Automatic bathroom exhaust fan rule in JRuby

With the latest updates on JRuby OpenHAB Rules System I’ve simplified my bathroom exhaust fan to this:

rule 'Humidity: Control ExhaustFan' do
  updated MasterBathRoom_Humidity, MainBathRoom_Humidity
  run do |event|
    humidity = event.item
    exhaust_fan = items[ '_Humidity', '_ExhaustFan']
    evo_rate = humidity.evolution_rate 4.minutes, :influxdb"#{humidity} #{event.state} evolution_rate: #{evo_rate}")
    if event.state > 70 && evo_rate > 15
    elsif event.state < 70 || evo_rate < -5


  • It is based on the rate of change in humidity levels. When it increased more than 15% (or any setting of your choice) in the past 4 minutes, it would turn on the exhaust fan.
  • Humidity levels are stored in influxdb persistence service on every change. This makes for easy retrieval using evolution_rate or any other persistence functions.
  • Persistence calls are attached to each item, e.g. humidity.last_update, MainBathRoom_Humidity.average_since, etc.
  • Simplified duration - instead of calling the cumbersome, simply specify 4.minutes
  • Number items can be directly compared against numeric e.g. MainBathRoom_Humidity > 70 instead of MainBathRoom_Humidity.state.intValue etc.
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