Solar Water Heater: Part 1- the Collector




About: A maker of things.

Find me on Youtube and Instagram(@themak5) for more projects!

The goal: Build a passive solar water heater to make the frigid water in the public bathroom less shocking to the human body. It needs to be a closed loop system because it freezes where we are a few times a year. Having water run through the collector will freeze the sitting water in the winter month nights and apparently scald hands in the summer months.

The background: We know a solar water heater can be made, but making it work everyday without requiring constant adjustment and fiddling is the tricky part. I'm lucky, my mother owns a hardware store (awesome place to grow up) and she decided to fund the project. She let me build it at the store and agreed to deal with the ridiculous time line I often fall into on pet projects like this. This will be installed to provide warmer than ice cold water to the hardware store bathroom and will only really be used for washing hands and cleaning etc...

I've broken the project into three phases:

1- The Collector: A heat exchanger that will collect the sun heat energy and efficiently transfer it to whatever fluid media we end up using to carry the heat to our desired location.

2- The Emitter: Yet another heat exchanger that will deliver the heat from my carry fluid to the water the end user will come into contact with.

3- The Install: making it all work as maintenance free as possible... probably the hardest part.

This 'ible covers Phase 1. I will not be as long winded in the steps as I was here.

Step 1: The Box

I found an old table that had been partially disassembled and re-purposed it for my box. I'm not much of a carpenter, but the box serves it purpose well. The final picture in this step is showing the black silicon I used to seal the inner joints of the box all the way around. I wanted to have a relatively air box when I was done.

A picture speaks a 1000 words, so I'm not going to step by step the box construction. I've opted not to go into great detail about quantities and sizes because I think those metrics would be governed by the specific application. There is so much to say about each picture that this 'ible could end up being a royal pain to read; so I'll keep it nice and general for the sake of easy reading. Hopefully the photos do the talking.

Step 2: The Coil

The coil is made up of 1/2 rigid copper. The 180 degree bends are made of one 90 degree elbow and one 90 degree street elbow. All connections and fittings are slip fit and soldered.

-Cut your runs so they'll fit inside your box taking into account the the " qty 2 elbow lengths" you'll add to each end.
-Dry fit everything together and put the coil into the box, see how everything fits. Adjust as necessary.

-The initial and final run of the coil are slightly longer so they'll exit the box and allow access for plumbing the rest of the system. I saved my longer pieces for the end so I could estimate where to drill the holes.  

At this stage, I began planning how I would suspend the coil off the back of the box to maximize the surface area of the coil and air flow in the box.

Step 3: Paint

I did not end up using the RUST STOP primer; I found a cheaper can a few shelves over. I did use the heat resistant black. I was not sure what temperature I would see and I did not want the paint to flake off once I sealed up the collector. I used the same paint for the box and the coil. The second picture is what I used to clean the coil prior to painting.

As with any paint job, the success of the finish is determined in the prep. Clean smooth surfaces take and hold paint better than rough, dirty, oily, greasy, sandy, dusty... you get the point.

The Box- I primed the entire box and then sprayed on two coats of black. I think more would have been better for water proofness etc... time will tell.

The Coil- This was an important step for me. I wanted the collector to have the best efficiency possible in all weather conditions. So I chose to paint the coil. I just felt a few of you cringe... yes, the heat resistant paint is a barrier in the convective heat transfer process. But I believe the radiative gains I'll get on the less that scorching hot days will out weigh the convective losses. There is a bit of math that could be done to quantify this. But I didn't do any of it because this is an experiment for fun and I chose to go with my gut.   

Step 4: Prep Coil

This coil was going to be soldered. Just like painting, when soldering anything (especially water tight connections) the qualtiy of preparation determines your success at the end.

This task was tedious, luckily I had a two trusty sidekicks that volunteered to help. We set up an assembly line. We wire brushed/sanded every single piece of the coil then washed them in soapy water . 

The sanding and wire brushing removes dirt and grit, but more importantly scores up the surface of the copper enough to take and hold paint well and helps with soldering. The soapy water removes oils and grease... which also helps with soldering.

Step 5: Assemble Coil

There are a lot of good 'ibles on here that teach how to solder so I won't attempt it. My tips: Start with clean parts. Use flux.  Make sure both pieces are hot and solder away. The fourth picture shows a good solder job vs a bad one.

I made sub-assemblies first: each straight piece gets one of each type of elbow on it, facing opposite directions.  Finally, solder all of your sub-assemblies together and your coil is complete. 

If this doesn't make sense, look back to the pictures of the fit up and visualize how it will all go together.

Step 6: Leak Test Coil

I used the items pictured below and the garden hose to leak test the coil.

I got it all hooked up, purged the air, the sealed the end. I let it sit under city water pressure(~35psi) for around 30 minutes.

No leaks, good to go. 

Step 7: Assemble Collector - Coil in Box

Again, pictures are worth 1000 words, so I hope I've included enough. The wooden blocks and copper wall mounts are used to suspend the copper coil in the center of the box. In hindsight I would have sized the blocks to secure two pieces of copper per block.


Step 8: Assemble Collector - Glass on Box

I chose to use real glass vs plexi because glass transmits light and radiation more efficiently. Plexi would make the whole project less delicate and less dangerous.  Use caution when working with larger pieces of glass like the one pictured. 

I made a handful of the modified clips, then applied a bead of silicone and placed the glass onto the bead. Once I was happy with where the glass was resting I used the clips to keep it in place until the silicone dried.

Step 9: Testing

I rigged up a test circuit to see if my master plan had worked. It was a 5 gallon bucket with an inlet at the bottom and an outlet at the top. I hooked the outlet (top end) of the collector to the inlet (bottom end) of the bucket using a garden hose. I hooked the other two ports up the same way.

The idea:
Heated water moves up the coil and through the hose into the bottom of the bucket. The same water then rises in the cooler temperature water bucket and is recirculated back to the coil out the top port of the bucket to the lower end of the collector. Hopefully that process repeats.

It worked! for awhile... The heated water began to soften the garden hoses so much they collapsed under any bend radius at all.

Before they collapsed I added food coloring that shows the water circulating and being pulled back into the circuit. Then all of a sudden everything stopped. I used my infrared thermometer and checked my outlet temp at the box and got a reading of ~250 degrees F... I thought this couldn't be right so I used a few other thermometers and got similar results...

I think the water in the coil may have vaporized and turned to steam (water boils at 212 F) which caused a vapor lock and halted my circuit flow. Over all I'm happy, better plumbing setup and less hasty testing will really tell me how well this thing works, but for now I'm satisfied.

The collector works very well, during dis-assembly of my test rig I ended up draining the coil onto my hand and burned myself pretty significantly. Blisters and the whole deal. just a heads up, be careful.

Anyhow, this was the build and a tid-bit on the testing. I'll come back through here some day and go into more detail so if someone wants to build one they can improve off what I've done.

Thanks for reading.

Feel free to shoot me questions if I've completely skipped something you want/need.

Find me on Youtube and Instagram (@themak5) for more projects!



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    18 Discussions


    1 year ago


    I didn't read the complete explaination yet, but it looks nice.

    I don't know the climate where you live but I'm wondering how it preforms in cold weather like 0C°. Is there a way to calculate it? If I can make something like this to heat a small greenhouse, it opens new possibility's.


    3 years ago

    to mae it a little simpler and quicker, you could make up the copper tube and dry fit it together all fluxed up and use a heat mat in the bow and just sweat it up in one go..

    did you try using a small pump to keep it from air loacking up?

    thanks for the ible...


    3 years ago

    Excellent design & workmanship! You're a gifted young man indeed!


    3 years ago

    and how exactly is the collector box sealed


    3 years ago

    how many parts does it have?Am looking for a solar water heater with 13 parts


    5 years ago on Introduction

    I know the cllector box is sealed, but if it's still possible to open it - i would suggest adding a light reflective film to the back of the box itself. I am not sure, but i guess heating the box itself doesn't make much sense, but reflecting the light to heat the backside of the black painted pipe seems quite reasonable. I have no experience and have not yet built any sun collectors, but you did and you can experiment with it :)

    Excellent project, I must say! I feel inspired to try something similar.
    Although I am curious about a couple of things:
    1.In what season did you test this and where is your geographical location?
    2.What diameter copper pipe did you use?
    3.Since this solar collector does not have an absorber plate, how do the pipes heat up primarily? Is it through the absorption of thermal radiation or conduction of heat from the warmed air trapped behind the glass?

    1 reply
    Mak5Chicken of Steel

    Reply 5 years ago on Introduction

    Thanks for the feedback.

    1. Early August in El Paso, Texas right on the US/Mexico border.
    2. 1/2"
    3. Good question, both are at work; but I'm not sure which method of heat transfer is carrying the load. I'm thinking it's convection from the air...


    Great job, really surprised it came out so hot. I know you said your mother funded it but is there a solid bill of materials and any way to know about how much this all cost?

    1 reply

    Reply 6 years ago on Introduction

    Thanks for the compliment.

    Unfortunately, in my haste to build I did a poor job of recording what I was doing as I went. I would think I can probably build a pretty accurate BOM from the photos.

    Good call, I'll do that and update the thread.


    6 years ago on Step 9

    Mak, great writeup and workmanship!

    One quick question tho: wouldn't it be more effective to connect inlet of the collector with an outlet at the bottom of bucket and outlet of collector in the top of bucket? this way, when rising the bucket a little higher than the collector, natural circulation should do the work. And also, due to the fact that you would take the cold(er) water from the bottom of the bucket to be heated, it should prevent the problem with the vapor lock (well, at least til the whole bucketfull is boiling :) ?

    1 reply

    Reply 6 years ago on Introduction

    Wow its been awhile since I've been on here... Really appreciate the feedback. Apologies for the late reply.

    You might be correct, I think there are likely a number of things that could be improved during testing. Your idea is probably the first place to start.

    I still have the the collector sitting in my garage, maybe some day ill pull it out and do some more testing.

    Again, thanks for the feedback!


    7 years ago on Introduction

    What are the dimensions on your box and how long did you cut the length of your pipes?

    Have you done further testing to on this hot water heater? Does it work?


    7 years ago on Step 9

    I would like to comment about using the glass to cover the collector.
    If you look at the edge of a piece of glass and it is a light green color, the glass contains some iron. The iron cuts down on the suns heat transmission. Clear glass lets the infared thru. Also when you seal the glass to the wood box , eventually you will get moisture trapped in the box.
    While studying the coverings for collectors several years ago I found that Visqueen (trade name) plastic sheeting allowed more of the desireable infared rays into the collector. I used 6 mil thick sheeting. An added benefit was that the collector needed to be cleaned of dust periodically. The plastic sheeting was cheap and replaced when the dust was cleaned out..

    Now that you have constructed a collector capable of producing steam, carry your experiments a little farther. Absorbtion refrigeration systems work on heat. An example is refrigerators in campers operating on propane.
    At one time Absorbion refrigeration was popular in the south for airconditioning homes. Industrial plants use large absorbtion systems operating on steam to cool their office spaces. Carrier makes 100, 200, and 300 ton units. Several companies made units for homes. Bryant, Arkala, Servel, and two others were popular brands. Their airconditioning units worked fine. Their problem was in the servicing. The units were sold thru Natural Gas and Propane suppliers and their heating techs didn't know how to service the absorbtion systems. I attended a very large Solar Expo in Phoenix several years ago and the Japanese had absorbtion units on display that operated on solar collectors. Google Absorbtion Refrigeration to find out more.
    You are to be commended for your achievements. The Instructable is well written and your pictures are clear and informative.

    1 reply

    Reply 7 years ago on Introduction

    Thanks for the feedback and the history lesson.

    As for the covering: there are definitely better light transmitters out there. I was limited to what was on the shelf (built and tested in less than two days) I chose glass over plexi for light transmission and for it's durability to the sun. Plastics will often break down very quickly under UV exposure. Not sure how the Visqueen is UV rated.

    Sealing the box: hindsight is 20/20 on that one. As you can see in the test photos the moisture trapped inside the box condensed on the glass covering. It was interesting to watch- the second I turned on the water hose and allowed cold water to flow through the coil the heat was immediately sucked out of the air in the box and the condensation was completely gone. It was a physics/thermo experiment all it's own.


    7 years ago on Introduction


    Why not save all that effort and money and simply repurpoue a discarted fridge radiator?
    Already designed for that purpouse, already black, adds extra blades that allow better heat exchange...

    1 reply

    Reply 7 years ago on Introduction

    AFAIK they don't work very well for this, fridges don't need so much heat flow, whereas with solar you need a kilowatt or so.