ASK THE EXPERTS: Active or Passive Solar Hot Water?

Which type of solar hot water (SHW) system—active or passive—is more applicable for large commercial buildings? I would assume that the answer would be an active system, due to its greater efficiency and the flexibility in where the tanks can be located. If you could provide any insight, it would be greatly appreciated.

Will Mullins • via email

In the United States, most commercial SHW systems are active types, with the water stored in large tanks. Systems with large ground-level tanks are less expensive if the tanks are unpressurized, since pressure vessels in the United States must have an approval from the American Society of Mechanical Engineers (ASME). (Pressurized large tanks—more than 120 gallons—can be five to 10 times the cost of unpressurized ones.) Heat exchangers are used in the unpressurized tanks to transfer the heat to the building’s potable water.

Many other countries in the world don’t have the roof loading codes of U.S. buildings or the ASME requirements, so passive thermosyphon systems with tanks on the roof are very popular in nonfreezing climates. These systems use traditional flat-plate or evacuated-tube collectors racked on the roof with the tank above the collectors to allow the passive thermosyphon flow of water from the collector to the tank.

Chuck Marken • Home Power solar heating editor