SHI Risk Assessment & Mitigation 2 — Questions and Answers
Question 1: Which property of propylene glycol heat transfer fluid makes it preferable over ethylene glycol in solar thermal systems?
- Higher thermal conductivity
- Lower freezing point
- Lower toxicity if leaked (Correct answer)
- Greater pressure tolerance
Correct answer: Lower toxicity if leaked
Propylene glycol is preferred because it is non-toxic, making incidental contact with potable water or skin far safer than toxic ethylene glycol.
Question 2: A solar collector reaches stagnation temperature on a hot summer day. What is the PRIMARY risk this creates in a glycol-filled system?
- Glycol freezing inside the collector
- Glycol degradation producing acidic compounds (Correct answer)
- Increased UV exposure to the manifold
- Reduced flow from the circulation pump
Correct answer: Glycol degradation producing acidic compounds
Stagnation temperatures can exceed 200°C, causing glycol to break down into acidic byproducts that corrode system components.
Question 3: When pressure-testing a newly installed solar thermal loop, what fluid should be used to safely detect leaks before commissioning?
- The specified glycol mix at operating concentration
- Clean potable water
- Compressed nitrogen (Correct answer)
- Air from a tire inflator
Correct answer: Compressed nitrogen
Nitrogen is inert, dry, and non-damaging, making it the safest medium for pressure testing solar thermal piping before fluid fill.
Question 4: An installer notices the expansion vessel pre-charge pressure is lower than the system static head. What risk does this create?
- The vessel will absorb too much fluid and cause low pressure
- The bladder will not compress, leading to pressure spikes that open the relief valve (Correct answer)
- Freeze protection will be reduced at night
- The collector efficiency will drop below rated values
Correct answer: The bladder will not compress, leading to pressure spikes that open the relief valve
If pre-charge pressure is less than static head, the bladder stays fully compressed and cannot absorb thermal expansion, causing the pressure relief valve to discharge repeatedly.
Question 5: What is the correct response when an installer finds that a pressure relief valve on a solar thermal system is weeping continuously?
- Tighten the valve cap to stop the leak
- Replace the relief valve and investigate the cause of overpressure (Correct answer)
- Increase the expansion vessel size only
- Reduce the glycol concentration to lower boiling point
Correct answer: Replace the relief valve and investigate the cause of overpressure
A weeping relief valve signals a real overpressure condition; the valve must be replaced (it may be damaged) and the root cause—such as undersized expansion vessel—must be corrected.
Question 6: Which chemical hazard is specifically associated with draining a solar thermal system that has been in service for several years without fluid maintenance?
- Chlorine gas release
- Exposure to acidic, degraded glycol with low pH (Correct answer)
- Ammonia fumes from bacterial growth
- Carbon monoxide from fluid oxidation
Correct answer: Exposure to acidic, degraded glycol with low pH
Long-service glycol degrades into organic acids with pH as low as 4–5, which can cause skin and eye irritation during draining.
Question 7: A solar domestic hot water system is set to deliver water at 60°C (140°F) to prevent Legionella growth. What additional risk does this temperature introduce?
- Increased risk of glycol freeze in the collector loop
- Scalding hazard at fixtures, requiring thermostatic mixing valves (Correct answer)
- Higher UV degradation of collector glazing
- Reduced heat exchanger efficiency
Correct answer: Scalding hazard at fixtures, requiring thermostatic mixing valves
Water stored or delivered at 60°C can cause rapid scalding; thermostatic mixing valves at point of use are required to blend water down to a safe delivery temperature.
SHI Risk Assessment & Mitigation 2
Which property of propylene glycol heat transfer fluid makes it preferable over ethylene glycol in solar thermal systems?