A solar thermal installer is conducting a system startup and observes that the circulating pump frequently turns on and off in short bursts, even though the collector temperature is consistently 20°F higher than the tank temperature. The differential controller's ON setting is 12°F and its OFF setting is 4°F. What is the most likely cause of this 'short cycling' behavior?
- AThe tank temperature sensor is reading too high.
- BThe flow rate through the collector loop is too low.
- CThe expansion tank in the solar loop is undersized.
- DThe collector temperature sensor is reading too low.
Show answer & explanationAnswer & explanation
Correct answer: B. The flow rate through the collector loop is too low.
Short cycling, where the pump turns on and off rapidly despite a sufficient temperature differential, is often caused by low flow. With low flow, the fluid in the collector heats up very quickly, reaching the ON differential. However, as soon as the pump starts, the small volume of hot fluid is quickly moved out, and cooler fluid enters, causing the differential to drop below the OFF setting almost immediately, thus turning the pump off again.
Why the other options are wrong
- A. If the tank sensor read too high, the differential would appear smaller, leading to the pump turning on less frequently or not at all, not short cycling.
- C. An undersized expansion tank primarily affects system pressure stability (overpressure/underpressure), not directly causing rapid on-off cycling based on temperature differentials.
- D. If the collector sensor read too low, the pump would either not turn on at all or would turn on less frequently, not short cycle.
Solar Pump Short Cycling
A condition where a solar thermal circulating pump rapidly turns on and off, often due to insufficient fluid flow, leading to inefficient heat transfer and premature pump wear.
- Pump turns on/off rapidly
- Commonly caused by low flow
- Can result from air locks, closed valves, or undersized pump
- Reduces system efficiency and component lifespan
Memory trick: Pump goes 'on!', then 'off!', then 'on!' again; low flow's the culprit, a heating pain.