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Did ASHRAE Specify Which Temperature Sensor You MUST Use in BMS? (The Truth Every Engineer Should Know)

November 15, 2025

This is one of the biggest confusions in HVAC Controls & BMS Design.
Many engineers think ASHRAE defines which sensor type (1kΩ Nickel, 1kΩ Platinum, 2.2kΩ Thermistor, 10kΩ Thermistor) must be used for air or water systems.

But the truth is very simple:
NO global standard (ASHRAE, IEC, ANSI) forces you to use a specific sensor type in BMS.

So what do standards actually tell us?

 What ASHRAE REALLY Specifies
1. Accuracy requirements
2. Sensor placement (where to mount in duct/pipe/room)
3. Response time
4. Environmental protection (IP rating, insulation, shielding)
5. Calibration & test methods

But they do not say:
“Use Pt1000 for water”
“Use 10k Thermistor for duct”
“Use Ni1000 for return air”
These choices are driven by engineering judgement + controller capability + project specifications.

Related Standards You Should Know
1. ASHRAE Guideline 36 - sensor accuracy & placement
2. ASHRAE Standard 41.1 - temperature measurement method
3. ASHRAE Handbook - application guidance (air vs water sensors)
4. IEC 60751 - platinum RTD accuracy classes
5. UL 873 - safety for temperature devices

None of them specify which resistance type to use.

So How Do We Decide Which Sensor to Use?
Here’s the industry-recommended best practice
a. Pt1000 (Platinum RTD)
1. Best for chilled water & hot water lines
2. High accuracy, stable, long cable runs
3. Used in BTU meters & critical applications
b. 10kΩ Thermistor (Type II / Type III)
1. Most common for comfort sensing
2. Supply air / return air / mixed air / room sensors
c. 2.2kΩ Thermistor
1. OEM units, VAV boxes, general duct sensing
d. Ni1000
1. Mainly for legacy Siemens/JCI systems

Final Takeaway
1. Standards tell you HOW to measure temperature.
2. They don’t tell you WHICH sensor to choose.
3. Sensor type selection is always engineering-driven.

If you're designing, submittal reviewing, or commissioning BMS systems, understanding this difference helps you avoid project conflicts and ensures accurate control.

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