Description
The Johnson Controls A421ABC-02C (replaces the older Penn A421 temperature controls) is a single-stage electronic temperature controller with an SPDT relay that switches heating or cooling loads directly at 120 V or 240 V—no auxiliary contactor required. A bright back-lit LCD and three-button keypad make temperature, delay, and setback settings easy, while an adjustable anti-short-cycle timer helps prevent compressor hard starts and trips.
Key Features
Wide control range
Adjusts temperatures anywhere from -40 °F to 212 °F (-40 °C to 100 °C), covering most HVAC and industrial needs.
Works on 120 V or 240 V
Direct line-voltage switching (SPDT relay) simplifies wiring for fans, pumps, heaters, or chillers.
Clear back-lit LCD
Large numbers and system icons are easy to read; brightness can be dimmed for dark mechanical rooms.
Easy On/Off set-points
Set heating or cooling cut-in and cut-out temperatures right from the keypad—no jumpers to move.
Anti-short-cycle delay
Adjustable time delay keeps compressors off for a few minutes after they stop, extending equipment life.
Temperature setback
A dry-contact input lets you shift both set-points by a chosen offset—for example, to save energy at night.
High/low limits & display offset
Lock the adjustment range for end users and fine-tune the displayed temperature to match a reference sensor.
Status LED
A front-panel light shows when the relay is energized, so you can confirm operation at a glance.
Durable enclosure
Type 1 (NEMA) / IP20 plastic housing mounts on DIN rail or directly to a flat surface; sensor with 6 ft 7 in. (2 m) lead is included.
Typical Uses
- Switching boilers, chillers, or circulation pumps
- Alarm or shutdown control when a process temperature drifts out of range
- Fan or damper operation based on duct or water temperature
- Cooling-tower fan cycling
- General heating or cooling control in greenhouses, walk-in boxes, or equipment cabinets
Key Specifications
Power / relay rating
120 / 240 VAC line voltage, single-pole double-throw (SPDT)
Sensor supplied
A99BB-200C, 10 kΩ NTC, 6 ft 7 in. (2 m) cable
Adjustment range
-40 °F to 212 °F (-40 °C to 100 °C)
Differential (hysteresis)
Adjustable
Anti-short-cycle delay
Adjustable
Enclosure Mounting
Type 1 (NEMA), IP20, high-impact plastic
Mounting
Surface mount or 35 mm DIN rail
How the A421 Series works
The A421 Series is essentially a smart, line-voltage thermostat built for equipment control duties. An external A99 sensor monitors temperature; when the reading crosses the “On” or “Off” set-points you program on the keypad, the onboard SPDT relay flips—energizing or de-energizing fans, heaters, pumps or condensing units at either 120 V or 240 V. Because the relay is rated for 1 HP, you can land the load leads directly on the controller instead of adding a separate contactor.
The A421 is adjustable for anything you come across in the field: cut-in and cut-out temperatures (-40 °F to 212 °F), a programmable temperature differential so you can tighten or widen the control band, an anti-short-cycle delay to spare compressors, and a dry-contact “setback” input for night-set or defrost cycles. You can also lock the keypad to keep from bumping set-points.
FAQs
What is the differential range on an A421ABC-02C?
The temperature differential—the span between turn-on and turn-off—is adjustable from 1 °F to 30 °F
Setup Instructions
- Access Setup Mode:
- Navigate the three-button interface via Menu, Up, Down to access Basic and Advanced menus.
- Set Temperature Differential:
- Under Basic menu, set OFF (cut-out temp) and ON (cut-in temp). The differential (Diff = OFF – ON) can be anywhere between 1 and 30 °F.
- Adjust Anti‑Short‑Cycle Delay (ASd):
- In Basic or Advanced menus, configure ASd (anti-short cycle delay) from 0 to 12 minutes to prevent compressor or heater short cycling.
- Enable Temperature Setback (tSb):
- In Advanced settings, you can program a setback offset (–50 to +50 °F). To activate it, wire a dry contact to the BIN input and enable setback via the parameter setting.
