SHI Case Analysis & Practical Application 2 — Questions and Answers
Question 1: A homeowner reports that their drainback solar water heater loses heat overnight even when the pump is off. What is the most likely cause?
- Insufficient insulation on the storage tank
- Check valve failure allowing thermosiphon reverse flow (Correct answer)
- Undersized expansion tank
- Incorrect tilt angle on collectors
Correct answer: Check valve failure allowing thermosiphon reverse flow
A failed check valve can allow cooler water to thermosiphon backward through the collector loop at night, draining heat from the storage tank.
Question 2: A closed-loop glycol system serving a radiant floor application shows a differential temperature of only 3°F across the collectors on a clear day. What is the most probable diagnosis?
- Collectors are properly sized and operating normally
- Flow rate is too high, reducing heat pickup per gallon (Correct answer)
- Collector array is undersized for the load
- Heat exchanger is oversized
Correct answer: Flow rate is too high, reducing heat pickup per gallon
An excessively high flow rate reduces the temperature rise across the collectors, resulting in a small differential temperature despite normal solar irradiance.
Question 3: During commissioning of a thermosiphon system, the installer notices flow occurs only intermittently. Which condition would cause this behavior?
- Storage tank is positioned too high above the collector
- Air trapped in the collector loop disrupting thermosiphon circulation (Correct answer)
- Glycol concentration is too high
- Expansion vessel is overcharged
Correct answer: Air trapped in the collector loop disrupting thermosiphon circulation
Air pockets in the loop create hydraulic resistance that can interrupt the gravity-driven flow of a thermosiphon system.
Question 4: A two-story building has solar collectors on the roof and a storage tank in the basement. The system uses a drainback design. What critical requirement must be met?
- Glycol must be used as the heat transfer fluid
- The pump must have sufficient head to lift fluid from the lowest drain-back point to the collector inlet (Correct answer)
- Collectors must face east to maximize morning gain
- A double-wall heat exchanger is mandatory
Correct answer: The pump must have sufficient head to lift fluid from the lowest drain-back point to the collector inlet
In a drainback system, the pump must generate enough head pressure to lift the heat transfer fluid from the drainback reservoir up to the collector level on every startup.
Question 5: A solar domestic hot water system in Phoenix, AZ is producing less than expected output in July. The storage tank reaches 160°F by noon and the pump shuts off. What is the most likely issue?
- Collector array is undersized
- High-limit control is shutting down the pump to prevent overheating (Correct answer)
- Freeze protection is activating incorrectly
- The heat exchanger is fouled
Correct answer: High-limit control is shutting down the pump to prevent overheating
When the tank reaches the high-limit setpoint, the controller shuts off the pump to prevent scalding and equipment damage, which is normal stagnation behavior in a hot climate.
Question 6: A customer's indirect solar water heater uses a heat exchanger coil inside the storage tank. After 5 years, output has declined by 30%. What should the installer check first?
- Solar collector glazing for dirt accumulation
- Glycol pH and inhibitor concentration for degradation (Correct answer)
- Pump motor amperage draw
- Expansion tank pre-charge pressure
Correct answer: Glycol pH and inhibitor concentration for degradation
Degraded glycol becomes acidic, deposits scale on the heat exchanger coil, and loses heat transfer efficiency — a common cause of long-term performance decline.
Question 7: An installer is sizing a solar water heating system for a family of four in Boston, MA. The design calls for a 2-collector system. Which factor most significantly reduces the required collector area compared to the same load in Miami, FL?
- Lower hot water consumption in cold climates (Correct answer)
- Higher solar irradiance in Boston during winter
- No factor — collector area requirements are identical
- Boston has higher irradiance in Boston during winter
Correct answer: Lower hot water consumption in cold climates
Seasonal usage patterns and occupancy behavior can reduce the effective daily load, but the primary driver of increased collector area in Boston is lower solar resource, not reduced consumption.
SHI Case Analysis & Practical Application 2
A homeowner reports that their drainback solar water heater loses heat overnight even when the pump is off.
What is the most likely cause?