Bard 2100-066 Rev. A - Manual (Page 9)

A third reason is that the upper limit control de-energizes the
control circuit. Under some circumstances, this secondary limit
could be open up to one hour preventing the control circuit from
operating the fan timer or burner circuits. For the thermostat to do
its job and get the furnace back in operation, power must be
provided to the control circuit.
Additive Cooling Relay
NOTE: When cooling is added to a standard furnace, an
additive cooling relay is required.
The line voltage side of this relay has a set of contacts which
bypass the fan control, delivering power directly to the blower
motor. The contacts are pulled in by a low voltage coil which
is energized on a call for cooling from the thermostat or if the
thermostat fan switch is set for constant blower operation. Note
that electrically both the 24 V and 120 V circuits are completely
independent, but one controls the other by pulling in its contacts.
The blower relay is necessary because on cooling, the temperature
of the air being circulated through the system would be below the
set point of the fan control and this would shut off and not allow
the blower to run. Under cooling without the blower running, the
evaporator coil would ice up and the condensing unit would go off
on its limit control.
Additive
coDling relay
24 Volt or Low Voltage Control Circuit
In almost all residential forced warm air heating systems, the
control circuits are powered with low voltage (24 volts). There
are several reasons for using low voltage: ease of installation, lower
installation cost, closer (better) temperature control and less
possibility of electrical shocks.
Transformer
In order to supply 24 volt power in control circuits, a “step-down”
transformer is used. Tlie line side is wired directly into the 115V
power supply.
Basically a transformer consists of two coils of insulated wire
wound on a common iron core. The coil connected to the line
voltage or input side is the primary coil and the output or load side
is always the secondary. If the voltage on the line side is greater
than that on the load side, it is a “stepdown” transformer. If the
voltage on the load side is greater than the line side, it is a “setup”
transformer. Therefore, to supply 24V on the load side from 115V
line, a stepdown transformer is used.
Transformers are always rated in volt amperes (VA), which is the
amount of electrical power (volts x amps) it can supply.
The transformer must be sized to handle the current (amperage)
requirement of the load or loads connected to the secondary side of
the transformer. The transformer select may be larger than the load
requirement, but the transformer can never be smaller than the load
requirement (amp-draw). Short circuit protection for the 24V side
of the transformer is provided in some cases by a replaceable fuse
and in others as fusible link. The fusible link is built in and
requires replacement of the entire transformer if it bums out.
Thermostat
The first load that will be attached to the low voltage circuit is the
thermostat. Basic operation of the thermostat and its internal
wiring has been discussed in a previous manual, and here the
common leg will be connected to one side of the transformer and
the fan and heating circuits will be connected to the other side of
the transformer. The thermostat can now control the fan circuit
independently from the heating and cooling circuits.
Basic Maintenance Procedures
Regularly scheduled basic maintenance calls are mutually
beneficial to both the service firm and the homeowner/customer.
Normally, this involves two maintenance calls per year—one in the
fall prior to the heating season and one in the spring prior to the
cooling season.
Maintenance Check Sheet
A maintenance check sheet has been developed which has
checklists for residential gas, oil and electric heating units and
cooling systems and accessories. The check sheet had checklists