Introduction: Solar Hydrothermal Collector

About: I am passionate about clean energy and sustainability. Please contact me if you want to talk about off-grid home design.

I built this to experiment with heating water with the sun.

I want to eventually build a house that is powered almost entirely by the sun.

This project also goes by the name Solar thermal-siphon. It works because of convection. The heated water will naturally rise and cold water will fall. This only works if your storage tank is above your collector otherwise the hot water in the collector will have nowhere to rise to. A solar thermal-siphon is nice because there is potentially no need for a water pump. CPVC pipe was used because of it's high temperature rating but if water were to ever stop flowing or the unit got too hot, it could potentially melt or shorten it's lifespan. Also keep in mind that if you live somewhere where the outside temperature drops below freezing, then you will need to use a glycol or antifreeze liquid and run that heated liquid through a heat exchanger that heats your potable domestic water.

This entire thing cost me about 43$. I was able to use some stuff I already had and acquire some free parts such as shipping pallets. I enjoyed the fact I could build it all out of reclaimed items (except the CPVC pipe which was one of the only things I had to purchase).

Potential uses for this are:

  • An outdoor shower with heated water.
  • A school campus (Think athletics department).
  • Beach or recreation facility.
  • To heat a water thermal mass inside of a greenhouse.
  • A demonstration display at a school campus, museum, or other.
  • Space heating. (Accessory preheat for your existing hydronic heater.)
  • Either primary or accessory heating of your domestic hot water.

This is fairly simple project that can potentially save you a significant amount of money over your lifetime..and help the environment to boot. I entered this into a contest on here but didn't even make finalists. I guess it's just another popularity contest as usual for these things. :(

I didn't take that many pictures while building so the afterward pictures will have to do in some instances.

Step 1: Make the Box

Turn a shipping pallet into a box by pulling planks off of one side and nailing them back onto the other side.

Then set the box face down on the floor and nail two boards to the ends. Setting it face down will allow the ends to be flush with the top and bottom sides of the box.

I reused the nails.

Note: Hammer and Flatbar is required, it helps to have a pry bar too. Pallets are held together extremely well. These ones had twist-shank nails. You have to use a lot of muscle and patience to pull them apart without breaking the boards up.

This turned the shipping pallet into a lidless box. Making the box first allows you to then measure for the piping.

Step 2: Make the Pipe Array.

(See image for flow path.) The water in the vertical pipe will heat and
rise then flow out the outlet. The lower inlet is a suction for colder water out from the bottom of the storage tank.

I used five 10-foot long pieces of 1/2"OD CPVC Tubing. (CPVC is for high temp/pressure otherwise it could melt in the sun with how hot it will get)

I used a chopsaw to cut the pipe but a simple hacksaw will work if you clean up the edges with a knife after cutting.

The shipping pallet I used was slightly smaller than standard and my 5 lengths of pipe cut perfectly into eighteen 36-inch long pieces. (4 pieces per 10 foot length with leftover for the connectors)

I cut several tiny short pieces for connecting the T's together to make the manifold.

I used "Flowgaurd Gold" glue and glued a T onto one end of each of the 18 pieces except one which got an elbow. I then glued all of the T's and 1 elbow together laying it on the floor so that they were all parallel (caution: the glue dries very fast have it planned well ahead of gluing)

I glued short pieces of CPVC into one side of each of the remaining T's, then carefully took the T along with the short piece sticking out and glued the rest of the array together.

Step 3: Complete Array Box.

I attached/glued lengths of pipe on the outlet and inlet then marked on the crate/box where to cut. I used a jigsaw to cut a slot for the array to go inside of the box. This slot can be covered and insulated later.

I used some pipe clips to hold the array to the back of the box (sorry for the poor quality pictures).

Note: Because we want water to flow as smooth as possible, consider rotating and installing the array slightly clockwise inside of the box before cutting the slots. Then the water will flow to the left when it hits the top manifold. I decided to mount it straight then I can tilt the box later if I want.

I went to my local glass company hoping to get some scraps I could somehow use and they ended up cutting and selling me an exact fit piece for only 5$ (because this was a school experimental project they gave me a deal).

I used my jigsaw and cut some of the extra pallet boards into pieces to mount the glass using some screws (see images). I had a channel mounting the entire bottom of the glass and only one block holding the top in place. It fit so tight that I did not need anything to keep it from sliding out. I can also pull the glass in and out easy this way to continue experimenting and working on the inside (I will be adding insulation later).

Step 4: The Tank Stand

I used almost an entire pallet to create the tank stand.

The pallet already had notches cut out that my tank fit into (see image). So I just cut the corner out of the pallet then disassembled the boards from the rest of it to nail together the stand. I was able to reuse the same nails. I also mounted the legs angled outward and using the floor traced and cut the legs to be flat at the bottom for more stability.

Step 5: The Tank

I was able to get this tank for free. It's original function was supplying chlorine to treat a pool.

I used a drill and drilled two holes in the tank for the outlet and inlet using a drill bit smaller than the CPVC pipe (largest drill bit I had).

I then used a dremel and carefully made the hole bigger so that it was smaller than the vinyl tubing but larger than the CPVC pipe.

I inserted the vinyl tubing into the hole by squeezing it (see image), then pressed the CPVC pipe into the hole so that it was a tight fit.

I was then able to use small hose clamps to clamp the vinyl tubing onto the CPVC so I could assemble and disassemble the array from the tank for easier transport.


  • During my first test I had leaks and in the future I plan to pull it back apart and add some silicone during assembly.
  • I predict the vinyl tubing will not hold up in the outdoors and when it gets hot it will droop. This is an experimental device but a more permanent one I would suggest piping the entire thing with CPVC pipe and using the appropriate silicone or glue.

Step 6: Finishing and Testing

You will want to paint the inside matte black. I had just enough heat resistant paint to cover it.

You will also want to insulate the box as much as possible. I just stapled some cardboard to the back for now and I had some sheets of aluminum that I put behind the array (they were promotional signs that I salvaged from the garbage). You can buy some rigid foam insulation from the hardware/lumber store or I was thinking carpet scraps might work well too. If you have access to a metal shop maybe some curl metal filings from a lathe would be good to absorb the radiant energy reflecting it onto the pipe as well.

Test 1:

It is winter here so I have not had much of a chance to test it. You can see from the picture that the sun is extremely low in the sky. For my first test in about 1.5 hours I was able to raise the temperature of the entire tank by 6 degrees F with a starting temperature of 70F and an outside temperature of about 34F. That was with the sun passing behind two trees putting the entire unit in the shade for a few minutes at a time. Even without any insulation, as soon as the sun hit the box the temperature shot up to 114F. A thermometer taped to the outlet hose had a temperature as high as 79F.

From another picture you can also see some green food coloring I added in. The food coloring flowed right down to the outlet.


I was very happy with how well the first test worked. I would like to use this unit for heating the school greenhouse or install permanent thermometers, a pinwheel to show water flow/movement, fill it with an antifreeze mixture and set it up as a visual display piece on campus.

I also did several physics calculations when building this to try to track the movement of energy. I would be happy to share and discuss them with anyone while I try to improve on them for designing my future dream house.

Physics Calculations/Concepts:

Radiation: Figure out Watts/m^2 from the sun at your location and how much surface area your panel is (this can be looked up often at local weather station data available online). You can use the specific heat of water (4186 J/kg*C) and Q = mc ∆T formula or you can do it by using Watts * Seconds = Joules to figure joules per hour which can be converted into BTU's/hr. Also use Stefan-Boltzman equation to figure out how much radiation energy is being absorbed and emitted by various parts.

Convection/Flow: This was a hard one. I used Density, Viscosity, Bernoulli's equation, and Poiseuille's equation.

Conduction: Q/t = kA(T2-T1)/d to find conduction rates of different materials in the system. (k is your thermal conductivity of the material)

Result: I roughly determined that with a perfectly system I should be able to get over 20F rise in 15 gallons of water over a 1 hour period.

Note: When doing all that I learned about how someone might be able to make these things more efficient. Since glass is the most conductive material in the box, you want to prevent air from circulating by it. Natural convection of the air in the box will circulate the air by the glass pane. Someone might be able to minimize this by installing horizontal partitions splitting the inside of the box into cells. This will help prevent your heat gained by radiation from being lost by convection and conduction.


This project was inspired by "Super Simple, CPVC Solar Hot Water Collector" as found on youtube and simply solar.

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