Bard 2100-066 Rev. A - Manual (Page 7)

heat exchanger. When this temperature reaches a predetermined
point, the bi-metal closes the normally open contacts thus making a
complete circuit, starting the blower motor.
Internal view of L4064, showing use of screwdriver to connect
to disconnect wires at push-in terminals, NOTE: Because the
dial turns when the eiement temperature changes, the lettering
on the dial may not be horizontal
There are basically two types of fan controls. One is called a
combination control. See illustration. The fan control portion is
provided with two adjustable levers—one to set the blower
“on” temperature and the other to set the blower “off” temperature.
Fan contro! switch*
upfiow, highboy furnaces.
The settings of the fan control are normally field adjusted according
to the desires of the homeowner and the “on” or “make” point is
usually somewhere around 100“. This means that when the air
passing over the heat exchanger reaches approximately 100“, the
switch will “make” and the fan will come on, delivering air to
the space.
The “break” or “off” point on the switch can be field set and
should usually be set about 25“ below the make point. If the switch
is set well below return air or room temperature during the normal
heating season, continuous blower operation would be
accomplished.
The difference between the fan "on” and “off” points is called the
fan control differential. The temperature difference between these
two points may be set from 15 to 25“. If the differential is set too
low, the control will cycle because of slight temperature
fluctuations. This causes undesirable fan cycling. If set at more
than 25“, the blower would have to wait too long before coming
“on” increasing the heat loss fi'om the furnace and reducing the
furnace efficiency.
The other type is the single fan control. It has just one adjustable
lever by which the desired blower “off” temperature is set. This is
a fixed differential control with a built-in differential of 25“ F,
Therefore, the blower “on” temperature will always be 25“ F higher
than the blower “off” dial setting.
Note that if the homeowner desires to have continuous air
circulation (CAC), it will be necessary to set the open or
break point below the normal return air temperature. In this
way, the air passing over the heat exchanger will remain above the
setting even though the burners are off and the blower will continue
to run.
Combination fan and limit control
Fan (on)
25^ Differential setting
Downflow and Horizontal Units
Downflow and horizontal units present a little different problem
than the upflow units. The fan control probe location in a downflow
imit will work fine on the “make” or “on” cycle, because the fan
would not be running and the heat exchanger would rise through
the heat exchanger actuating the probe. However, once the fan is