11
DRAINBACK-PROTECTED SOLAR COMBISYSTEMS
ANSWERS TO TEXTBOOK QUESTIONS AND EXERCISES
1. Unpressurized drainback-protected solar combisystems do not require an expansion
2. Because it conveys water directly from a tank open to the atmosphere, the collector
3. At the start of each solar energy collection cycle, the collector circulator must lift
water from the static water level in the tank to the top of the collector array. It must
4. Automatic air vents should not be installed at the top of drainback systems because
any ejected air will cause a drop in the system’s air pressure.
7. A single, variable-speed collector circulator is preferred to a double-pumped system
10. An automatic make-up water subsystem should not be used in a pressurized drainback-
protected system because air leakage may allow more water into the system. If such a
12. The difference between air elimination and air control is that air elimination seeks to
14. “Dead level” collectors or piping could freeze if the tubing sags between supports. A
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16. The black curve represents the net pump curve of the upstream circulator while it
pushes flow through the inoperable circulator. (The head loss curve is shown in blue.)
17. The maximum head loss curve is the sum of the lift head (15 ft) plus the total frictional
18. 1.58 inches
19. Use Equations 11.8a, 11.8b, and 11.8c. Assume the 30psi pressure relief valve refers
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continuously monitors the outdoor temperature sensor and calculates the target
temperature required by the distribution system, turning on and off the boiler as
needed to achieve the calculated temperature. The boiler is only used during the
heating season. The heat input to the storage tank during the cooling season comes
from the solar collector array. In combisystem #3, the outdoor reset controller
determines whether the boiler or the thermal storage tank will supply the space heating
load.
21. Using a heat exchanger between the tank water and space heating distribution system
22. When the outdoor temperature is 10°F, the boiler will turn on when the supply water
24. It is necessary because the external stainless steel brazed plate heat exchanger used to
26. At an outdoor temperature of 55°, the calculated target temperature for the supply