Electrical & Logic | Module 218
Thermistor Resistance Audit
Field Status: Most HVAC controllers utilize NTC (Negative Temperature Coefficient) thermistors. As temperature rises, resistance drops. “Thermistor Drift” occurs when the internal chemistry of the NTC sensor ages, causing the resistance curve to shift. A 5°F drift in an evaporator thermistor will cause the logic board to miscalculate superheat, leading to EEV hunting or icing.
Use the audit terminal to compare measured resistance against the expected value at ambient temperature.
SYS AUDIT: NTC Curve Analysis
AUDIT STATUS: —
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NTC Curve Dynamics
[Image of NTC thermistor resistance vs temperature curve]The resistance-to-temperature curve is non-linear. The most critical point for diagnostics is the “inflection point,” where small changes in temperature cause massive swings in resistance.
| Variance % | Diagnostic Meaning | Action |
|---|---|---|
| 0-5% | Precision Tolerance | System Normal. |
| 5-10% | Marginal Drift | Monitor or replace. |
| >10% | Failed Component | Replace Thermistor. |
Diagnostic Gateway Challenge
You test a 10kΩ thermistor. The resistance measures 7kΩ at 25°C. Is this sensor performing within the industry-standard 10% tolerance?