duplicate same what I can do on boiler itself) and send current room temperature to boiler, and boiler will calculate water temperature by 3 variables: target room temperature, outside temperature, current room temperature. Is it wrong?

What I want to achieve, it to be able to send to boiler (or to thermostat?) target room temperature, and may be also current room temperature (if possible).

[hvxl]

So your boiler can handle weather-compensated control of the heating on its own? That's a feature that is usually provided by the thermostat. I have no idea how that will work if you add a thermostat. I can only suggest that you try it and let us know.