• What Should I Do?

    Build a Solar-Powered Water Heater

    Solar water heater using a thermosiphon loop
    by bgarrett

    Wednesday, October 29, 2014, 9:18 PM

This is a very simple design for a batch-type solar water heater that uses a thermosiphon loop to move water between a solar thermal collector and a storage barrel. While it’s not the most efficient method for solar water heating and works rather slowly, it effectively demonstrates the operation of a thermosiphon and its connection to hot water storage.

The system can provide useful amounts of hot water for an outdoor shower, washing garden produce, keeping a biogas generator warm, or any number of other uses you can think of. Because it uses water, the system must be drained when the weather turns cold.

The parts list includes recommended materials and temperature ratings. Use materials that can withstand temperatures up to 180°F, such as metal, CPVC, or polypropylene fittings, as well as high-temperature water hose material, such as EPDM rubber. Black-colored hose will assist in absorbing solar heat. If you can’t find what you need at your local hardware store, plumbing supplier, or home center, shop online through industrial supply companies (such as Grainger or McMaster-Carr).


• Four 8-foot 2x4s
• Five 8-foot 1x4s
• 2 1/2″ deck screws
• 3/4″ roofing screws or galvanized sheet metal screws
• Two 2 x 8-foot corrugated metal roofing panels
• High-temperature, flat black spray paint
• One 100-foot, black EPDM rubber garden hose, 3/4″
• I.D. (inner diameter), rated for 200°F
• One hundred 8″ black plastic cable ties, UV-resistant, heat-stabilized, rated for over 200°F
• One 55-gallon barrel, black plastic, preferably with wide-mouth top
• Two 3/4″ NPT (National Pipe Thread) /GHT (garden hose thread) brass faucets, rated for 180°F
• Teflon tape
• Two 3/4″ polypropylene bulkhead fittings, rated for 180°F
• Two 3/4″ brass garden hose-to-tubing adapters
• Two hose clamps
• One 36″ length foam pipe insulation

1. Build the collector structure

Construct two A-frame supports using 2x4s and 2 1/2″ deck screws: Position the front leg of each support at the desired tilt angle for the collector panel (your latitude angle is a good starting point), and attach the rear leg at an opposing angle for stability. Join the two legs (front and back) of each side with a horizontal cross piece.

Space the A-frame supports about 6 feet apart, and join the two back legs with two diagonal 1x4s to keep the frame from racking. Install three 8-foot pieces of 1×4 horizontally across the supports. Locate one piece at the tops of the supports, one 24″ down from the tops, and one with its bottom edge 48″ from the tops. Fasten the 1x4s to the supports with the deck screws.

Mount the roofing panels to the wood frame, using 3/4″ roofing screws or sheet metal screws. Install the lower panel first, fastening it to the bottom 1×4. Install the upper panel so it overlaps the lower panel, then fasten through both panels into the center 1×4. Fasten the upper panel to the top 1×4.

Paint the top surface of the roofing panels with high-temperature, flat black spray paint (the kind used for painting wood stoves). Let the paint dry completely.


2. Install the collector hose

Lay out and mark the hose path on the collector panel, using full-length horizontal runs back and forth, working from the bottom of the panel to the top. Do not exceed the bending radius of the hose at the ends, as kinks in the hose will stop the flow and can lead to trapped air bubbles. A 3/4″ I.D. hose should allow for eight horizontal runs across the 4-foot-tall panel. Be sure to leave extra hose at the beginning and end for connecting both ends to the storage barrel.

Note: As the water in the hose heats up, air will be released from it. Keeping the hose runs reasonably level, with no kinks or sags, allows the thermosiphon to work effectively, eliminates trapped air bubbles, and facilitates draining the collector.

Secure the hose by drilling pairs of holes through the collector panel, one above and one below the hose, then loosely fastening the hose with a plastic cable tie (zip tie). Add a tie about every 12″ along the entire path of the hose. Make sure the hose is in full contact with the collector panel for best heat transfer. You will tighten the ties later, after the hose is connected to the barrel and all fits well.

3. Prepare the water storage barrel

The barrel gets two threaded bulkhead fittings to provide a watertight connection through its side. The fittings receive the tapered threaded ends (not the garden-hose ends) of the faucets that will connect with the collector hose. Make sure the faucets are compatible with the bulkhead fittings and the adapters for connecting the collector hose.

Cut a hole through the barrel for each fitting, using the appropriate size of hole saw. Position the lower hole as close to the bottom of the barrel as possible, and locate the upper hole about one-third of the way down from the top of the barrel. Choose areas with no raised markings and little curve in the barrel surface to ensure a watertight seal.

Wrap the threaded end of each faucet with Teflon tape, and thread it into the exterior half of a fitting. Fit each fitting into its hole and secure it inside the barrel with its nut.

4. Set the barrel and connect the hose

Position the barrel on a sturdy stand that is tall enough so that the faucets on the barrel are higher than their corresponding hoses mounted on the collector. Position the collector close to the barrel. Extend the lower end of the hose to the lower faucet, and cut the hose to length so it makes a smooth upward arch toward the faucet. Make sure the hose doesn’t sag, which can trap air or create a “heat trap,” stopping the thermosiphon action. Cut the upper hose end to connect to the upper faucet.

Install a tubing-to-faucet adapter on each end of the hose, securing it with a hose clamp. Thread the hoses onto the faucets. Tighten the cable ties on the collector panel so they hold the hose securely in place. Put a piece of foam pipe insulation on the hot water (top) hose to help reduce heat loss and increase the effectiveness of the thermosiphon.

5. Get started making hot water

Open the faucet valves, and fill the barrel to the top, leaving 1″ or 2″ of air space for expansion. You want to be sure that there is no air in the collector loop. Tilt the collector back and forth (one tilt for each hose bend) after the barrel is filled to be sure that all the air is out of the hose.

You’ll start making hot water as soon as the sun comes out. Cold water sinks to the bottom of the barrel and continues down the hose to the bottom of the collector. As the water is heated it rises up through the hose, through the top faucet, and into the barrel, where it rises to the top. There will be a noticeable temperature stratification within the barrel until the water is completely heated.

As an example, on a sunny 45°F day, I achieved a 40°F temperature rise through the collector loop using about 75 feet of 3/4″ hose laid out on the collector, plus another 8 feet leading to and from the storage barrel. This was at a fairly low flow rate, and resulted in a 10°F per hour temperature rise within the stored water during the hours just before and just after noon.

Ideas for Upgrades

With some additional effort, you can increase the efficiency of the system and even bring the hot water you make into your home, where it can preheat the water in your existing water heater. Check local plumbing codes before modifying your water heater. Using a heat exchanger in between your solar collector and water heater means you won’t be putting potentially non-potable water into the water heater.

• Insulate the water storage barrel to keep the water hotter for longer periods.
• Enclose the collector in a box for hotter water. Insulate the box on the bottom, and cover it with a piece of UV resistant clear Plexiglas or flexible fiber-reinforced plastic (FRP). Note: Enclosing the collector may make temperatures exceed the materials’ ratings, requiring the use of all metal components.
• Collect and store more solar energy by increasing the collector area and the length of the circulation hose.
• Switch to an active system with the addition of a small solar-powered 12- volt DC pump that moves less than 2 gallons per minute; this frees you from the constraints of a thermosiphon system.

Optional equipment for water circulation is available through local and online renewable energy dealers. This includes a low-wattage, 12-volt DC circulator pump that can operate from a 5- or 10-watt solar panel. If you make this investment, you will also want to spend a bit more to insulate the storage barrel and enclose the collector.

Interested in reading more tutorials like this? This project was used with permission by Paul Scheckel from his successful book “The Homeowner’s Energy Handbook: Your Guide to Getting Off the Grid” If you’re interested in getting his book, click here.

~ Brandon Garrett

Brandon Garrett is a preparedness consultant and team member of The Ready Store.  He writes informative articles and information for the ReadyBlog, the Ready Store's blog and educational section pertaining to topics of the economy, resiliency, and preparedness issues. 

Full disclosure: Based on our existing relationship with The Ready Store, PeakProsperity.com will receive a small commission as an affiliate for purchases made through the Ready Store. This will not impact the price you pay and the proceeds we received will be immediately invested to fund new features and functionality for this site.

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  • Fri, Dec 05, 2014 - 9:55pm



    Status: Bronze Member

    Joined: Feb 28 2013

    Posts: 339


    This is great....I saved our

    This is great....I saved our old hot water heater when we got a new one earlier this year with the intention of building a frame around it with glazing on the front to pre-heat water for our existing heater.  I haven't gotten to it yet in part because plumbing makes me nervous and in part because the logical place for it is right in front of our house and I can't get signoff from the wife for an eyesore like that.

    I might build something along these lines and stick it out in the back 40.....just in case.


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  • Sat, Dec 06, 2014 - 3:29pm



    Status: Bronze Member

    Joined: Aug 11 2010

    Posts: 195


    This looks so easy and so useful!

    I am going to print this one out and try it for next summer.  Maybe I can turn off my it water tank during the hot months, or create an outdoor shower room for my future tiny house guest cabin.  I like that it looks easy even for a non-technical person like me.



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  • Sun, Dec 07, 2014 - 2:51pm


    Taz Alloway

    Status: Bronze Member

    Joined: Feb 18 2010

    Posts: 464


    Temperature monitoring is critical for safe hot water

    The bacterium that causes Legionnaires disease loves warm water. People are exposed when they use the water. The bacterium: Legionella, is now the number one cause of waterborne outbreaks: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6235a3.htm

    Here is a description of the disease:


    WHO has prepared a document on risk factors and control of the bacteria: http://www.who.int/water_sanitation_health/emerging/legionella.pdf

    "The document also identifies necessary measures to prevent, or adequately control, the risk of exposure to Legionella bacteria for each particular environment. Outbreaks of legionellosis generally cause a high level of morbidity and mortality in the people exposed; therefore, the suspicion of an outbreak warrants immediate action. "

    "...naturally occurring L. pneumophila survived and multiplied in water at temperatures between 25 ºC and 45 ºC, with an optimal temperature range of 32–42 ºC. The study also found that legionellae were most commonly isolated at temperatures between 35 ºC and 45 ºC, with the greatest increase in viable counts occurring between 37 ºC and 42 ºC (Wadowsky & Yee, 1983; Schulze- Robbecke, Rodder & Exner, 1987). As the temperature falls below 37 ºC, the bacteria’s reproductive rate decreases and there is little or no increase in numbers of bacteria below 20 ºC. Therefore, to prevent Legionella infection, the recommended temperature for storage and distribution of cold water is below 25 °C, and ideally below 20 °C. Recent laboratory studies of mutant Legionella strains show that the bacteria may grow below 20 ºC under certain conditions (Soderberg, Rossier & Cianciotto, 2004). Legionella will survive for long periods at low temperatures and then proliferate when the temperature increases, if other conditions allow."


    Stay safe!


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