Override for zone control

[Dipapi]

Hi all,

I stumbled on the Opentherm Gateway in a search to make a zone controller for my house and I was wondering if and how it would work to enable zone control in my situation.

My situation is as follows:
- Nest thermostat V3 on ground floor controlling the boiler through opentherm.
- Ground floor where the Nest is located has underfloor heating
- Upstairs have multiple rooms with radiators and TRVs (controlled via Z-wave)
- Home assistant

What I'd like to do is independently control the underfloor heating and radiators. Currently the problem is that if I'd like to control the TRVs upstairs when the ground floor has already reached its desired setpoint, there is no hot water supply. What I now have to do is to raise the setpoint of the Nest to a degree or so higher, such that the boiler turns on, and heats the upstairs radiators. Now the ground floor is heated to a higher setpoint than desired.

How I'm planning to achieve this:
I am planning to install some servos on the heating system such that I can decouple it from the heating system (i.e. can turn the hot water supply off). My thinking to have the desired situation of independent zone control, is to have the Nest still control the boiler. If the setpoint is reached, and I want to heat upstairs, I would like to overrule the setpoint of the boiler, and at the same time close the servos to the underfloor heating. In this way, the ground floor is not heated unnecessarily.

My main questions are:

• Does above make sense or do you have a different suggestion?
• Does the OpenTherm Gateway allow me to overrule the Nest by having by setting the boiler to heat temporarily until the upstairs radiators have achieved their setpoint, without having this reported on the Nest as a raised setpoint?
• Any examples of the logic of such a system in Home Assistant? (otherwise I would be comfortable enough to make this from scratch, but maybe I can save some time

Thanks in advance for the great help!

[hvxl]

This is what happens when you send a CS=10 command to the OTGW:

Code: Select all

15:09:03.943966	T10010000	Write-Data	Control setpoint: 0.00
15:09:03.954308	R10010A00	Write-Data	Control setpoint: 10.00
15:09:04.073278	BD0010A00	Write-Ack 	Control setpoint: 10.00
15:09:04.084103	AD0010000	Write-Ack 	Control setpoint: 0.00

Considering that you don't own an OTGW yet, you may not know how to read that. The messages mean the following:

1. The thermostat sends a Control Setpoint request for 0°C to the OTGW.
2. The OTGW transforms that to a request for 10°C and sends that to the boiler.
3. The boiler responds that it accepted the request for 10°C.
4. The OTGW acknowledges the 0°C request to the thermostat.

So the thermostat doesn't see the modified control setpoint. It will see the fact that the boiler switches on (via bits in MsgID 0) and the resulting increase in water temperature. So, if your thermostat has an indication when the boiler is on, that will still light up. But that shouldn't affect the thermostat's control algorithm. What many people don't realize is that the thermostat requests a desired water temperature from the boiler, not a desired room temperature. So, even if the thermostat would be aware of the raised control setpoint, that doesn't translate back to a room setpoint.

Your idea definitely makes sense. The only difficulty will be to determine what control setpoint to request depending on the current temperatures and TRV settings upstairs. The proper way would be to implement some temperature control algorithm. This is usually done with a PID controller. But you could just request 50°C or so until the TRVs close. A problem may be that battery operated devices such as the TRVs don't always report changes immediately. So you may also have to monitor the return water temperature (if your boiler provides that). If all valves or servos are closed, the temperature difference between the water leaving the boiler and when it comes back will be small. Then it's time to switch off the boiler. Decent boilers will already implement this mechanism as well.

[Dipapi]

Hi Schelte,

Thanks a lot for your reply and clear feedback. I'll get a gateway and start playing around first with a static setpoint and then try to implement a decent controller. Luckily my boiler indeed modulates based on its return temp, so that'll already help. Indeed, it seems that my TRVs report the temperature with quite a delay, perhaps I'll get some better external temp sensors in the room such I can use that for the feedback to the controller. Sounds like I'm up for a nice project !

Thanks again!