State SBL85 390NE A - Manual (Page 33)

31 Servicing should only be performed by a Qualified Service Technician
VARIABLE FREQUENCY DRIVE – ALL MODELS
The SBL95 199NE, SBL85 275NE(A) and SBL85 390NE(A) models have a variable frequency drive (VFD)
that controls the speed of the blower motor. The VFD receives a signal from the thermostat and ignition
control that instructs the VFD to transmit the proper frequency to the blower to produce the proper blower
speed and proper input rate.
Two conditions must be met for the variable frequency drive to start the blower at low speed:
1. 100 - 120VAC must be supplied to the VFD input.
2. The thermostat closes a contact through the low speed I/O module – VFD terminals LI1 and +24V.