ASK THE EXPERTS: Active or Passive Solar Hot Water?

Most commercial solar hot water systems are active
In the United States, most commercial solar hot water systems are active (pumped), instead of passive (thermosyphon).
Most commercial solar hot water systems are active

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 MarkenHome Power solar heating editor

Comments (4)

Doug Kalmer's picture

It would be considerably more efficient to put the domestic water copper coil inside the tank, as the tank is unpressurized, if any leaks developed, it would flow from the pressurized coil to the tank, no chance of contaminating domestic water. Your SHW lines from the collectors will have antifreeze, they should flow thru a coil in the tank, so you will have a double walled HX. Doug

Homepower Issue #129, Feb/Mar 2009 Pg 26 Has a letter From Chuck Marken, Solar thermal Editor, containing this paragraph from the Uniform Solar energy Code Section 405.1 Appendix D 3.2 -

"Single-wall heat exchangers shall be permitted if they satisfy all of the following requirements:

1) The heat transfer medium is either potable water or contains essentially nontoxic transfer fluids having a toxicity rating or class of 1.
2) The pressure of the heat-transfer medium is maintained at less than the normal minimum operating pressure of the potable water system.
3) The equipment is permanently labeled to indicate that only additives recognized as safe by the FDA shall be used in the heat transfer medium."

Fred Golden's picture

80 liters, that is a pretty small tank, I live in Portland Oregon, and have a 52 gallon 240 volt electric water heater. It is considered average size, and will use up to 60 gallons when 5 of us take our daily showers, more will get used by the dishwasher and clothes washer. This works out to about 80 to 100 gallons of water daily, or 320 to 400 liters.

I am using a heat pump to warm my water to 105F then a 240 volt 28 gallon water heater to heat it to my required 120F in my house now. This is saving me a lot of money, as the heat pump is collecting heat at around 8,000 Btu's per KW to 12,000 Btu's per KW (Winter VS Summer) instead of the older electric heater at only 3,400 Btu's per KW.

When I build a home, I will include a 60 to 90 tube evacuated tube solar heater and 200 to 400 gallon unpressurized steel tank. Wrapped around that tank will be copper tubing to warm my hot water, that will be heated to 120F by a electric heater or heat pump on colder days. This provides the required dual heat exchanger, so that the solar heated water will not mix with the domestic water inside the copper tubing wrapped around the steel tank.

I would have to agree that Americans take showers using upwards of 20 - 30 gallons per shower, that would be 80 to 120 liters per person per day. Some take two a day, or fill a tub with 45 gallons (185 liters) of water to soak in for only about 20 minutes, then dump all that water down the drain. We get more than 1 meter of rain each year in this area, so we can use lots of water and still have plenty left over.

Anders Yuran's picture

That is ofc not true. We live in Cyprus, have a passive system with a 80 liter tank on the roof. We can take as long HOT showers as we want


Europeans do not have any idea what an American means by a hot shower. Even the warm Mediterranean only gets 5 minutes of hot water from those devices. The units are small and they deliver small quantities of hot water.

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