as usual, Rich is head on nail! 
one thing you to keep in mind, if you want to persist only with a threshold is how to avoid not persisting anything at all. Letās just assume, that your temperature is inertial and just stays under the threshold, but over the day, this cold then add up to a significant change, so you have to add that behaviour to your rule also. Since you donāt want to use persistence, you canāt use the cool āaverageSinceā with an item, so you must find other persistences to check, if an item changed since more than one interval.
I would go with a ācompactingā function within the persistence - or just delete old values, if you wonāt need them. I for myself also persist all my sensors (>150, if I count them correctly) within MySQL in my case. As my MySQL grew also big, I just added a stored procedure as described here: "Garbage Collection" on MySQL persistence: deleting old Item states. If i had some more time (and ran out of space with my DB), Iād like to come up either with a migration to InfluxDB or some stored procedure which would compact my data in junks of some hourlong intervals or something - I donāt knowā¦ But for the time being, I have around 500 items in my MySQL-Database running for approx. a year now and the size is 216,12 MB right now.