Batch Solar Water Heater




About: I like to make functional beautiful things

Aim: To build a solar batch water heater using some old materials and some new. The idea behind this heater is to use as many old materials as possible, keeping them out of landfills and to use new materials as efficiently as possible. For this reason the box has been designed to use exactly two sheets of plywood without any waste. This also gives the box a trapezoid shape, making it stronger and angling the sides to reflect light onto the bottom of the tank. This was a lucky accidental feature;-)
I think this is a very simple design most people are capable of making with fairly basic tools, cheaply and without advanced carpentry skills. it is a effective heater - I have seen the water on top reach 150F on a hot spring day.
I have built a few of these and am still working on improving the design. I will log in and update this as I have more pictures and info. If these are not completely clear directions, well, this is my first instructable, so it will only get better.

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Step 1: Materials Needed

Double glazed patio sliding glass door, 3' wide ideal. Should measure about 36” x 79”
Old water heater tank. I have only used 40 gal gas heaters so far, but I see no reason why larger tanks and electric tanks should not work, fine.

½ inch sheet of plywood
5/8 inch sheet of plywood
2 sheets of 1 inch foam insulation with reflective foil surface
Rustoleum or equivalent flat black metal paint, 1 pint
Stain or exterior primer and paint, 1 quart of each
¾ inch steel plugs for extra tank holes, maybe.
2 cut off pieces of 2 x 8, 16” long
One tube of latex or other caulking
Box of 1 5/8” drywall screws
10 3" wood screws for tank supports
16" long 1/2" copper pipe with cap and
2" long 3/4" threaded steel pipe
JBWeld for above
Teflon pipe tape


Step 2: Locate an Old Heater and Buy Materials

First locate an old water heater, preferably one without leaks, although leaks can probably/possibly be patched with JBweld or equivalent hardening sealant. A dump, or salvage yard may have one, also check on Craigslist, call local plumbers and be creative. The patio door can be found in similar locations, double glazed is best, although single will work too. 

All other materials should be at your local hardware store

Step 3: Prepare Tank

Remove all parts that protrude from the water heater casing. This includes the burner, relief valve, drain valve, etc.
Take of the top and bottom of the casing, then use a screwdriver or pry bar to open the crimp where the casing is joined together. This can also be done using a grinder fitted with a cutoff wheel, or a Sawzall and a very fine blade. If using these methods be careful not to cut too deep as the inner tank is about an inch below the casing.

Pull the casing off and remove the foam insulation below, by scraping with a putty knife if necessary.
Sand the tank down and use a wire brush until most of the rust is removed.
Paint tank with flat black metal paint. Rusty metal primer is recommended but optional.

Step 4: Building the Box

If you have an average tank, (40gal, 16” wide) the following dimensions should work, for larger or oddly shaped tanks, other dimensions need to be figured out. These dementions are the finished dimentions of the outside of the box, not the exact panel sizes. You will have to figure out the exact sizes of each panel deducting for the width of the sheets of plywood.

To build box, rip 5/8 sheet of plywood down the middle, i.e. 24”. Cut one side down to 68” long, or as long as your patio door is tall. This will be the bottom of the box.
   Cut out your trapezoid shaped box ends from the other half of the 5/8" plywood. with a top dimension of 36”, bottom of 24” and height of 24”.
   The sides of the box are made by ripping the 1/2" sheet of plywood down the middle, making them 24" wide and cutting them to 68" long. 
Screw trapezoid sides to bottom, and long sides onto these. Use about 1 screw every foot.
 Cut trim wood to fit rim of box and screw on.

 Caulk under rim to seal rim to box, and along corners, keeping moisture from entering plywood here. Do not caulk around the bottom of box as this would stop a leak from exiting the box.
When caulk drys, prime and paint outside of box and rim with exterior paint, or use two coats of high quality stain.
8.Lay tank on side raised up three inches and scribe contour of tank onto 2x8 scrap. Cut along line with scroll saw, jig saw, or saber saw. Make two of these, they are the tank supports.
9.Cut first sheet of insulation to fit in bottom of box, place tank supports at good distances from ends of box, mark around tank supports on insulation. Lift insulation out of box and cut out holes for tank supports.
10.Screw tank supports to box with five or more screws in holes cut in insulation.
11.Cut long side panels of insulation to fit snugly in box. Measure and cut trapezoid end pieces to fit angled into the box. 

Step 5: Making the Supports for the Tank

Take the length of 2x6 lumber and cut it into 2 16" long pieces. 
Prop the tank a couple of inches off of the table, hold the piece of wood against the end of the tank as it the diagram and mark the curve of the tank on the wood. Cut it with a jigwaw, saber saw, scroll saw or any other method you can make work. 

Step 6: Insulating Box and Supporting Tank

Cut one sheet of insulation so that it fits snug in the bottom of the box. Take the tank supports and lay them out on the insulation. lay the tank on top of them and move them around until the tank seems well supported, similar to the way they are in the picture. Mark and cut holes for them to fit through the insulation and attch to the plywood.
Screw them to bottom with the 3" wood screws. 

Step 7: Make a Cold Water Inlet Dispursor

To prevent the incoming water from mixing with the hot water in the tank, we need to make a disperser, done in the following way:
cut a 16 inch length of the 1/2 inch copper pipe. Drill small (about 1/8 inch) holes every inch or so along the top side of it. JB weld the cap to one end of the pipe, and JBweld the other end into the 3/4 inch steel pipe. I will take a picture of this when I get a new charger for my camera. The heater will work without this part, but it will be more effective with it, especially when it is not very hot. 

Step 8: Plumbing the Tank

Prop up one end of the box underneath the tank support until the tank is at about a 30 degree angle towards the South. Place the tank in the box with the bottom facing up. Rotate it until the 3/4 inch connection closest to the bottom of the tank is pointing upwards. Screw in a 3/4 inch angle pipe here.
Screw the Cold water inlet pipe into the lowest hole in the top of the tank. plug all remaining holes with things removed during the preparing of the tank, the drain cock and pressure release valve work fine.

Again, I need my camera working again before I can show you this part, and it is not easy to describe it without pictures. Forgive me. 

Cut a slot in the bottom of the lower insulation end panel, and drill two holes in the plywood, probably with a hole saw so your piping can get in and out of the box. 

Step 9: Using It

Place the double glazed glass panel on top, let it heat for a few hours, then enjoy your solar heater water. Ask me if you have any questions. 

Some modifications which you should try are:
Thinker insulation, maybe 2" would keep it hotter in the winter. 
To keep it warm at night for use in the morning glue a sheet of insulation to a think sheet of plywood and hinge it to be a closable cover for night time and other times of no sun.
A thermometer can be made to read the temperature inside the tank by drilling out a hole in a 3/4" steel plug and JBwelding in a little kitchen thermometer into the hole.
Thanks, sorry about the quality of this article, I am not really feeling it right now, and will probably re-write it later when I am inspired. 
Thanks Annie Rose for who you are, I love you like I love myself. XXXoooXXoooxXOxX

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50 Discussions


22 days ago on Step 9

you don't list the 'trim' for the top of the box in your list of materials needed...


2 years ago

what is the thickness of the water tank can i use ms steel sheet and making 40 gal tank ,we don't find old tank, what is the best thickness tank for batch solar water heater and gallon size?

1 reply

Question 6 months ago on Introduction

I certainly understand & commend your recycling of materials but using steel fittings instead of brass will create internal rust issues.
I will be posting my home made solar water heater soon. I live in Victoria Australia on the coast & using Facebook market place found & purchased 2x18mt (150ft) of 1/2 new soft drawn copper pipe for $90 Aud less than 1/2 price in the shop here, it will be the flat coil water heater type


2 years ago

Hi, I have an important question to ask you: I have a commercial vacuum tube solar heater and am very happy with it however this year I had a non-return valve get stuck open and the hot water almost disappeared completely from the system. Now I have just finished building a batch one like yours for my parents and would like to ask this: do I need to add some kind of non-return valve to prevent hot water to escape towards the cold water tank which sits higher in this setup? My guess is yes because hot water will try to rise up even though the batch tank's inlet and outlet on my design are at the bottom.

Thanks for posting this instructable and nice job.


3 years ago

Do you tap directly off the tank or do you transfer the heated water off into an insulated secondary storage tanks?


3 years ago

Do you tap directly off the tank or do you transfer the heated water off into an insulated secondary storage tanks?


3 years ago

Very excited to try this. Could you say a little more about how you plumbed this into your home? Do you have any safety measures to suggest if the temp goes dangerously hot?


3 years ago

Thanks for Sharing the blog,its really informative,its very safe during rainy season

water heaters</a>


3 years ago

Thanks for Sharing the blog,its really informative,its very safe during rainy season

water heaters</a>


4 years ago on Introduction

I had a solar heater on my roof. Worked great at 7 am it was 65 df and about 8 am all hell broke loose. I installed a water heater relief valve on the top of the system. Well it hit what ever the relief valve was set for and did its jobs. One thing you may want to due is to add it in with a temp control shower controler. This way you can add cold water mix. That way you do not scald anybody.

Mr Ganeshruskin

Can you pls mail me more pictures on Batch Solar Water Heater on my Mail ID

I intend to build one myself.


Satish Khabiya

Well, I love this Instructable, any time someone re-purposes something that is really hard to get rid of I am impressed. As far a Black is concerned, I once heated an entire pool to 76 degrees with a 100 foot roll of 3/4 I.D. black plastic Air Brake Hose for Truck Air Brakes. I just spread the loops across the roof of my house and plumbed the filter outlet to the tube and then to the pool. The pool was 58 degrees when I filled it and warm and comfy three days later. The pool is 45 feet by 26 feet and 10 feet deep on the deep end.


7 years ago on Introduction

just a thought, I keep seeing people say, "use flat black paint to increase the surface heat". I once saw a demo that compared flat black colored cardboard side by side w/ a very dark colored green cardboard both with thermometers placed under them, all inclosed in a Styrofoam box. The heat source came from 2 60wat bulbs placed above each piece of board. There seemed to be a significant difference in the temp, with the green being the higher temp. Made sense to me as nature itself uses green not black to improve the suns reaction in making the plants food. anyone interested can easily perform this same experiment to see for them selves..Just hoping this may help improve your heat exchanger.

2 replies

Reply 6 years ago on Introduction

Yes, but plants aren't trying to make heat, but carbohydrates out of water and CO2. in fact plants want to limit the amount of heat they produce. I've not done the experiment you talk of but my understanding of physics is that if there is color coming off something, then light is being reflected, meaning heat is being lost. Could be wrong of course, but i havn't taken the time to experiment with it yet, so....


7 years ago on Introduction

Mainstream science would disagree regarding the dark green theory:

Plants are green because that part of the light spectrum is reflected. So, to most plants, green seems to be kinda useless.
Black means the abscence of light, hence black objects (black holes) are considered to be the most absorbant there.

Maybe the dark green paint was matte and had a certain chemical setup while the black paint was glossy or less absorbant due to ingredients.

My main question would be where exactly the thermometers were placed. If "under" means inside the box while the painted cardboard was tightly against the wall then, of course, the temperature in the dark green box would have to be higher since the black one absorbs heat better, channeling it to the outside.


7 years ago on Introduction

@ (author) ganeshruskin, Your right, I wasn't as clear as I could have been,sorry. My idea was, if you could heat up a used water heater by putting it into a trough and heating it with the sun, what if you used a radiator from a used air cond or car or truck or? place it in such a way in a trough type design using Mylar to reflect the sun onto the radiator and heating the water. Radiators are used to get rid of heat, could we just reverse the idea and collect heat the same way? One other "idea," when heating a pipe becomes possibly dangerous, how about a one way valve? water expands when heated, therefore "pushes" the water through the pipe from hot to cooler "collector" where the water would be used or not. if not than circulated back to the "heater." Again, use a one way valve, the type used on lawn sprinkler systems to avoid reverse osmosis from lawn chemicals etc polluting your water supply . I've never done this, but the idea seems doable...

1 reply

Reply 7 years ago on Introduction

Yes, there is no reason you couldn't use an old radiator, but the point of this design is to store the water as it is heated, you know, 40 gal of it, and a radiator only holds maybe a gallon, so you need a separate storage tank, which changes the design. If you build one, please let me know how it works.
And valves are only needed if you have a situation where the water in the pipes could freeze while the water in the tank is brought to boiling, pretty unlikely ;-)
The tanks also come with a built in safety pressure and temperature valve.


7 years ago on Introduction

Has anyone else actually built one of these and installed it?
I am about to try manufacturing these again down in Flagstaff and just curious if anyone has developed any improvements on this?