Heat recovery extractor fans are generally continuously running with a ‘trickle’ function which can be boosted when needed using the trigger (normally a pull cord like classic extractor fans). Thanks to it’s specially designed heat exchanger included in the unit, incoming fresh air is heated by the outgoing hot air at a 75% efficient rate. This can also be switched off during summer months, and the fans can be run only on the boost (without the trickle function).