Terminal Block Tb2; K3 Indoor Blower Relay; Fan Timing Control A28 - Lennox G21Q3-40 Unit Information

If a two speed condensing unit is used and Harmony or CCB1
is not used jumper DS to Y2. The blower will operate on the
low speed heat/cool tap during first stage cooling (low speed
compressor). During second stage cooling (high speed com
pressor), the blower will operate on the high speed cooling tap.
If a single speed condensing unit and no Harmony or CCB1 is
used, jumper DS to G. The blower will operate on the high
speed cooling tap during the cooling mode.
CAUTION
Never jumper Y2" to DS" if a CCB1 control is
used. Damage to the CCB1 control will occur.
IMPORTANT
Y2" must be jumpered to DS" in two−speed, non−
zoned application when CCB1 is not used.
4 − Terminal Block TB2 (Figure 6)
Line voltage is routed to the unit through a power supply termi
nal block (TB2) located inside the control box. The terminal
block is energized at all times.
In Q" units the accessory terminal (ACC) is energized any
time there is a blower demand. In V" units the accessory ter
minal is energized by the VSP control and is powered when
the blower is running.
The accessory terminal can be used for accessories such as
an electronic air cleaner.

TERMINAL BLOCK TB2

ACC
FIGURE 6
5 − K3 Indoor Blower Relay Q" Models Only
A double pole, double throw indoor blower relay is located in
side the control box to provide power to the blower. K3 relay
contacts also control the 120V accessory terminal located on
terminal strip TB2.
L1 N
6 − Watchguard Control Board (A18)
GC1 Control Only
The watchguard control board is illustrated in figure 7.
Watchguard serves as an automatic electronic ignition
reset device. Watchguard is a N.C. SPST self resetting
timer wired in series with W1 thermostat demand. It is
built into the ignition control on all units equipped with
GC 3 or Johnson ignition control. On GC 1 equipped
units it is externally mounted.
For GC 1 equipped units Watchguard is enabled with a
heating demand. After one hour (unit locked out or run
ning) of continuous thermostat demand, watchguard
opens for two minutes then closes remaking thermostat
demand W1. This resets electronic ignition control A3.
After this break, a thermostat de
mand for heat will allow the unit to
fire. If ignition is not successful
unit lockout occurs. If successful
the unit operates. Lockouts are
usually attributed to low gas line
pressure. For units equipped with
a GC 3 or Johnson ignition con
trol, the Watchguard is enabled only after the unit locks
out because of failed ignition attempts (five tries). The
watchguard will then break and remake thermostat de
mand after one hour.
7 − Fan Timing Control A28 Q"Units Only
(Figure 8)
A fan timing control
(A28) located in the con
trol box is used in Q"
models to regulate fan
on and fan off timings.
Fan on timing is the
amount of time that the
unit operates in a heat
ing demand without the
blower running during initial start up. Fan off timing is the
amount of time that the blower continues to run after heating
demand has been terminated. Fan timing control part number
51K4601 (figure 8) has a factory set fan−on time of 45 sec
onds and is adjustable from 30 to 60 seconds. Fan−off timing is
factory set at180 seconds and is adjustable from 120 to 240
seconds. Fan timing control with early part number 76H31 has
a fan on timing set at 45 seconds and is not adjustable. Fan
timing control with part number 97H03 and 79J65 has an ad
justable fan on timing from 30 to 60 seconds. Fan off timing is
factory set at 180 seconds and is adjustable from 120 through
240 seconds. During fan off timing blower operates on low
speed heat/cool tap.
Page 14
WATCHGUARD
CONTROL BOARD
(A18)
FIGURE 7
FIGURE 8