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Multi-layer Heating Application
Posted: Fri Jan 07, 2022 8:34 pm
by AndrewFG
Background: My central heating system is a multi-layer application with a Viessmann Vitodens boiler supplying hot water to two sub-systems, namely a) a Heatmiser under floor heating control system with five room thermostats controlling the UFH heating coils downstairs, and b) a Tado radiator heating control system also with five room thermostats controlling the radiators upstairs. So I have in total ten room thermostats, where the function is that if any room thermostat is asking for heat, then the respective upstairs/downstairs sub-system controller opens a zone-valve to supply hot water to the respective part of the house. And those two zone-valves have an end switch which is wired in parallel to the "heat enable" contact input of the boiler. The boiler has its main hot water supply temperature setpoint is fixed manually via the console.
Question: The boiler has an OpenTherm input which is not currently used, but I am thinking of adding an OpenTherm gateway to it to a) monitor the boiler's actual state and temperatures, and b) possibly adjust the main hot water supply temperature setpoint. My question is if it is possible to use the OpenTherm gateway for this purpose, without any OpenTherm thermostat, and still using the "heat enable" contact input of the boiler as described above?
Re: Multi-layer Heating Application
Posted: Fri Jan 07, 2022 9:27 pm
by hvxl
Someone recently asked a
very similar question. Unfortunately he didn't report back any results of his experiments. As I indicated in that topic, I don't have high expectations that it will work.
Re: Multi-layer Heating Application
Posted: Fri Jan 07, 2022 9:42 pm
by AndrewFG
^
Thanks for the link to the other thread. I had in fact already read it before making my post, and I did not really understand the discussion, which is why I posted my own thread
> don't have high expectations that it will work
Can you please be a bit more explicit about where you see the potential / actual problem?
Re: Multi-layer Heating Application
Posted: Sat Jan 08, 2022 3:56 pm
by hvxl
That's the thing with expectations, they are not based on evidence. But if I were making the boiler firmware, I would ignore the on/off when OT messages are being received. Otherwise it is unclear what to do when the OT interface says "don't heat" and the On/Off interface says "heat". But hopefully for you I am wrong. Unfortunately I cannot test it for you, because my boiler uses the same terminals for both types of thermostats.
Re: Multi-layer Heating Application
Posted: Sun Jan 30, 2022 8:18 pm
by AndrewFG
I just want to report back that I was completely successful in getting it to work.
My OpenTherm Gateway is connected to my OpenHAB system, and the solution was actually quite simple, as follows..
- 1. No (master) thermostat connected to the OTGW.
2. Boiler (slave) connected to the OTGW.
3. Monitor the boiler "heat enable" input (in my case by a Fibaro ZWave device).
4. In OpenHAB, when the "heat enable" status changes, run the following rule..
Code: Select all
rule "41: Boiler OpenTherm Controls"
when
Item Heat_Enable changed or
Item OpenTherm_Gateway_Offline_Alarm changed to OFF or
System started or
Time cron "10 * * ? * * *"
then
val enable = (Heat_Enable.state == ON)
val actual = (OTGW_CH_Enable_Override.state == ON)
if (actual != enable) {
if (enable) {
// NB: Enable_Override must be ON before the Setpoint_Override value is sent!!
OTGW_CH_Enable_Override.sendCommand(ON)
OTGW_CH_Setpoint_Override.sendCommand(65)
} else {
OTGW_CH_Enable_Override.sendCommand(OFF)
OTGW_CH_Setpoint_Override.sendCommand(0)
}
}
end