CSLB C-46 Solar ContractorSolar Thermal System InstallationHard
A solar thermal installer is troubleshooting a closed-loop glycol system where the circulating pump often short cycles, turning on and off rapidly, even when there's a significant temperature difference between the collector and the storage tank. What is the most probable cause for this short cycling behavior?
- AThe pump is undersized for the system's hydraulic resistance.
- BThe storage tank's aquastat is set too low, reaching temperature too quickly.
- CThe differential controller's 'on' temperature differential is set too high.
- DThere is air trapped in the collector or sensor well, causing erratic readings.
Show answer & explanationAnswer & explanation
Correct answer: D. There is air trapped in the collector or sensor well, causing erratic readings.
Air trapped in a sensor well or collector can cause inaccurate and fluctuating temperature readings at the sensor. This erratic data can confuse the differential controller, leading it to rapidly turn the pump on and off (short cycling) as it struggles to interpret the false temperature differentials.
Why the other options are wrong
- A. An undersized pump would lead to low flow and poor heat transfer, but not necessarily rapid on/off cycling of the pump unless it's struggling to prime.
- B. A low aquastat setting would cause the tank to reach its set point quickly, leading to the pump shutting off, but not rapid on/off short cycling unless the tank cools just as quickly and the cycle repeats.
- C. A high 'on' differential would make the pump cycle less frequently, waiting for a larger temperature difference, not more frequently.
Differential Controller Short Cycling
Short cycling occurs when a solar thermal pump rapidly turns on and off, often due to control issues, sensor inaccuracies, or hydraulic problems, reducing efficiency and component lifespan.
- Can be caused by air in sensor wells or collectors.
- Incorrect differential settings or sensor placement are also culprits.
- Leads to increased wear on the pump and reduced system performance.
Memory trick: Erratic readings, pump's a-leaping.