IntroductionSeveral people have inquired about using the gateway without a thermostat. I personally prefer to use a thermostat for a couple of reasons:
Gateway operationWhen nothing is connected to the thermostat terminals of the gateway (X1), the gateway will send a fixed series of opentherm messages to the boiler. The sequence is:
The data bytes the gateway puts in the messages by default will not normally cause the boiler to start heating. To actually heat the house with this configuration, the gateway has to be instructed what to send to the boiler via serial commands.
External controlAn external program that is used to control the central heating system via the gateway can use the following serial commands:
On/off thermostatAs indicated, the gateway by default will instruct the boiler not to do any heating when the thermostat terminals are left open. However, when the terminals are tied together, the gateway sends its internal maximum central heating setpoint value to the boiler and sets the CHenable bit in MsgID 0. This allows the use of a simple on/off thermostat in combination with the gateway and an opentherm boiler.
The internal maximum central heating setpoint value starts out at 90 degrees. If the boiler has specified different boundaries via MsgID 49, the provided upper boundary will be used. The MM serial command can be used to prevent the boiler from always burning at maximum capacity when the thermostat indicates a heat demand.
ConsiderationsMaking a program to control the central heating may appear simple at first glance. But simply telling the boiler to start heating when the room temperature drops below the desired temperature and shutting it off when it reaches the setpoint again won't work very satisfoctory. That method will result in large temperature swings, because a central heating system inherently has a big response delay.
So the controlling program has to account for the time and amount the temperature will continue to rise or fall after the boiler has been switched off or on. To make things worse, those figures are influenced by external factors like outside temperature, open or closed doors and windows, curtains being drawn, other heat sources, hot water use, etc.
In addition to keeping the temperature as constant as possible there's also the matter of doing it in the most energy efficient way. Modern boilers are generally most efficient at low temperatures. So, when maintaining a desired room temperature it may be wasteful to use relative short burns with a high control setpoint rather than longer burns with a lower control setpoint.
Finally there's the matter of reliability. If the controlling program crashes, or the system running the program decides to reboot and install patches after the control setpoint has been set to 80 degrees, the house may get quite warm.