CSLB C-46 Solar ContractorSolar Thermal System InstallationMedium
A solar thermal system installer is troubleshooting a closed-loop glycol system where the circulating pump is running continuously, even when the collector temperature is below the storage tank temperature. The differential controller is set correctly for a 10°F turn-on and 3°F turn-off delta-T. What is the most likely cause?
- AOversized heat exchanger
- BInsufficient system pressure
- CA shorted collector temperature sensor
- DAir trapped in the expansion tank
Show answer & explanationAnswer & explanation
Correct answer: C. A shorted collector temperature sensor
If the pump runs continuously even when the collector is colder than the tank, it indicates the controller is receiving false information. A shorted temperature sensor (or sensor wire) would typically read a very low resistance, which the controller interprets as a very high temperature, thus continuously trying to circulate fluid.
Why the other options are wrong
- A. An oversized heat exchanger would improve heat transfer efficiency, not cause the pump to run continuously when the collector is cold.
- B. Insufficient system pressure might cause pump cavitation or flow issues, but it wouldn't directly trick the controller into misreading temperatures and running continuously.
- D. Air in the expansion tank would affect system pressure and potentially pump cavitation, but not directly cause the controller to misread temperatures and run continuously when it shouldn't.
Shorted Temperature Sensor
A fault in a thermistor-type temperature sensor or its wiring where the electrical resistance drops to near zero, causing the controller to interpret an artificially high temperature.
- Leads to erroneous temperature readings
- Can cause continuous pump operation
- Requires testing sensor resistance or replacing
Memory trick: Pump runs wild, temps are lies; sensor's shorted, the system cries.