Lennox LDT Series Information page 16

Temperature Sensors
The return air (RT16) and discharge air (RT6) duct probes
and the outdoor air (RT17) are all two wire thermistors. The
resistance vs. temperature table is shown below:
Temp. °F (°C) Resistance +/-2%
-40 (-40) 335,671
-20 (-28.9) 164,959
0 (-17.8) 85,323
20 (-6.7) 46,218
30 (-1.1) 34,566
Room Sensors
Room sensor (A2) is a two-wire thermistor with 1k series resistor.
Temp. °F (°C) Resistance +/-2%
40 (4.4) 27,102
45 (7.2) 23,764
50 (10) 20,898
55 (12.8) 18,433
Carbon Dioxide Sensor
The indoor carbon dioxide sensor (A63) is an analog sensor with a 0-10VDC output over a carbon dioxide range of 0-2000
ppm as shown in the following table. The sensor is powered with 24VAC.
Carbon Dioxide DC
PPM V
0 0
200 1
400 2
Relative Humidity Sensor - Optional
The indoor relative humidity sensor (A91) is an analog
sensor with a 0-10VDC output over a relative humidity
range of 0-100% relative humidity. The sensor is powered
with 24VAC.
Enthalpy Sensor - Optional
The optional enthalpy sensors (A7 and A63) used with the
economizer have an output of 4-20mA. The sensor is
powered with 18VAC provided by M3 unit control.
Economizer Differential Pressure Sensor - Optional
Rooftop units installed with Smart Airflow™ will have a
Pressure Transducer (PT5) present in the economizer.
PT5 requires 5VDC power supply (P266-5 and {P266-6)
and gives 0.25 VDC to 4 VDC output (P266-4)
corresponding to 0" water column and 2" water column
respectively. For all practical purposes the output should
be less than 1.2" water column if not an error code is stored
and service alarm output is turned on.
Resistance vs. Temperature
Temperature °F
(°C)
40 (4.4)
50 (10)
60 (15.6)
70 (21.1)
80 (26.7)
Two-Wire Thermistor
Temperature °F
(°C)
60 (15.6)
65 (18.3)
70 (21.1)
75 (23.9)
Carbon Dioxide Range
Carbon Dioxide
DC Voltage
PPM
600 3
800 4
1000 5
Table 1
Resistance +/-2%
26,106
19,904
15,313
11,884
9,298
Table 2
Resistance +/-2%
16,313
14,474
12,882
11,498
Table 3
Carbon Dioxide
PPM
1200
1400
1600
15-Second-Stage Power Exhaust Relay
K231 (Staged-Blower units equipped
with power exhaust)
The second power exhaust fan is controlled by K231. A133
will enable K231 only when the blower reaches 70% of full
speed (adjustable ECTO). This prevents a negative
building pressure when the blower is operating in low
speed. Refer to the Unit Controller manual and ECTO
labels on the unit.
16-Fuse F61 (Higher SCCR units only)
Fuse F61 is used on units with higher SCCR rating. F61
provides overcurrent protection to compressor and other
cooling components. F61 and S48 are located inside a
sheet metal enclosure in the unit left front corner mullion.
17-Blower Motor Overload Relay S42
The relay (S42) is connected in line with the blower motor to
monitor the current flow to the motor. When the relay
senses an overload condition, a set of normally closed
contacts open to de-energize pin #1 in plug P299 of the A55
Unit Controller. A55 de-energizes all outputs. Units will be
equipped with a relay manufactured by Telemecanique
figure 5 or Siemens figure 6.
Page 16
Temp. °F (°C) Resistance +/-2%
90 (32.2) 7,332
100 (37.8) 5,826
120 (48.9) 3,756
130 (54.4) 3,047
Temp. °F (°C) Resistance +/-2%
80 (26.7) 10,299
85 (29.4) 9,249
90 (32.2) 8,529
Carbon Dioxide
DC Voltage
PPM
6 1800
7 2000
8
DC
V
9
10