Bard 2100-066 Rev. A - Manual (Page 8)

running, the air is flowing in the opposite direction from the probe
and when the burners would go off, the probe would sense the cool
return air rather than the heat in the heat exchanger. This would
fool the probe into thinking that the heat exchanger had cooled
down and could actuate the contacts to their normal position,
cycling the fan off too soon. Once off, it would sense the residual
heat in tlie heat exchanger being transferred to the air around it and
again, actuate the contacts energizing the blower and cycling it
until the heat exchanger has been cooled sufficiently,
The same type of problem would be true for a horizontal type
furnace, since it would not be possible to locate the blower
control in an ideal position for both the “make” and “break”
actions.
Limit Control
A safety device is connected into the line voltage circuit next. It is
called a limit control. The purpose for this “limit” is to turn off the
burner and control circuits if the air temperature becomes too high.
Some limit controls are combined with the fan control. See
photograph. In this case, one bimetal actuates both the fan
control switch and the limit control switch. As the temperature
rises, the bimetal will first turn on the fan switch and start the
blower. If a condition exists which causes overheating, the bimetal
will continue to warp or turn until the limit cutout temperature is
reached and it trips the limit switch.
The limit switch in the combination control is provided with an
adjustable lever. A step is installed which will not allow the lever
to be set above the safe cutout temperature—usually 200° F. The
lever should not be adjusted below this high limit temperature
since it will cause false or nuisance burner cycling.
The limit control prevents overheating of the furnace which could
cause a fire or damage to the fiimace components. It shuts off the
burner if conditions or failure should occur such as blower failure,
dirty air filter, blockage of the duct system or any condition which
abnormally restricts air flow through the fiimace.
The limit control is actuated by a bimetal element in the discharge
air stream. Therefore, it monitors the air temperature leaving the
heat exchanger. The bimetal is linked to a normally closed switch
which has an SPST action. If the air temperature rises to the limit
cutout temperature, the bimetal opens the switch and breaks the
line voltage circuit to the burner and controls.
Limit Control
Pictorial symbol
On most furnaces, the limit control is calibrated to shut off the
burner if the discharge air temperature reaches or exceeds 200° F.
The limit control will automatically recycle (reclose its contacts)
when the temperature drops 25° below the cutout point. Therefore,
the limit control will recycle the burner if the thermostat is calling
for heat, but will not allow the temperature to exceed 200° F.
The limit cutout temperature is factory set and the differential is
built into the switch. Neither should be readjusted or changed in
the field. Otherwise the equipment warranty will be voided and a
hazardous condition will be created.
Another common limit control is illustrated. This limit has its own
bimetal element and is located at a different point on the furnace
than the fan control. It acts the same as the combination limit but
does not have a dial setting. It is a fixed setting and fixed
differential control. Again the setting is usually 200° F and the
differential is 25° F. These are set and sealed by the control
manufacturer.
Upper Limit Control
As was outlined imder the discussion of the fan control, for
downflow and horizontal heating imits, an additional limit
control is used. This upper limit is installed in the blower
outlet or between the blower outlet and the heating section,
or very near the blower section of the imit.
The reason is that the high temperature limit is at the discharge end
of the heat exchanger. Before the blower comes on, the warm air
rises away from this high limit preventing it from sensing the
temperature and providing the protection needed, During this
period, the upper limit provides backup protection by sensing the
heated air that rises upward in the unit by gravity.
This upper limit control provides protection in case of blower
failure or excessive restriction in the system which prevents the
movement of air. Usually the limit cutout temperature is 140° F
with a differential of 25° F.
The switch in the upper limit control is an SPDT action. The
normally closed contacts are located in series ahead of the
high limit contacts in the electrical circuit. Therefore, if either
limit is “open” the burner and control circuits become deenergized.
The upper limit does one other thing. When the limit contacts are
open, the other normally open contacts then close. This completes a
circuit directly to the blower motor starting the blower.
There are several reasons for this feature. One is to put the upper
limit back in step if it should trip open from residual heat after the
fan control has turned the blower off. It simply overrides the fan
control to provide cool air on its own elements. When the limit
contacts reclose the blower contacts open, shutting off the blower.
A second reason is to back up the fan control timer if it should fail
to operate as it should.