Goodman [A/G]PG, Service Instructions Manual

Page: 18 / 52

18
SYSTEM OPERATION
HEATING CYCLE
When the contacts of the room thermostat close, the termi-
nals R to W on the control board are energized.
The heating cycle is accomplished by using a unique tubular
design heat exchanger which provides efficient gas heating on
either natural gas or propane gas fuels. The heat exchangers
compact tubular construction provides excellent heat transfer
for maximum operating efficiency.
Inshot type gas burners with integral cross lighters are used
eliminating the need for adjustable air shutters. The same
burner is designed for use on either natural or propane gas
fuels.
The induced draft blower draws fuel and combustion air into
the burners and heat exchanger for proper combustion. A pres-
sure switch is used in conjunction with the I. D. blower to de-
tect a blocked flue condition.
Blower operation is controlled by the ignition control module.
The module allows for field adjustment of the blower delay at
the end of the heating cycle. The range of adjustment is for
120, 135, or 150 seconds. The factory delay setting is 30 sec-
onds delay on 150 seconds delay off.
Direct Spark Ignition (DSI) Systems
APG/GPG units are equipped with a direct spark ignition sys-
tem. Ignition is provided by 22,000 volt electronic spark. A
flame sensor then monitors for the presence of flame and closes
the gas valve if flame is lost.
The system may be controlled by most good heating and cool-
ing thermostats with an adjustable heat anticipator.
NOTE: Some night setback thermostats that do not have a
common terminal, use a power robbing circuit in the off cycle
to maintain the batteries. This type of thermostat may interfere
with the operation of the ignition control module and should not
be used.
HEATING SEQUENCE
In order to illustrate the heating sequence, the following has
been simplified to give a better understanding of the pressure
switch operation.
The figure below is a view of the induced draft blower showing
the location of the pressure tap. The induced draft blower is
mounted on the collector box, where the combustion gases
from the heat exchanger are drawn through the collector box.
PRESSURE TAP
COOLING
The refrigerant used in the system is R-22. It is clear, color-
less, non-toxic, non-irritating, and non-explosive liquid. The
chemical formula is CHCIF
2
. The boiling point, at atmospheric
pressure is -41.4°F.
A few of the important principles that make the refrigeration
cycle possible are: heat always flows from a warmer to a cooler
body, under lower pressure a refrigerant will absorb heat and
vaporize at a low temperature, the vapors may be drawn off and
condensed at a higher pressure and temperature to be used
again.
The indoor evaporator coil functions to cool and dehumidify the
air conditioned spaces through the evaporative process taking
place within the coil tubes.
NOTE: Actual temperatures and pressures are to be obtained
from the expanded ratings in the Technical Information Manual.
High temperature, high pressure vapor leaves the compressor
through the discharge line and enters the condenser coil. Air
drawn through the condenser coil by the condenser fan causes
the refrigerant to condense into a liquid by removing heat from
the refrigerant. As the refrigerant is cooled below its condens-
ing temperature it becomes subcooled.
The subcooled high pressure liquid refrigerant now leaves the
condenser coil via the liquid line until it reaches the indoor
expansion device.
As the refrigerant passes through the expansion device and
into the evaporator coil a pressure drop is experienced causing
the refrigerant to become a low pressure liquid. Low pressure
saturated refrigerant enters the evaporator coil where heat is
absorbed from the warm air drawn across the coil by the evapo-
rator blower. As the refrigerant passes through the last tubes
of the evaporator coil it becomes superheated, that is, it ab-
sorbs more heat than is necessary for the refrigerant to vapor-
ize. Maintaining proper superheat assures that liquid refriger-
ant is not returning to the compressor which can lead to early
compressor failure.
Low pressure superheated vapor leaves the evaporator coil and
returns through the suction line to the compressor where the
cycle begins again.
COOLING CYCLE
All Models
When the contacts of the room thermostat close, the termi-
nals R to Y and G on the control board are energized.
The control board recognizes this as a demand for cooling and
energizes the compressor contactor and indoor blower motor.
The blower delay is an integral part of the control board.
When the thermostat is satisfied, it opens its contacts, break-
ing the low voltage circuit, causing the compressor contactor
to open and indoor fan to stop after a 60 second delay.
If the room thermostat fan selector switch should be set to the
"on" position then the indoor blower would run continuous rather
than cycling with the compressor.