CSLB C-46 Solar ContractorSolar Thermal System InstallationHard
A solar thermal installer is designing the piping for a drainback system where the collectors are located on the roof and the storage tank and drainback reservoir are in the basement. To ensure proper drainage of the collectors at night or during stagnation, what critical piping component must be included and properly sized?
- AA tempering valve on the hot water outlet.
- BA vacuum breaker at the highest point of the collector array.
- CA check valve in the collector supply line.
- DA pressure relief valve on the drainback reservoir.
Show answer & explanationAnswer & explanation
Correct answer: B. A vacuum breaker at the highest point of the collector array.
In a drainback system, when the pump turns off, the fluid drains back to the reservoir by gravity. For this to happen effectively, air must be able to enter the top of the collector loop to break the vacuum that would otherwise hold the fluid in the collectors. A vacuum breaker at the highest point precisely serves this purpose.
Why the other options are wrong
- A. A tempering valve is for scalding prevention in DHW, not for drainback system function.
- C. A check valve in the collector supply line would prevent fluid from draining back, which is the opposite of what's desired in a drainback system.
- D. A pressure relief valve on the drainback reservoir would protect against overpressure, but it does not facilitate the draining function itself.
Drainback System Vacuum Breaker
A device installed at the highest point of a drainback solar thermal collector array that allows air to enter the system, breaking the vacuum and enabling the heat transfer fluid to drain back to the reservoir.
- Crucial for proper drainage
- Installed at highest point
- Prevents fluid from being held in collectors
- Essential for freeze and stagnation protection
Memory trick: Drainback's secret, a vacuum's end; air in, fluid descends.