HVAC Temperature Sensor - How They Work

A temperature sensor measures temperature within a heating, ventilation, and air conditioning system. Its primary function is to provide feedback to the system's control unit, enabling it to maintain or adjust the indoor environment to the desired temperature settings. They are critical for ensuring comfort, energy efficiency, and the overall performance of the system. This article discusses the main types of HVAC temperature sensors, their features, and typical selection criteria.

Table of contents

• Role of an HVAC temperature sensor
• HVAC temperature sensor types
  • Average temperature sensors
  • Duct temperature sensors
  • Temperature immersion sensors
  • Contact temperature sensors
  • Cable temperature sensors
  • Outdoor temperature sensor
• Selection criteria
• HVAC temperature sensor calibration standards and procedure
• FAQs

Role of an HVAC temperature sensor

Temperature sensors in an HVAC system are connected to units like thermostats, air handlers, and furnaces; they provide precise temperature readings to monitor and regulate the temperature in an HVAC system. They are responsible solely for sensing and measuring the ambient temperature, then transmitting this data to a control unit. This control unit, equipped with the logic to interpret these readings, makes decisions to regulate the temperature within the HVAC system accordingly. For instance, in smart homes, these sensors enable zone-based temperature control, allowing different rooms to be heated or cooled based on occupancy or time of day, significantly reducing energy consumption. In commercial buildings, they integrate with building automation systems to adjust HVAC operations based on occupancy levels, improving comfort while minimizing energy use. Their application in data centers is vital for preventing overheating of servers, ensuring system reliability and performance.

HVAC temperature sensor types

Each type of HVAC temperature sensor plays a vital role in the overall performance and efficiency of HVAC systems.

HVAC average temperature sensors

• Purpose: These sensors measure the average temperature in a large space or across a section of ductwork. They are essential for ensuring that the HVAC system provides consistent comfort throughout a building.
• How they work: They typically consist of multiple sensing elements distributed across an area or length of duct. The sensor averages these readings to provide a more accurate representation of the overall temperature.
• Applications: Used in large rooms, long duct runs, or areas where temperature gradients are expected.

HVAC duct temperature sensors

• Purpose: Specifically designed to measure the temperature of the air flowing within HVAC ductwork
• How they work: These sensors are usually placed in the ductwork and directly measure the air temperature inside. They can be simple, single-point sensors or more complex, averaging types.
• Applications: These sensors are essential for the basic operation of HVAC systems, ensuring that air distributed throughout a building is at the desired temperature (Figure 1).

HVAC temperature immersion sensors

• Purpose: Used to measure the temperature of liquids, such as the water in a boiler or chiller system
• How they work: These sensors are immersed directly into the liquid they are measuring. They are typically encased in a protective sheath to prevent corrosion or damage.
• Applications: Important for systems that use water or another liquid for heating or cooling, such as hydronic heating systems or chilled water systems.

HVAC contact temperature sensors

• Purpose: Designed to measure the temperature of surfaces, such as pipes, ducts, or equipment casings.
• How they work: These sensors make direct physical contact with the surface being measured. They can be clamped, taped, or otherwise attached to ensure good thermal contact.
• Applications: Useful for monitoring the temperature of specific components within the HVAC system, such as refrigerant lines or heating coils.

HVAC cable temperature sensors

• Purpose: These sensors are used to measure temperature along lengths of cable, providing a distributed sensing capability.
• How they work: The sensor element is integrated into a cable, allowing for continuous temperature measurement over long distances or large areas.
• Applications: Ideal for tracing temperature changes along ducts, pipes, or even throughout entire rooms or zones. They can be particularly useful in complex systems where pinpointing temperature variations is critical.

HVAC outdoor temperature sensor

• Purpose: These sensors are designed to monitor the outdoor temperature, providing essential data to the HVAC system to optimize indoor heating and cooling based on external conditions.
• How they work: Outdoor temperature sensors are installed outside the building, typically on a north-facing wall or in a shaded area to avoid direct sunlight. These sensors measure the ambient temperature and send this information to the HVAC control system.
• Applications: Essential for HVAC systems that adjust indoor climate settings in response to outdoor temperatures. They are particularly useful in environments where external temperature fluctuations significantly impact indoor heating and cooling requirements.

Note: Temperature can vary significantly within a room due to air stratification (warm air rising, cool air settling). Placing sensors in stagnant areas or near air vents can lead to inaccurate readings and inefficient system operation. Also, sensors should be located away from direct sunlight, drafts, doorways, windows, and heat sources to avoid false readings.

Selection criteria

Consider the following parameters when choosing a temperature sensor for HVAC applications:

1. Sensor type: Select between contact, cable, duct, immersion, or average temperature sensors based on the discussion in the previous section.
2. Temperature range and accuracy: Select a sensor that covers the required temperature range with the accuracy level that meets the application's precision needs.
3. Material and construction: Opt for stainless steel for corrosion resistance or brass for specific liquid measurements. Ensure the sensor's length and diameter fit the installation requirements.
4. Environmental protection: Ensure the sensor has an appropriate IP rating (IP65 or IP67) for protection against dust and water, and is UV resistant if exposed to sunlight.
5. Mounting and application suitability: Verify the sensor's compatibility with the mounting needs (duct, tube, or immersion) and its design for use in air, liquids, or both.
6. Connectivity and installation: Decide between fixed cables and terminal blocks based on the installation preference. Consider the cable length for connectivity and whether passive (no external power required) or active measurement suits the system better. Some sensors are designed for straightforward installation, with options such as mounting flanges for duct applications and thermowells for immersion in water. This flexibility facilitates easy integration into existing HVAC systems.
7. Additional features:
8. Assess the need for housing for extra protection, and the relevance of display and wireless capabilities for the monitoring requirements.
9. Consider the sensor's compliance with safety and quality standards (like IEC/EN 60730-1 , ISO 9001 e) and the availability of accessories for installation flexibility.
10. Sensors directly exposed to sunlight can register higher temperatures than the actual room temperature, potentially leading to overcooling. Consider shading them or using sensors with built-in radiation shields.
11. Some sensors combine temperature and humidity readings, allowing for more precise control of both factors and improved comfort levels.
12. Wireless sensors eliminate the need for extensive wiring, simplifying installation and maintenance. However, they require reliable battery power or alternative power sources.

HVAC temperature sensor calibration standards and procedure

Over time, some sensors can experience drift, meaning their readings become less accurate. Regular calibration is crucial to ensure optimal system performance.

Calibration standards

1. ISO/IEC 17025: This international standard specifies the general requirements for the competence to carry out tests and calibrations, including sampling. It covers testing and calibration performed using standard methods, non-standard methods, and laboratory-developed methods.
2. ASTM E2877: Provides guidelines for the selection, use, and calibration of digital thermometers, including those used in HVAC systems, to ensure they meet required accuracy and reliability.

How to calibrate a temperature sensor

1. Preparation: Ensure the sensor is clean and functional. Choose a suitable calibration method (e.g., fixed-point, comparison).
2. Calibration equipment: Use a reference thermometer or calibrator with traceability to standards, having superior accuracy to the sensor.
3. Environment control: Calibrate in a stable environment to minimize temperature fluctuations, matching the sensor's operational conditions.
4. Calibration process:
   1. Fixed-point: Immerse both sensor and reference in a medium at a known temperature until equilibrium.
   2. Comparison: Expose both sensors to identical conditions, comparing readings.
5. Documentation: Record results, including date, conditions, and deviations.
6. Adjustment: Correct any discrepancies per manufacturer's instructions.
7. Verification: Verification involves re-calibrating the sensor to confirm its accuracy aligns with the standard after adjustments have been made.

FAQs

What is an HVAC remote temperature sensor?

An HVAC remote temperature sensor allows for temperature monitoring from a distance, enhancing system efficiency by providing precise control over indoor environments.

How do you install a temperature sensor in an HVAC system?

To install, select a location that represents average air flow, away from direct sunlight or drafts. Mount the sensor securely and connect it to the HVAC control unit following manufacturer instructions.

Where is the HVAC compressor temperature sensor located?

Typically, it's mounted on the compressor housing or near the discharge line to monitor temperature accurately.

What does an HVAC compressor temperature sensor do?

It monitors the temperature of the HVAC compressor, ensuring it operates within safe limits to prevent overheating and potential damage.

Where is the HVAC coolant temperature sensor located?

Typically, it's located near the coolant inlet or within the cooling system of the HVAC unit to accurately measure the temperature of the circulating coolant.