Tekmar 421 Mixing Reset Module One tN4, Mixing The Mixing Reset Module 421 is designed to operate all of the mechanical equipment in a hydronic heating system, except for zones, coordinating their operation through network communication. It can be used in applications ranging from a single zone of mixed radiant with an on/off boiler to multiple radiant floor zones with an on/off boiler. This control regulates one mixed heating water temperature through Outdoor Temperature Reset and Indoor Temperature Feedback. It is capable of controlling a single on/off boiler, floating action or variable speed injection mixing, and can expand up to 24 zones in total.