The iO-Twin-MM is designed to be used with the iO-Twin, universal twinning kit. It monitors both the control voltage that turns on the fan and the current draw of both fan motors. If the 24 volt control voltage is present but there is no current draw at both motors after six seconds, the unit will open a set of dry contacts and shut off both blowers.  iO HVAC Controls iO-Twin-MM Motor Monitor to be Used with iO-Twin