In many temperature- and humidity-sensitive environments—such as data centers, laboratories, museums, or precision 

manufacturing workshops—environmental control equipment is critical. However, “constant temperature and humidity 

units” and “standard precision air conditioners” are often conflated, potentially leading to misguided selection decisions. 

In reality, these are two distinct types of equipment designed for different requirements. This article will clearly analyze 

their fundamental differences across three core dimensions—control objectives, core functions, and component configurations

—to help you make a more precise selection.

I. Control Objectives: Humidity Precision as the Divide

The primary distinction lies in their fundamental control objectives.

Standard Precision Air Conditioners: Their primary and core task is precise temperature control. While they possess some 

dehumidification capability, this is typically viewed as a “byproduct” of temperature regulation. Their design emphasizes 

rapid response to thermal fluctuations, stabilizing temperature within a set range (usually ±1°C or narrower). For humidity, 

it typically employs only coarse control—such as preventing excessive humidity through incidental dehumidification during 

cooling—but lacks active, precise humidification capabilities. It cannot maintain humidity within a narrow setpoint range. 

Its humidity control accuracy often falls within ±5%RH or wider, and it struggles to increase humidity in dry environments.

Constant Temperature and Humidity Units: As the name implies, their design mission is to simultaneously and independently 

control both temperature and humidity with high precision. They elevate humidity to equal importance alongside temperature. 

Beyond precise temperature control (also achievable at ±1°C or higher), they must possess the capability to stabilize humidity 

within a narrow user-defined target range, such as ±3%RH, ±2%RH, or even higher precision. This means the equipment must 

actively and precisely respond to demands—whether dehumidifying during humid seasons or humidifying in dry winters—

to achieve true “constant humidity.”

II. Core Functionality: Presence of Active Humidification Capability

The differing control objectives directly lead to the second major distinction: whether the unit possesses an independent, 

controllable active humidification function.

• Standard Precision Air Conditioners: Typically lack active humidification. Their humidity regulation primarily relies on the

  refrigeration cycle. As air flows over the chilled evaporator coil, moisture condenses and is removed, achieving dehumidification.

  However, in inherently dry environments or during low-temperature/low-humidity operation, it cannot add moisture to the air.

  Some models may include basic humidification modules, but these are rarely standard equipment and offer limited control

  precision and humidification capacity, failing to meet stringent constant humidity requirements.

• Constant Temperature and Humidity Units: Active humidification is their standard and core functional module. The unit incorporates

  a capacity-matched humidification system (e.g., electrode-type, electric heating-type, infrared-type, or wet-film-type). Based on

  real-time feedback from humidity sensors, it automatically and precisely activates and outputs water vapor when air humidity

  falls below the setpoint, rapidly raising and stabilizing humidity at the target value. This “proactive” approach to humidity

  control fundamentally differs in logic and capability from the purely “passive” dehumidification method.

III. Component Configuration and Control Logic: Complexity and Integration

To achieve the aforementioned high-precision constant humidity functionality, constant temperature and humidity units feature 

more complex hardware configurations and system control logic.

Component Configuration:

• Humidifier: This is the most noticeable external distinction. Constant temperature and humidity units necessarily integrate a

  sufficiently capable humidifier, complete with water supply and drainage systems.

• More Complex Sensors and Control Systems: While standard precision air conditioners primarily rely on temperature sensors,

  constant temperature and humidity units require high-precision humidity sensors (such as capacitive polymer film sensors) as

  core feedback elements. Their control systems employ dual-variable (temperature and humidity) PID control logic, enabling

  simultaneous processing and decoupling of temperature and humidity control commands to prevent mutual interference.

• Specialized heat exchange component design: To ensure effective dehumidification in low-temperature environments

  (preventing evaporator frosting failure) and achieve finer dehumidification control, constant temperature and humidity units

  may incorporate reheat components (e.g., electric reheat, condenser heat recovery) or utilize variable-frequency compressors

  and electronic expansion valves. This enables broader, more flexible refrigeration capacity adjustment.

Control Logic:

• Conventional precision air conditioners employ relatively linear control logic: detect temperature → determine cooling/

  heating/ventilation.

• Constant temperature and humidity units utilize multidimensional, coordinated control logic: continuously monitor t

  emperature and humidity → assess deviation from setpoints → simultaneously or sequentially activate multiple execution

  units (cooling, heating, dehumidification, humidification, fan speed control) to achieve equilibrium with maximum energy

  efficiency or speed. For instance, when cooling and dehumidification are both required, it precisely controls refrigeration

  capacity. When cooling is needed but excessive dehumidification must be prevented (or even humidification is required),

  it may activate refrigeration alongside humidification or enable reheat functionality.

Summary and Selection Recommendations

In short, standard precision air conditioners are primarily temperature-control devices with limited dehumidification capabilities, 

while constant temperature and humidity units are environmental control solutions that equally prioritize both temperature and 

humidity with bidirectional high-precision control.

When selecting equipment, clearly define your core requirements:

• If your environment primarily concerns heat accumulation and is insensitive to humidity fluctuations (e.g., standard server rooms,

  electrical distribution rooms), standard precision air conditioners may suffice.

• If humidity is equally critical as temperature in your scenario, and both excessive dryness and humidity could cause severe

  consequences (e.g., precision instrument calibration, pharmaceutical storage, archival preservation of valuable documents and

  artwork, specialized material production workshops), then a constant temperature and humidity unit with active humidification

  and high-precision humidity control capabilities is essential.

Understanding these three core distinctions helps you cut through the confusion surrounding terminology. By aligning your selection 

with the physical requirements and process standards of your actual environment, you can choose truly compatible equipment. 

This ensures your critical operations and assets remain in a stable and reliable environment.