Hi, I'm not sure if the NEST thermostat development team use this forum for input but I would very much like to make the following product enhancement request.
The NEST currently allows for the setting of an upper limit for the Opentherm Control setpoint. I would like to request that a similar option be added to allow for a lower limit / minimum to also be set.
The reason for this is to allow control to minimise short cycling where a boiler is not able to modulate its power output down sufficiently to maintain the low setpoint temperature request from NEST thermostat.
With this control the boiler would be able to continue to provide output at its minimum level, until it got to a point the NEST would learn when the boiler should be told not to fire at all and the radiator residual heat will be sufficient for it to reach the desired room temperature.
With properties becoming better insulated it is going to be a case that boilers (furnaces) are not able to modulate down low enough to provide a continuous output that satisfies the heat low requirements. If as the case the NEST tells the boiler to output at 36 degrees C, but the lowest output of the boiler results in a flow temperature of 45 degrees C, the boiler will just fire for a few seconds and then cut out because it has exceeded the setpoint. This short cycling will continue for maybe 30 mins until the residual heat in the radiators is sufficient to reach the room setpoint. It would be better if the boiler just carried on firing at a flow of say 45 degrees C and then shut of properly in say 5 minutes times the room temperature will have been reached that bit earlier.
I hope that makes sense and really would be most grateful as I know many others would too.
Here is the ID from the Opentherm spec that is involved.
ID | Msg | Name | Type | Range | Description |
1 | -W | Control setpoint | F8.8 | 0..100 | Degrees C (see notes below |
Note : The master decides the actual range over which the control setpoint is defined. The default range is assumed to be 0 to 100.
There is only one control value defined - data-id=01, the control setpoint. The control setpoint ranges between a minimum of 0 and maximum of 100. It represents directly a temperature setpoint for the supply from the boiler. The slave does not need to know how the master has calculated the control setpoint, e.g. whether it used room control or OTC, it only needs to control to the value. Likewise, the master does not need to know how the slave is controlling the supply.
