Solar Water Heater From Scratch




About: I'm a web developer and I enjoy outdoors a lot

Hi there!

Five years ago when I moved to Sonora (the most sunny state in Mexico) I decided to build a solar water heater (SWH) and save money and CO2 on fossil energy.

It took me 6 months to figure out which system to choose according to the local climate, to size the collector and the tank and to design the 3D taking into account the material available locally.

I built it on my free time and 2 months later I had it installed and producing year-round crazy-hot water for the whole family!

Since then it never stopped heating water without problem, saving tons of $$ and CO2!

At that time I documented almost every step in the conception and construction process. I'm writing this Instructable so anyone can built one on his own, adding some enhancements based on experience of those 5 years of use.

You shouldn't spend more than 400USD using this design.

If you like this Instructable please vote in the Solar Contest 2016!


Step 1: Sizing, Modelling and Designing

This step is crucial. The results you will get will depend on how seriously you performed the sizing and the design.

The design I chose is direct and passive, see Wikipedia, it uses the principle of thermosiphon which is based on the density differential of the water when it's heated.

This design presents some drawbacks and advantages but it's particularly adapted to the climate in the region I live.

First thing: the 3D model. It is important to have an idea of the material that you can find locally in order to create your 3D model. I used Sketchup to create it (enclosed). This way I can compute some of the data requested below.

To size this design I found some good information and a very helpful spreadsheet on Hervé Silve's website. I built my spreadsheet around it (enclosed).

Here is how to fill it: *fill only clear-blue cells*

  • In the "INSTALLATION" tab:
    • Column B: enter the average need of hot water per person and per day in litter
    • C: the average number of person
    • D: the average hot water storage temperature you need
    • E: the diameter of the pipes you chose to use for this project (1/2" is fine)
    • F: the pipe material you chose to use for your collector (copper is better)
    • H: the total pipe length used
    • Check that columns I and J are green at all time (red is bad)
  • In the "collector" tab:
    • B11: the optical efficiency of the transparent material that will cover your collector, this value is ok for glass.
    • C11: this is the average temperature of the inside of the collector. This is an approximation because you don't really know it until you make the SWH...
    • D11: the outdoor temperature in the worst case scenario
    • E11: the heat loss coefficient due to conductivity of the material (current value is fine for glass)
    • F11: the heat loss coefficient due to convection through the material (current value is fine for glass)
    • G11: solar radiation on your location (See
    • J11: optimum angle for the coldest month of your location (See
    • K11: coldest month (information only)
    • L11: solar energy received during a day over a square meter (See
    • E14: average temperature of the cold tap water in your house in the coldest month
    • G14: global efficiency of your SWH. This is an approximation because you don't really know it until you make the SWH...
  • In the "tank" tab:
    • Column F: desired temperature of hot water
    • H: a safety coefficient, this should cover all the approximations we've done.
  • You can leave the "geometry, "losses" and "Mv" tabs as is.

Basically, this spreadsheet will allow you to obtain (tab "INSTALLATION"):

  • the minimum height "H": cell G3
  • the tank capacity you need: cell K3
  • the collector area: cell L3

Feeding those values in your 3D model you should be able to have the final design.

Step 2: Build the Collector Box

The list of material will strongly depend on your design and sizing. Basically I used the following:

  • 1 plywood sheet (12mm x 4' x 8')
  • 1 R5 rigid foam insulation sheet (1" x 4' x 8')
  • 16sqft of reflective foam
  • glue
  • screws (1")
  • water proofing sealant

The first picture shows how to cut the plywood in order to minimise the scrap.

Assemble the box as show in picture #2 and 3. The inner borders are lower than the outer ones, the glass window will rest on top of it. Put 2 or 3 layers of water proofing sealant.

Glue the insulation into the box and cover it with the reflective foam.

Step 3: Build the Captor

Material list:

  • 6 copper pipes (1/2" x 3m)
  • 2 copper elbows (1/2" x 90°)
  • 2 copper coupling (1/2")
  • 26 copper Tee (1/2")
  • 1 copper cap (1/2")
  • Copper solder and paste flux
  • Plywood scrap from previous step
  • 1 iron 24 gauge laminate sheet (4' x 4')
  • 1 iron rod (1/2")
  • 60 self drilling hex screws
  • 1 cartridge of transparent silicone caulk
  • 2 high heat matt black paint spray cans
  • sand paper

Cut several sections of copper pipe:

  • 28 of 68mm long
  • 12 of 1.07m long
  • use the remaining to for 2 more 1.07 long with the couplings

Sand each pipe outer extremity and elbows and coupling inner extremity.

Assemble as described in picture #2. Solder and test for leaks (pics #3, 4 and 5).

Cut fourteen 1m x 3" strips from the laminate sheet.

Create a mold to bend the sheets using the plywood and the 1/2" iron rod.

Bend the laminate strips to create the fins: you will need some clamps and a heavy hammer. This takes some time (it took me one day) so either be patient or find someone that has the machinery to do it for you.

The fins will be rigged to the captor as in picture #8, to prevent air insulation between the fin and the copper as well as electrolytic premature oxidation, put a layer of silicone in the concave part of the fins.

Rig the fins to the copper tubes using unfolded pieces of copper tube and self drilling screws (picture #9).

Paint each side with a very thin layer using the spray cans. Don't put to much paint because it could act as a insulant. Choose black and matt so it will absorb most of the infrared rays from the sun (heat).

Step 4: Install the Captor Inside the Collector Box

Material list:

  • some 1" x 1" wood scrap
  • 6 pipe fasteners (1/2")
  • screws (1")
  • 2 copper-PVC couplings (1/2")

Drill 2 holes on the sides of the box, this is where the captor input and output go.

Insert the captor inside the box and mark where the fasteners could be placed. Remove the captor and cut the foam at those places so you can use some 1" x 1" wood to anchor the fasteners. Screw the fasteners and cover them with some reflector.

Insert the captor and tighten the fasteners. Insert the couplings at each end of the captor.

Step 5: Tank

Material list:

  • A custom made stand (I had my father in law helping me with that, be creative! haha!)
  • A recycled water heater tank (110L)
  • Fiberglass insulant.
  • 1 roll of clear stretch film wrap

We need to create a well insulated tank to store the water heated during the day so it doesn't loose too much heat during the cold nights of winter. This way you will still be able to take a warm shower in the morning.

The stand must be made so the "H" value is greater or equals to the one described in the first tab of the spreadsheet. This is very important, the natural flow of the water depends on it. Forget about the wooden things in the pictures, trial and error your know...

Place the stand so your collector will face south and no shade will cover it at any time of the year (as much as possible). Lay the tank at the top of it. The uppermost connector must be on the side of the collector output (uppermost end).

Lay some fibreglass around the tank and maintain it with the plastic wrap.

Attach the collector to the stand so it forms a 45 degrees angle with the vertical. It is important that you give the collector a slight tilt so the output is a bit higher, this will prevent water stagnation in the "horizontal" copper pipes.

Step 6: Connections

Material list:

  • A tarp (size depends on your tank)
  • PVC tubing (1/2") (qty depends on your stand design)
  • PVC glue
  • Threaded adapters (qty depends on your tank inlets)
  • 1 PVC tee (1/2")
  • Elbows (1/2") (qty depends on your stand design)
  • Pipe wrap insulation (3/8" x 1/2" x 6')
  • Duct tape
  • 1 can of polyurethane foam spray
  • Flexible water lines (1/2") (qty and length depend on your house)
  • 1 check valve (1/2")
  • 1 clear glass sheet (5mm x 1.185m x 1.21m)
  • Some transparent silicone caulk
  • 3 control valves (1/2")

Using the PVC pipe and fittings connect the uppermost tank inlet to the uppermost collector end. Do the same to connect the lowermost tank inlet to the other collector end but this time add a tee and a control valve so you can drain entirely your SWF by gravity in case of needing some repairs.

Use some more PVC pipe, fittings and flexible water lines to connect your house's cold water and hot water grids to the tank. It is important that the cold water enters by the lowermost remaining tank inlet so it won't disturb the heat gradient inside the tank. Also the hottest water is laying at the top of the tank, place where you want to collect hot water for your house.

On the house-tank cold line place a check valve so the tank pressure wont go back in the cold grid.

You also might want to put some control valves to your house's connections so you can stop a potential leak in the future.

Seal the glass sheet over the collector with the silicone caulk.

Inject some polyurethane foam to fill the gap between the captor connections and the box.

Cover the insulated tank with a tarp so the moisture won't reduce the insulation capacity.

At this point you should be good to go!

Step 7: Conclusion

Usually it doesn't take much more than 2hrs in summer to get a nice and warm shower :)

You already understood that the main advantages of this SWH are:

  • Produces completely free hot water $$
  • No CO2 emission
  • Year-round hot water available

Now the drawbacks are:

  • This design won't work in freezing temperature and will be damaged if the water gets to freeze.
  • Needs some maintenance time to time (base and box coatings, leaks, glass cleaning, drainage etc.)

If you have any question feel free to comment and I'll do my best to answer and add some modifications.

I hope you enjoyed this Instructable!

Thanks for reading.

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    40 Discussions


    Question 6 weeks ago

    How do you avoid getting legionellas?

    1 answer

    Question 11 months ago on Step 2

    Could you please tell me the weight of the solar collector?


    1 answer

    Reply 11 months ago

    It is hard to tell, my guess would be that the collector only should weight about 50kg.


    1 year ago

    do you have social media e.g facebook/google+? i'd like to ask further more due to the tight area of my rooftop. i've also drawn it on sketchup

    Left-field Designs

    2 years ago

    great instructable, this is something that I have had in my head to do for some time now. I have built a space heater from beer cans that works great. one thing that always put me off this was that we have a regulation in ireland, I'm not sure if it stands elsewhere, that stored hot water must be brought above 70 degrees c at least once a week. this is to kill off any chance of legionaries developing in the water. if I wanted to stay fully off grid which I do then that would mean a substantial solar and battery arrangement to run an electric water heater for long enough to achieve this.


    2 years ago

    Looks complex but doable!


    2 years ago

    well done


    2 years ago

    I've built solar water heaters like that, but what I found worked better, and can tolerate freezing better- is to put a bare hot water tank. painted flat black. under glass in a insulated box. you can still thermosiphon to a second tank if you want. but all you do is input the cold water at the bottom of the solar collector tank. the water would reach +60c fairly easy on a sunny day

    5 replies

    Reply 2 years ago

    Hi, thank you for your comment!
    You wrote "worked better" which means that you compared both designs. Could you add more details of that comparison please?
    I usually get +94C at the output of the collector on a sunny day and +70C at the hot water tap.



    Reply 2 years ago

    oops I guess I press wrong one
    sure the tank solar water heater worked better from my experience-
    example your radiator style has 15 pipes that is about 7 inches in absorbtion area, plus a alittle from the tin backing the rest is from the internal ambient air temp transfer
    the hot water tank in a insulated box with a glass front- is surface area is about 16 inches, the same ambient air transfer as the front glass would still be about the same size that you are using- though it probably a bit better as the large mass and larger surface are at lower temp would absorb more readily

    your benefit from night time temperature retention, but with the tank one you can compensate with double glaze or insulated blanket at night...

    also being that you are in Mexico you can also lay the take fairly flat for better passive solar tracking...

    all I can say give it a try it very cheap and easy to build and I am sure you will find it preforms better then the radiator style


    Reply 2 years ago

    Hi, I think I'm getting what you mean by a "tank".
    I thought about building a square and thin/flat metallic box instead of pipe radiator, I saw some prototype like this but with plastic of something similar, in order to optimise the absorption area. This is not something easy to make without proper tool and knowledge so I abandoned the idea.

    Do you have pictures of your SWH please?

    I was also wondering about the heat loss of your system during the night, you say it stores the hot water in the collector, how do you manage to reduce the loss by radiation? The flat black is the most emissive color...

    BTW, you should document your SWH by writing an Instructable! I'm sure you would have a lot of success!



    Reply 2 years ago

    not a flat tank - an actual hot water tank. you remove the the insulation and the shell. paint the tank flat black . insert into box i prefer north south orientation that tilted upwards about 30 - 45 degree - I do not use it any more as i bought evacuated solar tubes. which is much more suitable for my climate ( ie middle of canada - they work equally as well at -40c as they do at +40c ) -- here an example

    though he orientates his east to west- it not nearly as efficient as north south orientation


    Reply 2 years ago

    Got it! Thanks for the explanation, I have to try this someday and do some comparative tests!


    Reply 2 years ago


    I used common water copper pipe (different from the natural gas pipes). I couldn't tell exactly the grade. Anything that hold the water pressure is ok.


    2 years ago

    Our water has a moderately high mineral content. I see you didn't add a drainage port at the bottom for maintenance draining. Would you consider this a good modification?