Temperature probe

A temperature probe is a sensor that measures the temperature of air, a fluid, or a surface and transmits this information to a control system (thermostat, BMS, PLC) to manage an HVAC installation: weather compensation, fan speed, defrosting, frost protection, alarms, etc. When properly chosen and placed, it provides a reliable measurement (accuracy, stability, response time) — an essential condition for consistent comfort and controlled energy consumption. If poorly installed, it tells a different story: inaccurate setpoints, cycling, and overheating.

Temperature probe: definition, technologies and uses

Current technologies

- RTD (Pt100 / Pt1000): very precise and stable, good for hydraulic networks and reference measurements.

- Thermistors (NTC 10k, 20k…): economical, reactive, widespread in ambient environments and on HVAC equipment.

- Thermocouples (K, J): large temperature range, useful in process/flue gas, less common in standard HVAC.

- Surface/contact probes : collar/magnet/plate for measuring pipes and heat exchangers without drilling.

- Ambient probes : wall-mounted unit or globe (operating temperature) for occupied areas.

Shape & assembly

- Immersion sleeve (well) on piping or tank to measure water without direct contact with the fluid (easy maintenance).

- Thermowell on CTA/air duct (sufficient depth, downstream of mixing).

- Outdoor probe (outdoor temperature) for water law: North/Northwest facade, in the shade, ventilated.

Key features : measurement range, accuracy (±0.1…±0.5 K), response time (τ), IP rating, compatibilities (cable, line length, 2/3/4 wire input), calibration and drift.

Selection, installation and integration: the right choices for a temperature probe

Selection

Adapting technology and precision to the use: Pt1000 for network supply/return, NTC for environments/equipment, globe probe if operational comfort is targeted.

Check PLC input compatibility (type, curve, 2/3/4 wires), temperature range, IP (outdoor/humid premises), cable (shielded if interference).

Provide an immersion duct (brass/stainless steel) of the correct length to be at the heart of the flow.

Laid

- Ambient : 1.2–1.5 m from the floor, representative area, away from sunny windows, radiators, hot/cold appliances and drafts.

- Hydraulics : probe downstream of mixers / exchangers, in main line rather than on bypass; thermal paste in the sheath.

- Aerodynamics : depth ≥ 1/3 of the duct, after mixing zones, moderate speeds.

- Exterior : cold, shaded, ventilated facade, excluding extraction exhausts / hot ducts.

Integration & maintenance

- Clean wiring (shielding to ground on the PLC side), reduced base, avoid ground loops.

- Periodic calibration/verification (ice point, reference comparison), GTB logging.

- On critical networks, double probe (safety) and consistency diagnosis (Ta/Tr deviations, supply/return).

- Take inertia into account : software filtering (moving averages, soft PID) to avoid yo-yoing.

Advantages, limitations and points to consider for temperature probes

Interests

• Comfort: instructions kept and stable (fewer complaints).
• Energy: reliable measurements → relevant water law/free-cooling, fewer cycles.
• Maintenance: deviations detected early (departure/return discrepancies, ambient vs operational).

Boundaries

• Drift/poor placement = persistent errors (over/underheating).
• Inappropriate response time (slow surface on fast process, “aspirated” environment) → unstable regulation.
• Interference (EMC, cable lengths) if unshielded cabling.

Points to consider

• Always document type/curve and position (DOE + GTB).
• Test after installation (comparison with reference), verify physical direction (e.g. start > return).
• Replace aging (drift) probes and protect sheath against corrosion/scaling.
• System coherence: To (operative) sensors in radiating zones; avoid a biased probe controlling an entire site.

Anecdote — "The probe that saw too much in Saint-Malo"

In Saint-Malo, an office building heated by an air-to-water heat pump fluctuated between lukewarm and overheating. The outdoor sensor, installed facing south (full sun on the cladding), was underestimating the temperature. After moving it to a northwest-facing location (shaded and ventilated) and recalibrating the heating curve, the heating outputs became consistent, the heat pump reduced its cycles, and the complaints stopped. The moral of the story: in HVAC, the right measurement in the right place is worth half a degree of adjustment to the setpoint.