The Selectra SERIES 20/30 electronic gas flame modulation systems are designed primarily for commercial and light industrial space heating, as components of indirect fired units with atmospheric burners. All fuel gases are compatible.The SERIES 20 is designed for single furnace operation, and the SERIES 30 is capable of handling up to four furnaces. They may be field installed on existing equipment or specified for new equipment installation.The systems utilize Modulator-Regulator valves. Amplifiers are available for high-fire ignition with integral or remote temperature selection.A discharge air temperature sensor is mounted within a mixing tube housing.Optionally, a room override thermostat provides space temperature control by raising the discharge air temperature to a pre-selected point - when used in conjunction with the remote temperature selector. Also optional, an inlet air sensor (and mixing tube) provides inverse change in discharge air for each degree change in inlet air - when installed in a convenient duct location upstream of the burner.