A DIY Thermosyphon Water Heating System


This is a very nicely done and uncomplicated thermosyphon solar water heating system. 


No pumps, no controllers -- just sun and gravity.


New: Update August 2011...

Thanks very much to Lonnie for sending the pictures and description.

For more DIY Solar Water Heating systems...



System schematic -- Thermosyphon Solar Water Heater

As shown in the schematic, when sun is on the Solar Panel, the water in the panel is heated, becomes less dense and rises up into the Storage Tank.  The heated water leaving the panel is replaced by cool water flowing from the bottom of the Storage Tank into the lower connection on the collector.  This continuous natural circulation of the water through the collector and tank heats the tank water over time. 


Sun and gravity take care of the circulating the water, so no pump is needed.  Control is automatic, as flow stops as soon as the collector cools below the tank temperature.


The system as shown here does not have freeze protection, so it must be used in non-freezing climates, or be drained for the winter. It is possible to use thermosyphon systems in freezing climates by circulating antifreeze through the collector and using a heat exchanger to transfer the collected heat to the storage tank, but this does add some complexity.



Building the Collector

I used a roll of aluminum 10 " wide, which I cut to length and fit under the tubes. I then added the fins over the tubes and roll aluminum.  This fin design seems to give better heat absorption. This is all held together with stainless steel screw and washers.


There is a 3/4 insulation sheet behind the plywood and another thinner plywood on the back to protect the insulation sheet.


There are two sheets of sun tuf corrugated polycarbonate on top.


The collector box/frame

Collector manifold using 3/4 by 1/2 inch
reducing T's to adapt from 3/4 inch manifold
to half inch risers.

The copper grid was first laid down over
aluminum sheet metal.  The fins with grooves
to fit over the copper tubes where then
fastened down with stainless steel screws.



In this picture, the grooves have been formed in
the fins, and the fins have been pressed on over
the copper pipe grid.

Detail showing 1) flat alum backing strip under tubes,
2) grooved aluminum fin over tubes, and 3) the
stainless steel screws and washers
holding  it all together. 
(note that the blue wires and support block were
for a test and are not part of the finished collector)

The absorber painted with flat black high temperature
BBQ paint.  Ready for glazing.



For more more construction details on on a similar type of collector construction here...  and here...



Collector Installation


Outside plumbing for the collector.
Note that all the plumbing slopes up toward
the storage tank (which is just behind
the wall and above the collector)
Especially in cold climates, insulating the
pipes would be a good idea.

Flex pipe thru wall in case panel should move.

A valve at bottom of collector allows
collector to be drained for freezing weather.


The panel is drained for snow and freezing temperatures.

Some more pictures of the finished collector.



Storage Tank and Plumbing

The large tank is a new 85 gal elect, the storage tank is 50 gal. and was my old gas water heater tank. The shut off's are shown with the elect. heater in use only. When I move the three shut off levers then hot water will flow from the storage tank into the cold side of the electric  tank. The solar panel gets it's cold water from the old drain outlet on the storage tank, the hot water from the panel goes into the old poppet valve outlet. I am installing a poppet valve into the storage tank and the solar panel, also temp sensing gages on both.  There should be no problem with overheating in the winter but summer temperatures will have to be monitored closely as it can get up into the 100's here.



Click on pictures for full size


This is the old gas hot water tank that was
converted to a solar heated water
storage tank.

The solar storage tank for a thermosyphon
system must be located above the collector
for thermosyphoning to occur.



The pictures above show the hot water storage tank that is heated by the collector, which is located just on the other side of the wall where the two pipes go out.  For thermosyphon systems, the collector must be located below the tank level, and the plumbing lines must slope upward from the collector toward the tank in order to establish the thermosyphon flow.



Click on pictures for full size

The new electric backup tank.

The three valves to control whether thermosyphon
solar water heater is in or out of the system.


The pictures above show the new backup hot water tank.  Preheated water from the solar storage tank comes into the cold water inlet for this tank.  This backup tank provides any additional heat that may be required to get up to the target temperature. 



Today I insulated all the pipes in the system inside and outside with poly pipe insulation and alum. tape. I plan to add a thermal blanket onto the storage tank tomorrow. This should keep any heat from getting out of the thermosyphon heating system.

With insulation applied to all the plumbing runs.

The newly placed insulation on the
outside lines from the collector to the tank.

Insulation close up.



Results To Date

Finally got thru our winter and back onto my hot water system. It has been up and running for about a week and performing just great. The temperature is usually around 140 degrees going into the storage tank. Temperature holds all night. The elect. tank is turned off and we have hot water to do dishes, clothes, and showers. Completely automatic.


Hot Water !!


The outdoor temperature here has been in the upper 80's and low 90's. I will let you know if higher temp in July / Aug. have much effect on the water temperature.



Update August, 2011

These are some updates and suggestions from Lonnie after about a year of operation:



I continue to get e-mails on the Thermosyphon system from all around the world. Just wanted to thank you for the great site at build it solar. System is working and has had no problems.

Several things I did not mentioned in the article, that are extremely important:

  1.  The solar panel must be tilted to an angle equal to builders latitude.
  2.  No air can be in the solar panel line.
  3.  The bottom of the storage tank needs to be above the solar panel.

These question have come up from the e-mails I have received. I have received question about using diesel truck gas tanks, PEX tubing, insulation and many others from Africa, South America, France and some places I have never heard off.

Thanks again, 







May 26, 2010


Lonnie will answer email questions at:  lh.smith93 AT yahoo DOT com    (replace AT with @ and DOT with a period)



Casey -- supervising the work.




Gary  May 26, 2010