CSLB C-46 Solar ContractorSolar Thermal System InstallationEasy
A solar thermal installer is performing a system startup on a new closed-loop glycol system. After filling and purging air, the differential controller is turned on. The collector temperature is 160°F and the storage tank temperature is 90°F. The controller's ON differential is set to 15°F and the OFF differential is set to 5°F. What should the pump do, and why?
- AThe pump should remain off because the tank temperature is too low.
- BThe pump should cycle rapidly due to a short-circuit in the sensor wiring.
- CThe pump should turn on because the temperature difference exceeds the ON differential.
- DThe pump should turn off because the collector temperature is too high.
Show answer & explanationAnswer & explanation
Correct answer: C. The pump should turn on because the temperature difference exceeds the ON differential.
The pump turns on when the collector temperature significantly exceeds the tank temperature by the 'ON differential' setting. Here, 160°F - 90°F = 70°F, which is much greater than the 15°F ON differential, so the pump should activate to transfer heat.
Why the other options are wrong
- A. The tank temperature being low is precisely why the solar system should be active to heat it.
- B. Rapid cycling due to a short circuit is a troubleshooting scenario, not the expected behavior under these normal operating conditions.
- D. A collector temperature of 160°F is well within normal operating range for heat transfer and does not indicate a condition for the pump to turn off, especially with a cold tank.
Differential Controller Logic (ON)
The operational rule for a solar thermal differential controller to activate the circulating pump when the collector temperature exceeds the storage tank temperature by a set 'ON' differential.
- Compares collector and tank temps
- Activates pump when ΔT > ON setting
- Transfers heat from collector to tank
- Prevents reverse thermosiphon
Memory trick: Collector hot, tank is cold; differential met, pump takes hold.