Johnson Controls A421 Installation Manual
A421 Series Standard Electronic
Temperature Controls Installation Guide
Applications
Important: Use this A421 Series Electronic Temperature Control only as an operating control.
Where failure or malfunction of the A421 Control could lead to personal injury or property
damage to the controlled equipment or other property, additional precautions must be
designed into the control system. Incorporate and maintain other devices, such as supervisory
or alarm systems or safety or limit controls, intended to warn of or protect against failure or
malfunction of the A421 Control.
Important: Utiliser ce A421 Series Electronic Temperature Control uniquement en tant que
dispositif de régulation. Lorsqu'une défaillance ou un dysfonctionnement du A421 Control
risque de provoquer des blessures ou d'endommager l'équipement contrôlé ou un autre
équipement, la conception du système de contrôle doit intégrer des dispositifs de protection
supplémentaires. Veiller dans ce cas à intégrer de façon permanente d'autres dispositifs,
tels que des systèmes de supervision ou d'alarme, ou des dispositifs de sécurité ou de
limitation, ayant une fonction d'avertissement ou de protection en cas de défaillance ou de
dysfonctionnement du A421 Control.
The A421 Series Electronic Temperature Controls are single-stage, electronic temperature controls
with a single-pole, double-throw (SPDT) output relay.
A421 Controls feature a backlit LCD with adjustable brightness and three-button touchpad interface
that you can set up to restrict user adjustments. An LED indicates the output relay's on and off
status.
A421 Controls have simple on and off temperature settings for heating or cooling, an adjustable
anti-short cycle delay, temperature setback, and sensor offset capability. The temperature control
range is -40°F to 212°F or -40°C to 100°C.
The A421 Controls are available either in Type 1 (NEMA), IP20 (CE), high-impact plastic enclosures
suitable for surface or DIN rail mounting (Figure 1), or in Type 4X (NEMA), IP66 (CE) watertight,
corrosion resistant surface mount enclosures (Figure 2).
Part No. 24-7664-3019 Rev E
2018-11-13
A421ABC-x, A421AEC-x, A421GBF-x, A421GEF-x
Dimensions
Figure 1: A421 Control with Type 1 (NEMA), IP20 enclosure dimensions, in. (mm)
Figure 2: A421 Control with Type 4X (NEMA), IP66 enclosure dimensions, in. (mm)
Parts included
Each A421 Control includes a Johnson Controls® or PENN® A99 Series temperature sensor. See A99
Series Temperature Sensors, Wiring, and Technical specifications for more information about A99
sensors.
A421 Series Standard Electronic Temperature Controls Installation Guide2
A99 Series Temperature Sensors
The A421 Controls require an A99 sensor, and each A421 Control includes an A99 sensor. Any A99
Series sensor works with the A421 Series Controls. Do not replace an A99 Series sensor with any
other brand, series, or type of temperature sensor. See Ordering information for available A99
Series sensor models.
You can extend the sensor leads in the field. See Table 1 for information about wire sizes and
lengths. On long sensor cable runs, use shielded cable to reduce electromagnetic interference
(EMI). Observe EMI best practices when you route sensor leads.
Do not immerse the A99 Series sensors in water or any other liquid. The A99 sensors are moisture
tolerant and splash resistant but if you immerse the sensor, liquid can enter the sensor probe
where the steel tube meets the wire cable and result in sensor failure, which voids any warranty.
In applications where the sensor may be exposed to a lot of moisture, splashing, or rain, mount the
sensor in a vertical position with the cable at the bottom routed downward so that moisture can
drain away from the stainless steel probe. Use a suitable bulb well for complete fluid immersion
applications. See Ordering information for information about bulb wells.
The A99 Series sensors are positive temperature coefficient (PTC) sensors. To test an A99 sensor,
disconnect the sensor from the control and measure the resistance between the sensor leads in the
following situations:
• When the temperature at the sensor is 77°F (25°C), the resistance should be 1,035 ohms.
• When the temperature at the sensor is 32°F (0°C), the resistance should be 855 ohms.
See Troubleshooting for more information.
When you connect an A99 sensor to a standard A421 Control, the control restricts the range of
usable values from -40°F to 212°F or -40°C to 100°C.
See Wiring, Technical specifications, and refer to the A99B Series Temperature Sensors Product/
Technical Bulletin (LIT-125186) for more information about A99 Series sensors.
Mounting
Observe the following guidelines when you locate and mount an A421 Control:
• Make sure that the mounting surface can support the control, DIN rail, mounting hardware, and
any user-supplied panel or enclosure.
• Mount the control in a vertical, upright orientation wherever possible. It is best practice to use
DIN rail mounting for type 1 controls.
• In direct-mount applications, mount the control on a flat and even surface.
Mount the control in a location free of corrosive vapors and observe the ambient operating
conditions listed in Technical specifications for the A421 Control and the A99 sensor.
• Allow sufficient space to connect and route wires, view the LCD, and use the touchpad.
• Do not mount the control on surfaces that are prone to vibration or in a location where highvoltage relays, motor starters, other sources of electromagnetic emissions, or strong radio
frequency may cause interference.
• Do not install the control in an airtight enclosure.
• Do not install heat generating devices with the control in an enclosure that may cause the
ambient temperature to exceed 150°F (66°C).
3A421 Series Standard Electronic Temperature Controls Installation Guide
Mounting a Type 1/IP20 control on DIN rail
1. Provide a section of 35 mm DIN rail that is longer than the control width. Mount the DIN rail in
a suitable location and use appropriate mounting hardware.
2. Clip the control module on the rail, position the module’s upper DIN rail clips on the top rail,
and gently snap the lower clips on to the bottom of the rail.
Direct-mounting a Type 1/IP20 control to a wall or other flat surface using the four keyhole slots
1. Disconnect the power and remove the enclosure cover. Place the control vertically against
the wall surface in a suitable location, and mark the keyhole slot locations on the mounting
surface.
2. Install appropriate screws or fasteners and leave the screw heads approximately one or two
turns away from flush to the mounting surface.
3. Position the control mounting slots over the screw heads, and then tighten the mounting
screws to secure the control to the surface.
Note: When you mount the control on an uneven surface, use shims to mount the control
evenly.
Additional guidelines for mounting Type 4X/IP66 controls
You can mount the Type 4X models to flat vertical surfaces using the four holes at the enclosure
corners. Place the control against a flat wall surface in a suitable location, and mark the mounting
screw hole locations on the mounting surface. Use appropriate screws and shims to mount the
control evenly on the surface.
On Type 4X models, select the knockout for removal. Place a screwdriver blade on the knockout
near the edge. Apply a sharp blow to the screwdriver handle to loosen the knockout. Be careful not
to damage the control’s interior components.
The A421 Control's temperature setback (BI) function requires an additional low-voltage, two-wire
cable for operation. On Type 4X/IP66 enclosures, you must install a suitable watertight fitting in an
available knockout to pass the two-wire cable through the enclosure wall.
You can rotate the control enclosure base on the Type 4X/IP66 models 180º relative to the control
enclosure cover and LCD, to bring the electrical connection to the top or bottom of the mounted
control.
Note: Do not twist the wiring harness between the enclosure base and cover more than 180º.
A421 Series Standard Electronic Temperature Controls Installation Guide4
Wiring
WARNING
Risk of Electric Shock
Disconnect or isolate all power supplies before making electrical connections. More than one
disconnection or isolation may be required to completely de-energize equipment. Contact with
components carrying hazardous voltage can cause electric shock and may result in severe personal
injury or death.
Avertissement
Risque de décharge électrique
Débrancher ou isoler toute alimentation avant de réaliser un raccordement électrique. Plusieurs
isolations et débranchements sont peut-être nécessaires pour -couper entièrement l'alimentation de
l'équipement. Tout contact avec des composants porteurs de tensions dangereuses risque d'entraîner
une décharge électrique et de provoquer des blessures graves, voire mortelles.
Important: Use copper conductors only. Make all wiring in accordance with local, national,
and regional regulations.
Important: Do not exceed the A421 Control’s electrical ratings. Exceeding the electrical ratings
can result in permanent damage to the control and void any warranty.
Important: Run all low-voltage wiring and cables separate from all high-voltage wiring.
It is best practice to use shielded cable for input (sensor) cables that are exposed to high
electromagnetic or radio frequency noise.
Important: Electrostatic discharge (ESD) can damage A421 Controls. Use proper electrostatic
discharge precautions during installation and servicing to avoid damaging A421 Controls.
Important: Do not connect supply power to the A421 Controls before checking all wire
connections. Short circuits or improperly connected wires can result in damage to the modules
and void any warranty.
Important: When you connect an A99 sensor with a shielded cable to an A421 Control,
connect the cable shield drain lead to the common (COM) terminal on the sensor and binary
input terminal block (TB3). Do not connect the shield at any other point along the cable, and
isolate and insulate the shield along the entire length of the sensor cable. If you connect a
cable shield at more than one point, transient currents can flow through the sensor cable
shield and cause erratic control operation.
Observe the wire size restrictions listed in Table 2 and the electrical ratings listed in Technical
specifications.
Observe the following guidelines, procedures, and illustrations when you wire an A421 Series
Control and A99 Series sensor.
• Select the appropriate A99 sensors for the ambient operating range that the A421 Control
monitors and controls, as shown in Table 7. See Technical specifications for more information.
• Keep the sensor leads as short as possible in your application. The additional resistance in long
5A421 Series Standard Electronic Temperature Controls Installation Guide
sensor cables creates an offset between the actual temperature and the displayed temperature.
See Table 1 when you extend sensor leads.
• A99 sensors are not polarity specific. You can connect either lead to the SEN or COM terminals.
• It is best practice to use 22 AWG, stranded, twisted-pair cable with a shield to extend sensor cable
runs.
Table 1: Maximum sensor cable lengths and wire sizes
Wire gauge
1
Maximum sensor cable length
1
16 AWG 500 ft (150 m)
18 AWG 300 ft (100 m)
20 AWG 200 ft (60 m)
22 AWG 125 ft (40 m)
1 The maximum sensor cable lengths have less than 1°F (0.6°C) error between the temperature sensed at the A99 sensor
and the temperature displayed on the LCD.
TB2 Terminal Block and SPDT relay output
The terminals LC, LNO, and LNC on the TB2 terminal block connect to an SPDT dry-contact relay in
the A421 Control (Figure 3). The control does not provide any internal power to the TB2 terminals
or relay contacts. The A421 Control energizes and de-energizes the relay to open and close the
contacts based on the On/OFF temperature values.
• Relay De-energized (Off) = LC open to LNO as shown in Figure 3, and the relay status LED is off
• Relay Energized (On) = LC closed to LNO and the relay status LED is on
Figure 3: TB2 Terminal Block showing connections to the internal SPDT relay
Figure 4 shows how to wire the A421 Control to use the same power source that powers the
controlled equipment to also power the A421 Control.
You can also provide an independent power source for the A421 Control on the TB1 terminals and
then wire the TB2 relay terminals to a separate power source to switch and power the controlled
equipment circuit.
A421 Series Standard Electronic Temperature Controls Installation Guide6
Figure 4: Wiring the A421 Series Controls using the same power source
to power the control operation and power the controlled equipment
Table 2: A421 Control wiring terminals and wire size information
Terminal
block
TB1
24 VAC
Models
TB1
120/240
VAC
Models
TB2
Label Description, function, and requirements Wire sizes
LN Low-voltage 24 VAC control power (common): Connect the
24 VAC supply power to operate the control.
28 AWG to
12 AWG
24 V Low-voltage 24 VAC control power (hot): Connect 24 VAC
supply power to operate the control (via jumper from LC in
Figure 4).
LN Line-voltage power source (common): Connect the neutral
wire for 120 VAC supply power applications. Connect the
L1 supply power lead for all 208/240 VAC supply power
applications.
120 V Line-voltage 120 VAC control power (hot): Connect the
0.08 mm² to
4.0 mm²
28 AWG to
12 AWG
0.08 mm² to
4.0 mm²
120 VAC supply power (hot) for 120 VAC supply power
applications through the jumper from LC. (Figure 4).
240 V Line-voltage 240 VAC control power (L2) terminal: Connect
the L2 supply power connection for 208/240 VAC supply
power applications through the jumper from LC. (Figure 4).
LC Line-voltage SPDT relay common contact: Connects power
supply to power the controlled load. Connect 24 VAC
(hot) for 24 VAC applications; 120 VAC (hot) for 120 VAC
applications’ and L2 for 208/240 VAC applications.
LNO Line-voltage SPDT relay normally open contact: Connects
28 AWG to
12 AWG
0.08 mm² to
4.0 mm²
controlled equipment to the line-voltage normally open
(LNO) contact on the SPDT relay. When LC is closed to
LNO, the relay is energized and the green LED is on. The
LNO terminal typically provides power to the controlled
equipment in cooling and heating applications.
7A421 Series Standard Electronic Temperature Controls Installation Guide
Table 2: A421 Control wiring terminals and wire size information
Terminal
block
TB3
Label Description, function, and requirements Wire sizes
LNC Line-voltage SPDT relay normally closed contact: Connects
controlled equipment to the line-voltage normally closed
(LNC) contact on the SPDT relay. When LC is closed to LNC,
the relay is de-energized and the green LED is off. The LNC
terminal is not typically wired to the controlled equipment.
BIN Detects a switch closure between the BIN and COM
terminals and enables the selected temperature setback
(tSb) value.
COM Connects the low-voltage common from the sensor and
binary input.
SEN Connects the low-voltage input signal wire from control
sensors.
22 AWG (0.34
mm²) stranded,
shielded cable
Replacing an A419 Control with an A421 Control: Terminal locations and labels
The A421 Series Electronic Temperature Controls are the next generation of the A419 Series
Electronic Temperature Controls.
If you need to replace an A419 Control with an A421 Control, be sure to note that wiring terminals
on the TB1 and TB2 terminal blocks on A421 Controls are in different positions and have different
terminal labels from the wiring terminals on the A419 Controls. Table 3 provides a cross-reference
for matching the terminal labels on A419 Controls to the terminal labels on A421 Controls.
Note: The low-voltage signal terminals on the TB3 terminal block are labeled the same on both
A419 Controls and A421 Controls.
See Figure 4 for terminal block and terminal positions on the A421 Control.
Table 3: A419 and A421 wiring terminal labels
Terminal block A419 Terminal label A421 Terminal label
(24 VAC)
TB1
(120/240 VAC)
TB2
(All Voltages)
T1 LNTB1
T2 24V
120 120V
240 240V
AC COM LN
C LC
NO LNO
NC LNC
Setup and adjustments
The front panel of the A421 Series Electronic Temperature Control has an LCD and a three-button UI
(Figure 5).
A421 Series Standard Electronic Temperature Controls Installation Guide8
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