I've been luring the Dyson hot and cool air purifiers, it's pretty silent from what I've heard, it also serves as both an electrical heater during the winter and a cooler during the summer. The price tag is the reason I've never bought one...
Only criticism is that the U300 does not have wifi connection - only IR control... I am hacking up a esp8266 + some IR LEDs and photoresistors to monitor and control it via MQTT though which is fun. The "wifi connected" units seem to add several hundred GBP to the price for the same filtering performance.
We're not sure if we're noticing any actual improvements in air quality, or just imagining it...! :-) The placebo effect is quite good if nothing else. Noses feel less congested, and the air in the room feels less stuffy in the morning.
1 - https://winixeurope.eu/air_purifiers/winix-u300/ No personal connection to these guys, just a satisfied customer (so far after a few months)
I have no way to really measure its effectiveness, but it very quickly removes any cooking smells and has fixed my year-round allergies while indoors.
Where does the heat go?
I have one. It works pretty well. The air filter isn't as good as the massive Austin Air I use for my living room, but it's okay for a smaller room... not to mention much quieter, smaller, and easier to integrate with home automation.