After living with the system for a while, you might observe that as the fire in the wood boiler goes out and the wood boiler cools down, it's not a simple on/off process. The temperature may fluctuate a bit, and it takes some time before it's certain that the wood boiler is really and finally out.
What we'd like to do is keep the backup heat source disabled until the wood boiler has been below its minimum temperature for some period of time - perhaps half an hour. This is a good example of a situation where a timer rule would be useful.
Timer rules use two variables. One contains the desired time interval in seconds. The other is the timer itself. It will count down to zero. We'll call them 'Wood Boiler Delay' and Wood Boiler Timer'.
For control purposes, any non-zero value of a timer counts as 'TRUE'. When the timer reaches zero, it is 'FALSE'.
When a timer rule is triggered, it sets its associated timer variable to the desired interval. Timers can be triggered whenever a monitored element has is either true or false. or when a transition happens from true to false or vice versa.
In this example, we want the timer to be set whenever the boiler is hot. That means it will be reset continually until the boiler finally becomes 'not hot'. At that point it will start to count down. Once the specified interval has passed, the timer will reach zero and become 'false'.
If at any time during that interval the boiler becomes hot again, the timer will reset and stay reset until the boiler is no longer hot.
Here's the syntax for our timer rule:
Set Wood Boiler Timer to Wood Boiler Delay when Wood Boiler Hot is true
Now, we need to change our rule for backup disable. We want backup disable to be true any time that the wood boiler timer is true. This is a simple change:
Set Backup Disable to TRUE if Wood Boiler Timer is true
Now the backup heat source will be disabled until the wood boiler has been below its minimum temperature for at least 30 minutes (1800 seconds). Of course, this interval can be changed at any time from the web control panel, using the 'Data Elements' tab.