Home Automation Domotica Forum Europe, Bwired Forum

Hello,

I'm trying to build my own thermostat. Now, my first plan was to make it a on/off thermostat, make some software to calculate when to stop heating and start heating to make the temperature as constant as possible.

But, i was thinking, is it possible to use the opentherm Gateway with this? I mean, my thermostat keeps sending the current temperature to the boiler and the set temperature, so that the boiler can manage the rest on itself? Or doesn't it work that easy?

No, unfortunately(?) it doesn't work that way. According to the opentherm specs the boiler is controlled via the control setpoint, which is the desired temperature for the water coming out of the boiler. A thermostat may report the room temperature and room setpoint, but nearly all boilers ignore these values. They are more for the benefit for other equipment, like an OTGW. For that reason some thermostats don't even send these values.

So, to make your own OT thermostat, you have to determine a control setpoint to send to the boiler. You may also use the Max relative modulation level for more accurate control. And for a complete story you should also be aware of the CH enable bit in the master status byte that's like a master switch. It prevents or allows the boiler to heat the water in the central heating system at all. The OTGW will automatically set this bit when you provide a control setpoint via the CS command (unless you override it with a CH=0 command).