These air temperature sensors maintain precise, stable gas-fired temperatures. Selectra's unique electronic Modulator
or Modulator-Regulator valves control gas flow with instantaneous response and continual adjustment. They are the superior alternative to mod motors and butterfly valves.
Selectra's Series 21/31 systems are designed primarily for make-up air heating, as components of indirect fired units with atmospheric burners. Series 21 is designed for single furnace operation while Series 31 is capable of handling up to four
furnaces.