Make a Cheap Solar Batch Water Heater (Solar Preheater)




Introduction: Make a Cheap Solar Batch Water Heater (Solar Preheater)

Today we are making a Solar Batch Water Heater as cheaply as possible.

What is a solar batch water heater? I don't know why it is called a batch, but regardless it is effectively a tank painted black that uses the sun to preheat water before it is supplied to your hot water heater. This causes your hot water heater to work less in heating your hot water. The nice thing about this process is that it is very simple. There are no pumps or electrical need and almost nothing to maintain.

There are Solar Batch Heater plans out there and quite a few videos that cover this process. They are neat, but I wanted to make something a bit easier and cheaper, yet still get solar heated water. The main difference with my solution is that I am skipping the insulated box that is typical in other designs. This makes the job quicker, easier and cheaper. My design gives up some of the heat production as well as most of the insulating ability that a box and insulation around the tank would provide. I live in north Florida, and I think that I get ample sunlight to make 120+ degree water without enclosing my water tank. Any water above 120 degrees is not desired, as this is the temperature my home hot water heater is set to. I am not interested in making water that is above 120 degrees. I am also willing to give up the heat at night to keep the design cheaper and easier to build. During a rainy or cloudy day, there is still benefit to this design as long as the outside temperature is higher than the temperature of tap water entering your house. In my case, typical tap temperature in the summer is 74 degrees. Most summer days and nights in FL are hotter than 74 degrees. During winter, I plan to monitor the system to see how it reacts to sun warming and cooling at night. I can bypass it of course, but I will be curious to see just how warm my tank can get.

I was inspired by the excellent Tiny house build series of videos by chud327 on Youtube. In his finale video linked he discusses his solar hot water heater. His was a simple black painted tank that was reaching 121 degrees and was still 98 degrees well into the night. I recommend watching his series of videos. He makes everything look easy.

So that chud327 video got the wheels turning and I decided that I wanted to do one at my house. I have a hybrid heat pump hot water heater, so making hot water is about 60% more efficient for me than a standard electric hot water heater. However free hot water to help it out and keep it from running as much sounds good to me. My hot water heater is located in the garage and the side of my garage faces south, so this is a convenient spot for the solar hot water tank and with it facing south, it should gather the most sunlight. (Although my particular spot is not ideal, as it is near a privacy fence that will provide some afternoon shade.)

So I have a spot to put the tank and a plan in my head, now I just need a tank.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Find a Donor Hot Water Heater

After looking for cheap new tanks and finding a $250 galvanized tank at Tractor Supply that might work, I decided that I needed a more creative and much cheaper solution.

I then decided to use a hot water heater that had been 'shucked' of its outer sheet metal skin and insulation. If I could find a good donor tank for free or cheap, this would be ideal.

I got on the phone and called a bunch of local plumbers. None had any old hot water heaters they were willing to give up. I was getting discouraged until I called a plumbing supply company and they had several they would give me for free. I went to check them out and was really surprised at the selection and quality of the heaters they had. They sell them to scrappers, but not for much money and so the guy let me pick one out. I chose one that still had the blue caps indicating that it was brand new. (The AC Smith unit in the picture.)

It had some dents, so I suspect a customer returned it for one without damage. The dents were not a problem for me. They were just in the sheet metal on the outer shell of the unit. I assumed (and later found to be correct) that the inner tank was not damaged.

The most important thing is to find one that is not rusty at the bottom indicating that the tank is rusted and not worth messing with.

Step 2: Shuck the Outer Shell

In this picture, you can see that I have removed the sheet metal outer shell.

This is an easy process. Choose to start at the top or bottom, and remove the sheet metal screws holding the top and bottom caps on.

Once these are removed, use sheet metal shears and cut the sheet metal length wise from top to bottom. It is a pain, but you will get through it. Be sure you are using gloves, as you could slip and make a nice messy cut for yourself.

Once the outer skin sliced, start peeling it away until you have it removed.

Step 3: ​Remove the Foam Insulation, Sand and Paint the Tank

Ok I'll just come right out and say it: This step sucks and makes a big mess.

The method I used to get this done was to take a 12 inch prybar and hammer and work the prybar between the tank and the insulation and get it to break off in about 6 inch pieces. I worked my way around the tank until it was all generally removed. It wasn't perfect. This took about 30 minutes and made a huge mess. I do not know why the word LAST was written on this tank, but I am glad that these seem to be something that is still made in the good old USA.

I used some 220 grit sandpaper and hand sanded the tank to remove leftover foam and prep the surface for painting.

Once ready, I used a can of satin black spray paint that could handle 200 degrees and was a 1 step primer/paint. I put a light 1st coat on, waited about 10 minutes and then put a heavy 2nd coat per the can's instructions. I used nearly the entire can. The finish is not great, but it is black and that's all I care about.

Step 4: Form and Pour Concrete for Your Pad

I didn't get any pictures of this in progress, but you should have something solid for your tank to sit on. I chose to make a quick 2' x 2' concrete pad. This is the only picture of the pad, we will get to the wood in the next step.

Step 5: Make Something for Your Tank to Sit On

I suggest that you not have your tank resting directly on the concrete pad because this could cause it to rust faster. I had some leftover landscape timbers laying around that are pressure treated, so I decided to use them. They are strong and durable.

I made them into an X by cutting out the middle of the two timbers. Find the center of both and then use a circular saw set to the depth that you want that will be roughly 1/2 of the width of the wood. Cut several times through this section and then use a hammer to knock out each piece until they are all gone. Clean up remaining ridges with a chisel as needed.

I chose to coat the timbers with a quick coat of polyurethane for additional weather resistance.

Step 6: Anchor Timbers to the Concrete Pad

Drill holes in your timbers and into the pad for the anchors. I anchored one timber at a time. Drill the holes in the timber first with a 1/2 inch bit. Place the timber on the concrete pad where you want it and swap drill bits, switching to the concrete bit. Put the concrete bit through the timber hole and begin drilling the concrete so that you have a mark. Do the 2nd hole the same and then remove the timber and continue to drill the concrete to the desired depth following the depth instructions for the anchor.

Pound the anchor into the concrete and then pound your timber down over the anchors. Tighten down with supplied anchor hardware.

Step 7: Start Running Pipe

My existing water heater was plumbed with Pex pipe. I had no experience with Pex, but had heard good things so decided to stick with it.

I soon discovered that the Pex stuff was going to be several times more expensive than if I was to use CPVC. I decided to gut it out though since I got the water heater free. I was money ahead and I only needed to go about 22 feet.

Here is where I made my first mistake.

The Pex is great quality and great to install in theory, but these are the things they do not tell you:
1. If you get a 50 foot roll of Pex pipe, it will fight you the entire time to stay rolled. It sucked bit time. I fought non stop trying to straighten it out. You can see as I got towards the end, I just gave up and let it curve.
2. You need 3 hands to crimp the connections.
3. The tool is terrible to work with in tight spaces.

Step 8: Splice Into Your Existing Cold Water Supply Line

I ran the lines to the water heater in the garage, trying to make the visible portions look as good as possible, but I was getting sick of that damn curved Pex pipe. Installed a thermometer T into the return line pipe to monitor water temps coming from the solar tank. (This only works when running hot water in the house of course. Otherwise, the hot water will sit in the lines and cool down as expected.)

One 3/4 inch pipe is the cold supply line running from the city water. This is spliced into the cold supply line pipe going to the existing water heater with a T fitting. I then installed a ball valve for shutting off the water supply to the solar water heater.

I then installed a ball valve below the T I just installed so that I could cut off supply going to the existing water heater. This forces the water to go to the solar heater and allows me to bypass the solar heater in the winter if I desire.

The hot line coming from the solar heater is then given a ball valve as well and then T's into the existing cold water supply line to feed solar heated water into the the supply line of the existing water heater.

As pictured here: I am currently bypassing the supply of water to the existing hot water heater and sending it first through to the solar tank. If I wanted to bypass the solar tank, I would turn all of the handles down. This stops supply from going to the solar tank and allows it to go directly to the hot water heater.

Step 9: Place the Solar Tank, Plumb It in and Check for Leaks

Set your solar tank in place and run your water lines.

Ensuring your new ball valves are set to closed, open your water supply and check for leaks.

If you have no leaks, open an air vent on your solar tank (mine was at the very top, a 3/4 inch hole that I later filled with a 3/4 plug.) and let it fill up with water while allowing the air to escape. Once it is full and water is shooting out of your air vent hole, shut off the water and install your 3/4 plug.

Now you are in the home stretch. Open the hot water return line valve from your solar heater and close the valve you installed between the solar heater supply and return line ensuring that any water coming into your hot water heater has been preheated by your solar heater.

As you can see in the pictures, my tank is not ideally positioned and will receive shade from the A/C compressor a bit in the morning and from the privacy fence at night. This impacts operation no doubt, but the tank is tall, and will get at least some sun hitting it during most hours of daylight.

Also you see more of that damn Pex pipe curved and pissing me off. Also, I chose to pop holes in the siding near the center of each length. I didn't want to fight with making a hole where it was not idea. I used a 7/8 paddle bit, which the 3/4 Pex pipe fits through nice and tight.

Now remaining to do is the bracing to hold the tank upright so that it doesn't fall over.

I will update this instructable when I complete the simple wood frame that will help keep the tank in place.

Be the First to Share


    • Heart Contest

      Heart Contest
    • Fiber Arts Contest

      Fiber Arts Contest
    • Paper Contest

      Paper Contest

    8 Discussions


    2 years ago on Introduction

    Hey Chuck, this is nice, have you done any new calculations as to savings for just the pre heater? I am removing a 22 year old electric water heater this week and adding a heat pump water heater like yours. I would think that this pre heater and the new HPWH will drastically drop my water heating bill. Hopefully about $30 per month. Let us know your savings if any. Thanks! Nice work!


    Reply 2 years ago

    No I don't have any more information as to actual savings numbers pre and post water preheater for a couple of reasons:

    1. I have been out of the country for work now for about 1.5 years. and the batch preheat system was bypassed via the pictured ball valves to make sure my girl at home didn't have to mess with it in my absence. (turn it off in winter)

    2. Even before I left, there were other factors involved in my power bill and impacting the measurement of savings. These include an electric car and a plug in hybrid as well as a 5kw solar array on my roof.

    My power bill, even with the electric cars charging every day was not too high thanks to the solar array and heat pump hot water heater already there.

    I can say that I saw water temperatures from the batch tank higher than the water coming out of my tap, so I was saving money for sure but who knows how much. The batch system with free tank didn't cost me much money, so it surely will pay for itself once I return back to American and put it back to work.

    I do plan to run this type of system in future living situations and will be placing the water tank in a less than vertical orientation in future builds based on what I learned with this system.


    3 years ago

    The only issue I see is the fact that unless you require any hot water, your exterior water heater tank is out of the picture. I would suspect a thermostatic setup that when the external tank had the water at some set temperature, it would switch on a simple pump and pump that heated water into the inside tank. And that process would continue. And if the outside tank didn't get to the required temp, your inside tank would simply take over. Yes, that does mean some sort of electric circuitry, but nothing very difficult to build. But you would then get some real return from the system. JMHO


    Reply 3 years ago

    Thanks for the suggestion. I agree that this could be an option, but I think that this would be a better option for the enclosed type batch systems with a box and insulation. I think my system is not a good candidate for this because I think that my water temps are going to be less than my home hot water heater's 119 degrees most of the time due to lack of insulation, the vertical orientation of the tank and lack of reflective boost that the more complicated batch systems typically consist of.

    Feel free to make one how you have suggest and post about it so that I can see it in action and maybe change my mind however.


    3 years ago

    Thanks for the comments. As suggested, I will cover the outdoor exposed Pex piping. Thank you for the recommendations there.


    3 years ago

    Great write up, nice set up considering you are going for most cost effective. As PJCRUX mentioned, PEX is not UV stable, so get a layer or two of covering on it, or you could home to a mess. Nice Job


    3 years ago

    PEX is not great at withstanding UV and the weather. Do you plan to insulate or cover the PEX?


    3 years ago

    Nice write up. Pex- if you un-coil it in the front yard in the sun it will straighten out. I used a heat gun(lightly) and it straightened right up. Also, you may want to make a note to Bleach the tank prior to running water throught it. Great Ible tho.