Johnson Controls Single Stage Electronic Controllers

Johnson Controls A421Single-Stage Electronic Temperature Control from Johnson Controls

The A421 Series single-stage temperature controls are available in low voltage (24VAC) and high voltage (120 or 240VAC) models. Packed with an A99 series PTC temperature sensor with an ambient operating temperature range of -40 to 212⁰F*, they provide a Single Pole/Double Throw relay output. The A421 series are available with NEMA 1 or NEMA 4X enclosure. Simple to install and set up, the A421 is loaded with features and benefits you need:

  • Control Front Panel LCD
  • Basic and Advanced Programming Menu
  • On/Off Temperature Adjustment
  • Switch-activated Temperature Setback
  • Adjustable Anti-Short Cycle Delay
  • Adjustable Sensor Offset
  • Optional Restricted Adjustment Mode
  • Sensor Failure Mode
  • Backlight Brightness Level Adjustment
  • Off-Cycle Defrost Models available.



  • Temperature monitoring and alarming
  • On/off control of boilers and chillers
  • Boiler and chiller pump control
  • Heating and cooling control
  • Cooling tower fan control based on water temperature
  • Supply, makeup, and mixed air temperature control
  • Temperature actuated valve control
  • Supply and makeup air damper and fan control

Temperature units can be displayed in ⁰F or ⁰C. The temperature adjustment range is -40 to 212 ⁰F or -40 to 100 ⁰C. The controls are available in 1P/IP20 high-impact plastic enclosures suitable for surface or DIN rail mounting and Type 4X/IP66 watertight, corrosion-resistant surface mount enclosures.


*Optional extended range sensors are available (-40 to 248⁰F).

- Models are available with convenient 120VAC plug-in power cord.

For product selection help, call us at (800)-227-9800.