You can teach an old Watchdog new tricks

My home unusually uses HomeKit smart automation for central heating control among other things. One feature that I’ve not seen documented elsewhere is use of a watchdog to improve robustness of the automations. Many people of course will use HomeKit as a fancy remote control, but in my case HomeKit automations have an important role in heating control linking heat demand from rooms to enabling the boiler to provide heat. It’s thus important to me that this link works reliably. However in my experience sometimes changes in state can be missed leaving the boiler not running when it should be, or running when it shouldn’t be, an error which could last for hours.

Some two-and-a-half years ago I created a means to improve the robustness of such automations. My watchdog is a HomeKit smart plug which cycles on and off periodically. Two timers alternately turn the plug or or off every few minutes. The change of state of the watchdog is used as a second trigger for the rules in the automations causing the rules to be reevaluated every few minutes.

To illustrate what this achieves let’s imagine that the HomeKit ecosystem misses one trigger in ten or 10% of triggers. That would mean that one night in ten the boiler would fail to turn off when the last radiator valve closed, and would instead run all night. With the watchdog concept the rules are re-evaluated every few minutes, not just at the moment a valve closes. Thus, within a few minutes the rules are evaluated again and then ninety percent of the missed ten percent of events corrected – the error rate is now down to one percent from ten percent. A few minutes later the rules are evaluated a third time and ninety percent of the remaining one percent of errors corrected – the error rate is now a tenth of one percent or once in every thousand days. The risk of a continuing error state thus becomes vanishing small in minutes.

Previously the period of the cycle was five minutes i.e. the timer repeating an on/off cycle every five minutes. Five minutes was chosen as that’s the minimum cycle time available in the Eve app that I use to write rules. Today I realised that I could improve this significantly.

New HomeKit timers

The illustration above shows the new solution. Here I created 3 on and 3 off rules which each repeat every six minutes, which causes the state of the watchdog to change every minute..

  1. watchdog off (off rule #1)
  2. watchdog on (on rule #1)
  3. watchdog off (off rule #2)
  4. watchdog on (on rule #2)
  5. watchdog off (off rule #3)
  6. watchdog on (on rule #3)
  7. watchdog off (off rule #1).. and repeat indefinitely.

The illustration below shows a typical rule which turns off the watchdog and repeats every 6 minutes.

Example rule

The net result is that my watchdog smart plug now turns on every even minute and off every odd minute which I think provides the minimum possible delay before the system responds after any missed change of state.

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