Defrost Control Components - Lennox HP21 Unit Information

TABLE 12
CMC1 DEFROST INITIATION
NOTE−The symbol ∆T (pronounced delta T) is used to represent
the difference between liquid line temperature and ambient tem-
perature. The defrost control compares actual ∆T to a calculated
∆T to determine when to initiate defrost.
The defrost control calculates ∆T by inserting the measured am-
bient temperature into the following equation. You can use the
equation to calculate the ∆T for any ambient temperature.
∆T = 0.30(Ambient Temp. − 15.0) + 1
Once the defrost control has calculated ∆T, it compares the cal-
culated value to the actual measured ∆T. When the actual ∆T ex-
ceeds the calculated ∆T, defrost is initiated.
Actual Ambient
Control will initiate defrost when measured
°
Temperature ( F)
35
30
25
20
15
10
0
As long as the liquid line remains above 65°F, the defrost
control remains turned off. If, during cooling operation, the
liquid line drops below 65°F, the defrost control will initiate
timing sequences, monitor liquid line and ambient temper-
atures and initiate a forced defrost after six hours of
compressor run time. If defrost is initiated while the unit is in
cooling mode, the outdoor fan will de-energize and the in-
door auxiliary heat will energize. A defrost initiated while
the unit is in cooling mode is an indicator that the unit may
be operating in low ambient conditions or further trouble-
shooting is required.

Defrost Control Components

1− Defrost LED
An LED on the circuit board lights to indicate the control
is in defrost mode.
2− Ambient Temperature Sensor
Ambient sensor (red cable) is permanently attached to
the circuit board. The sensing element is attached in-
side the ambient air port located in lower right corner of
compressor compartment (see figure 1).
3− Liquid Line Temperature Sensor
The blue cable is also permanently attached to the cir-
cuit board. The sensing element is attached to the
liquid line next to the expansion valve.
∆
Calculated T
∆
T exceeds this value
7
5.5
4
2.5
1
0
0
SOLID STATE DEFROST CONTROL
AMBIENT TEMP.
SENSOR (RED)
24VAC COMMON
24VAC INPUT
LED (INDICATES
DEFROST MODE)
4− 24V Terminal
Terminal 24V" receives 24VAC from the control trans-
former in the indoor unit (transformer T1 in CB21 series
units). This terminal powers the control's internal timer,
relays and temperature probes. Terminal 24V" is pow-
ered anytime the indoor transformer is powered.
5− DEF RLY" Terminal
Terminal DEF RLY" controls defrost when connected
in series with defrost relay coil. An internal relay con-
nected to terminal DEF RLY" closes to allow external
defrost relay (K4) to energize and initiate defrost. At the
end of defrost period, internal relay connected to termi-
nal DEF RLY" opens to de−energize external defrost
relay.
6− COM" Terminal
Terminal COM" provides 24VAC Common.
7− HLD" Terminal
Terminal HLD" holds the internal timers in place be-
tween thermostat demands and allows the unit to
continue timing upon resumption of thermostat de-
mand. Terminal HLD" is connected directly to 1st
stage thermostat demand.
Page 18
LIQUID LINE
TEMP. SENSOR
(BLUE)
TEST"
DEF RLY"
HOLD"
FIGURE 13