Solar Water Heater




Introduction: Solar Water Heater

This was a project mostly just to see if I could do it-

I was living full time in my motorhome, and I wanted to see if I could:

1: Build a thermal solar water heater

2: Build one primarily out of reclaimed/ reused resources

3: Produce enough hot water to be useful.

Step 1: Supplies and Materials

The basics, mostly resourced from Habitat for Humanity ReStore (r)

  • a sliding glass-door, or some other large sheet of glass, I like door glass because it is strong. (I took a hail storm with 3/4 inch hail and my glass didn't break).
  • some lumber and plywood, I used 2x4 and 1/2 inch ply, but use whatever you have or want!
  • a roll of copper roof flashing.
  • Aluminum tape.
  • a 50 foot coil of 1/2in copper tubing,
  • copper tube straps
  • Barbecue Black Spray Paint (Cheap) or Solar Black Spray paint(Expensive)
  • a couple of sheets of pink styrofoam insulation. (I lined my box so I had a couple inches on the bottom and the sides. The better insulated the better this will perform).
  • PEX pipe
  • pipe insulation
  • GatorBite (tm) connectors make working with PEX and copper tube a lot easier.
  • 2 Faucet type flex lines(at least two, depending on how you set things up you may want four)
  • a Washer Supply Valve (you could use two regular quarter-turn valves, but the washer supply valve opens/ closes two sides with one lever) You will also need some thread adapters to use the WSV.
  • Blow-out inlet and drain valves (Check at an RV store)
  • A mechanical thermostat (capillary and bulb type, see picture during later steps)
  • Two automotive temp gauges.
  • One critical piece for this design is the pump, I experimented with a lot of pumps until I settled on the El SID Brushless DC Circulating Pump for 5W Solar Panel, SID5PV. Use anything else at your own expense!

Step 2: Construction

The box size and shape is going to be determined by the glass you have available to cover the box with.

  • I used one pane of glass out of an old-school single glazed sliding glass door.
  • A double glazed door would be good to keep the heat in the box, but if you use a more modern door, you may have to strip off any UV protective coating (you want all the solar energy you can get through the glass- the glass is there to keep the air from stealing your heat away.)

Frame the box with lumber to match your glass cover, use plywood for the bottom of the box.

Line the box with styrofoam insulation.

  • Cover the insulation in the bottom with copper flashing.
  • Cover any exposed insulation with aluminum tape. I discovered that any "exposed" styrofoam melted under direct sunlight. Reflective aluminum tape seemed to help prevent this.

Gently hand bend the copper tubing into a coil spaced as evenly as you can get it.

  • Be super careful making the turns, if you crimp the pipe you will have to repair the pipe.
  • You want to get as near as one continuous piece of pipe inside the box as possible, you don't want to have to open the box up later to repair any leaks.
  • You can see in the picture the "pipe T" at the end of the tubing in the center of the box. This allowed me to connect the temperature gauge bulb in the flow of water,"wet"
  • Using copper pipe tube straps and a type of self drilling screw that has a neoprene washer I anchored the copper tube, through the foam and into the plywood.
  • I tightened the screws just tight enough that the copper tube was actually pressed into the copper flashing.
  • Make sure the screwws are long enough to go through the strap and the styrofoam and anchor firmly in the plywood- BUT- be careful of the screws coming through the bottom of the plywood!

The copper flashing is the heat collector. A tight bond between the flashing and the pipe transfers the heat into the water.

Wait to install the thermostat bulb until after the interior is painted black.

Paint all the copper in the box with black paint

  • Through several design versions I experimented and found no significant difference between barbecue grill paint and "Solar Black"- except the price.

Lastly install the thermostat bulb. Be careful with the capillary, if it gets damaged the thermostat is ruined.

  • I placed small piece of thermal reflective insulation on top of the the bulb, just so it wasn't being heated by direct sunlight.

Step 3: Control

I wanted the operations to be as analog/ mechanical and non-digital as possible. There are a few approaches you can take here.

  • You could connect the ELSID pump directly to a 5W or (larger) solar PV panel.
  • This gives a direct control based on solar input, and, is not a bad way to go.
  • The pump speed will be directly proportional to the amount of solar energy input.

However, a well insulated holding tank may be holding more heat energy in the morning than the solar is providing.

  • I didn't want my pump to come on until there was actually more heat in the box than in the tank.
  • Of course this would be way easier to do with a digital solar differential control, but like I said- I want to keep this analog.

So, I used a mechanical thermostat with a capillary and bulb. This allows me to place the bulb "dry" in the box and run the capillary to my control location.

  • When the temperature in the box gets to the preset temp the pump turns on. I found that my tank temp was usually down to 100 degrees by morning, so I set my thermostat to 140.

This isn't perfect, if the ambient temp starts dropping the pump will effectively be keeping the box warm (which DOES provide a moderate level of freeze protection). It can radiate a lot of my heat energy out until the thermostat gets down to 140 and shuts off, but at least it keeps 140 degrees in my tank.

I used a Washer Supply Valve (WSV) to isolate the coil.

  • The flow goes through the WSV both on the supply side and the return side.
  • This way I can shut off the water completely to the coil, which prevents the heated water from rising and radiating my heat during cold days, and lets me isolate the coil if I want to blow the water out of the coil completely.

The temperature gauge bulbs are installed "wet" in pipe "T"s, both in the box and on the "hot-out" side of the water heater.

  • Be careful running the capillary lines, if they get crimped or broken the gauge is ruined.

I installed a Single Pole/ Double Throw/ Center Off switch, so I can manually over-ride the thermostat if I want to turn the pump on or off.

Step 4: Plumbing.

This is an open loop recirculating design, that means the water circulating through the coil is the water that you are going to use.

  1. I placed a "T" on the "hot out" side of the water heater.
  2. Flex line runs from the T to the washer supply valve (WSV)
  3. PEX from the WSV to the coil.
  4. The water flows through the copper coil, then PEX to the other side of the WSV.
  5. Flex line returns the solar heated water from the WSV to the pump.
  6. Flex line leads from the pump back to a "T" installed on the "cold inlet" side of the water heater.

Since this is an open loop recirculating system, if outside temperatures are going to freeze you want to drain the coil.

  • If you want, add a drain valve and a blow out valve above the washer supply valve.
  • Install the blow out inlet on the supply side to the coil.
  • Install the drain valve on the return side.
  • Do this above the washer supply valve so you can isolate the coil from the tank before you blow out the coil.
  • You probably want to put some plastic hose from the drain valve to wherever you want the drained water to go, the coil does hold a couple gallons, so don't forget this little step!

Did I mention? Use copper tube for the coil. I discovered that condensation on the glass inside the box acts as a magnifying glass and focuses a beam of concentrated sunlight on plastic pipe. It will melt it enough that the water pressure will blow it out. Stick to copper!

As you can see from my temp gauge I easily exceeded 212 degrees in the coil.

  • The water heater tank seemed to act as an expansion tank, so nothing ever blew out.

I didn't find any difference in performance regardless if the pump was on the supply side pushing "up" or the return side pulling "down". In my mind it just makes more sense to be on the "pull" side.

My tank and pump are only about 7 feet lower than the solar box, I'm not sure how far I could get away with.

Benefits of using the EL SID pump-

  1. The pumping chamber is completely sealed from the driver, there is no shaft from a motor to the impeller. The impeller is magnetically driven. These pumps are virtually impervious to leaking. The driver can even be replaced without shutting off the water.
  2. This pump has a very low flow rate which gives the water a fairly long dwell time in the box.
  3. This pump only draws 5 watts, so it can run on a small solar PV panel, and can tolerate a fairly wide voltage range so no battery or controller would be required.

Step 5: Seal Everything Up Tight

Wait! Of course you are going to wet-test everything to ensure you don't have any leaks! Then seal up the box.

The glass is sealed to the box with silicone, and I used steel corner braces just to ensure the glass wont lift off from any strong wind.

The screws are just beyond the edge of the glass- they are not drilled through the glass!!

Step 6: It Worked.

Consider this was running on a 10 gallon RV water heater.

I was the only person using it, and I was at work all day nearly every day.

But, without using propane or electric to heat the water, most days I had enough hot water for everything a person needs hot water for in the evening, and just warm enough to use first thing in the morning.

I felt the project was a huge success-

However, there are some super cool closed loop systems out there that don't cost much more to buy (than buying all of these parts retail) and work better.

If you can build this out of mostly reused/ recycled materials the overall cost isn't too bad, but to scale this up for a full household may not be as cost effective.

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    5 years ago

    I also live full time in my 27' Shasta. I replaced my RV hot water heater with a residental 20 gallon, 110 ac. I tried building a solar water heater last year with 2 100' black heavy duty water hoses. I included 4 ball valves to the in/out puts of the hot water tank. I put brass adaptors (for garden hose treads) on 2 of the ball valves, these go to the hoses, then i attached the others to the cold water input and hot water out put. I took the hoses behind my trailer, coiled them up as tight as i could then just layed them on the ground. Worked great, i used them for a year summer (got really hot) and winter (warm enough to shower). Never had any leaks or problems, only the first few weeks I could smell rubber then the smell went away. Since I volunteer as a host I move the trailer every 6 months i need to redo it again but the hoses make it easy to break down and put back together quickly. I will try your sliding door idea this time. Sounds like i could use the sliding doors for the solar panels i am building right now. Thank for your ideas, I will use the WSV and termo this time as well. Great job.

    hi andy, great job mate .. i have built lots of these panels very similarly, they work amazingly well .. my only caveat is to watch the temps that your panel produces, it can easily exceed the operating temperatures of pex and push lock fittings


    Cool design. It looks similar to what I have seen sitting on top of peoples pool houses and circulating water to heat their pools.


    Reply 6 years ago on Introduction

    It is slightly similar, however this is a direct circulation design, where most passive solar pool heaters rely on the convection of the heat to rise in the solar panel to recirculate the heated water.

    Thanks for the feedback, much appreciated!

    Are you going to enter it in the 'Making Energy' competition? I would vote for it!


    Reply 6 years ago on Introduction

    I did enter it! Thank you for the encouragement!


    You could always store the thermal energy from the pump in a secondary tank to keep your hot water hot over night. Phase change materials have been used to great effect for this purpose. Nice job :)


    Reply 6 years ago on Introduction

    With the pump off when the temp cools it holds the heat well in the primary tank, i could have achieved better efficiency with digital controls, but I was aiming for all analog controls, and a build with more or less easily sourced materials. An earlier design a backed the coil with drywall- gypsum is a fairly good material to store heat in- but it retained heat for so long that I actually bled more heat out while it circulated before the temp dropped enough for the thermostat to shut off the pump. The cheapest way to gain efficiency in this type of design is better insulation on the holding tank.

    Thanks for the feedback!

    Looks good! Is it frost proof? If it's not, you could use anti-freeze and an indirect tank?


    Reply 7 years ago on Introduction

    It is not designed to be freeze-proof. It is an open-loop design. A closed loop design adds complexity and cost that I felt would outweigh the benefits gained. If I had intended to use this in a colder climate I may have planned it differently.