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C24 DIAGRAM AND B34 DIAGRAM
Electromechanical or Electronic Thermostat
Connected to Residential units (GCS16R and GCS16H)
without Economizer Harness or Unit Terminal Strip
A-RESIDENTIAL THERMOSTAT SYSTEMS
This flowchart is used to show the step by step sequence that takes place when thermostat
demand is sent to the GCS16. The sequence describes the actions of devices in the unit which
control blowers, fans, gas valves and other components in the system. The sequence is out
lined by numbered steps which correspond to circled numbers on the adjacent diagram.
1- C24 SECTION and B34 SECTION (electromechanical or electronic thermostat wired
to residential unit with pigtails)
The following is an explanation of Lennox' model number designations:
GCS16R: Residential unit without crankcase heater, high pressure switch, loss of
charge switch and low voltage terminal strip.
GCS16H: Same as GCS16R except in horizontal only (non-convertible) cabinet.
GCS16: Commercial unit with crankcase heater, high pressure switch, loss of charge
switch and low voltage terminal strip.
GCS16R and GCS16H units are designed for residential use only and are not equipped with
the necessary hardware for connecting optional control systems or economizer. The C24 ther
mostat section is a straightforward residential thermostat design for the GCS16R and
GCS16H exclusively. The thermostat used may be electromechanical or electronic.
511 and 651 units are factory equipped with hard start components (start capacitor and poten
tial relay). 261, 311 and 411 units are not. All units except for 411s use separate compressor
and fan run capacitors. 411 units use a single dual" capacitor for both the compressor and
condenser fan motors.
NOTE - This is a basic operation sequence for a single phase GCS16. The sequence shows
a single phase GCS16R or GCS16H connected to a C24" thermostat control section.
Operation Sequence
Cooling:
1- Line voltage energizes transformer T1. Transformer T1 provides 24VAC power to all unit con
trols and thermostat.
2- Cooling demand energizes Y1 and G in the thermostat. Y1 energizes compressor contactor
K1. G energizes relay K3.
3- K1 is a single pole contactor, allowing compressor terminal R and one leg of the condenser fan
to be powered at all times. K1-1 closes to energize compressor start circuit, compressor ter
minal C and condenser fan. Condenser fan immediately begins operating.
4- K3 contacts 7-4 close to energize the indoor blower on cooling speed. Contacts 6-9 close to
energize the economizer.
5- As the compressor gains speed, compressor terminal S is powered by start capacitor C7.
When the compressor nears full speed, potential relay K31 is energized and the start capaci
tor is taken out of the circuit. K31 remains energized during compressor operation. Run ca
pacitor C5 remains in the circuit between terminals R and S to optimize motor efficiency.
Heating:
6- Heating demand initiates at W1 in the thermostat and energizes relay K13. Heating demand
also passes through high temperature limit S10 to combustion air prove switch S18.
7- Relay K13 terminals 4-7 close to energize combustion air blower B6. Terminals 6-9 close to
energize the economizer (commercial units only). When the combustion air blower nears full
speed, prove switch S18 closes. Heating demand continues through S18 and through high
temperature limit S47 to energize ignition control terminal 1.
8- Ignition control A3 then waits 30 to 40 seconds to allow combustion air blower B6 time to draw
exhaust gas from combustion chamber and to introduce fresh air. Combustion air blower B6
operates throughout the heating cycle.
9- After the ignition control delay, A3 activates gas valve GV1, time delay K25 and the spark elec
trode. When flame is sensed by the flame sensor (minimum 5 microamps) the spark electrode
stops. If flame is not sensed after the first trial for ignition, controller A3 repeats steps 8 through
9 up to two more times (depending on controller make) before locking out. Delay relay K25
delays 60 seconds before closing.
If the control locks out, it can be reset by breaking and remaking thermostat demand.
10- After the 60 second delay, relay K25 closes to energize the indoor blower on heating speed.