Service Manual

SiUS281811E Special Control
Part 4 Functions and Control 83
3.3 Defrost Control
Pc: High pressure sensor detection value
Pe: Low pressure sensor detection value
Tb: Heat exchanger deicer temperature
Tc: High pressure equivalent saturation temperature
Te: Low pressure equivalent saturation temperature
Ts1: Suction pipe temperature detected by thermistor R3T
Defrost control is performed to melt frost on the outdoor heat exchanger when heating, and thus
recover heating capacity.
Starting conditions
Defrost operation is started referring to the following conditions.
 Outdoor heat exchanger heat transfer coefficiency
 Timer setting (Two hours at the minimum)
In addition, outdoor heat exchanger coefficiency is derived from Tc, Te, and compressor load.
 Between (A) defrost preparation control and (B) defrost control, and between (B) defrost control
and (C) control after defrost, the compressor stops for 1 minute to reduce noise on changing of the
four way valve.
Outdoor unit
actuator
Electric Symbol
(A) Defrost preparation
control
(B) Defrost control (C) Control after defrost
18/24
class
30/36/42/48
class
Compressor M1C M1C Upper limit control 140 Hz Full load Increases 2 steps every 20
seconds from minimum
frequency until Pc – Pe >
0.4 MPa (58 psi) is
achieved.
Outdoor fan M1F M1F
M2F
STEP 8 OFF STEP 8
Main electronic
expansion valve
Y1E Y1E SH control 480 pulse (Fully open) 55 pulse
Subcooling
electronic
expansion valve
— Y3E 0 pulse 0 pulse 0 pulse
Four way valve Y1S Y1S ON OFF ON
Hot gas bypass
valve
Y2S — OFF ON ON
Ending conditions 2 minutes
x 12 minutes
x
Tb > 11°C (51.8°F)
x Ts1 – Te < 5°C (9°F)
OR
&
x 160 seconds
x
Pc – Pe > 0.4 MPa
(58 psi)
OR
Indoor unit actuator Defrost control
Fan
Thermostat ON unit OFF
Non-operating unit OFF
Thermostat OFF unit OFF
Electronic expansion valve
Thermostat ON unit 416 pulse
Non-operating unit 256 pulse
Thermostat OFF unit 416 pulse