An air purifier is a fan with a filter and, these days, an app that would like to know which continent you live on. Ours disagreed with our answer twice before it accepted the device at all, after which the device was politely told to speak to the house directly, on the local network, and the app was left to think whatever apps think. Everything below happens without a cloud, which in this category is the feature.
A purifier is, in other words, a fan with opinions. The job was to give it better ones than "Auto", which on this model means "run at a speed you can hear, whenever the sensor has a feeling about it."
Off until there is a reason
The regime is simple: the purifier is off unless something has happened. Three things count. The indoor smoke detector raises its flag. Cooking is detected. Or the room's own particulate sensor sits above 30 for five minutes, which is the number at which the air in a small flat has stopped being a rumour. Any of those turns it on, always at speed two, because "start in the middle and watch" is the whole design.
Regulate on the trend, floor on the level
Every five minutes, while it runs, the house compares the room's particulate reading with the reading from the previous check. Up by three or more: go to speed two. Down by three or more: drop to one. In between: leave it alone. Speed three exists, and it is a speed you can hear from the next room, so it is reserved for two situations: the air is genuinely dangerous, or the room is empty and the air is bad, in which case nobody is there to mind and the fan may roar. A presence trigger drops it back to two the moment somebody walks in, rather than five minutes later.
Trend alone had a hole, and the hole was found the way holes are: by asking why the fan was running at two in a room with pristine air. Because it had been switched on into pristine air, and pristine air, having nowhere to fall to, produces no downward trend. So there is a floor: below the "clean" threshold the fan goes to one regardless of what the trend says, and twenty minutes of clean air with no smoke and no cooking switches it off entirely. That last check runs on a timer, not on an edge, for the reasons on this page.
Three small bugs, exported for your convenience
The first check. The "previous reading" helper starts life at zero. On its first run the regulator compared a reading of 4 with a memory of 0, concluded the air had worsened by 4, and went to speed three in a clean room, loudly. The fix is one clause: no previous reading means no trend. Every regulator that remembers something must first decide what it believes before it has remembered anything.
The renamed device. Renaming the purifier's entities froze its readings for two hours while the device stayed perfectly reachable. Frozen numbers are the most dangerous kind, since they look like calm. There is now a watchdog: four readings unchanged for an hour while the device answers is not calm, it is a stuck integration, and the integration is reloaded and the log says so. (The traps page has cousins of this one.)
The beeper. The device beeps on every command. An automation that changes speed four times an evening therefore beeps four times an evening, at people trying to watch a film. The beeper is off. A device that announces every automation is a device that gets unplugged, and then it announces nothing.
The one it got wrong, and why we left it
One afternoon a pot of something steamed for twenty minutes: humidity in the kitchen up ten points, volatile compounds tripled, particulates flat as a pond. The cooking detector, which was taught after an earlier mistake to distrust volatiles without particulates, declined to call it cooking. The smoke detector, which watches a different compound, called it a smoke session instead. The purifier ran either way, which is the point: it does not care who made the cloud. The detector's caution stays; the alternative is a purifier that starts whenever somebody opens a bottle of cleaner.
The ioniser that was never off
One of us asked why the ioniser had not been on. Its switch had read off for most of the week, which looked like a decision. It was not one. The ioniser and the ultraviolet lamp are settings of the machine, not of the house: they are on whenever the fan is, and they report off whenever the whole appliance is off, which, thanks to the rules above, is about eighty per cent of the time. Nothing in the house has ever touched either switch. We flipped the ioniser off and on with the fan running, to be sure it was a real switch and not a reflection of the power state. It is. The fan did not blink.
The question underneath was whether to run the machine at its lowest speed round the clock, for the ions. We declined. An ioniser does not remove particles; it charges them so they settle on the nearest shelf, which is a redistribution service rather than a cleaning one, with a whisper of ozone thrown in. The filter does the work. Running the fan all day would spend the filter five times faster in order to dust the furniture more evenly.
The purifier now runs less, more quietly, and for reasons it could explain if asked. Nobody asks. That is what a good appliance sounds like.