Our passive greenhouse solar water heater

It can't get much simpler:
An old window framed with old 2 x 4s, an old piece of OSB for the backing, 20' of 3/8 cooper, 3/8 and 1/2″ plastic tubing, a piece of garden hose and a couple of 1/2″ plastic barbs reducing to 3/8″.  Black spray paint for the OSB and copper and of course you need a water barrel.

1/5/10 solar water heater

This isn't nearly enough copper tubing to actually get the water in the barrel “warm” in winter with freezing temperatures. However, it helps make the water a little warmer and this really was an experiment to see whether it would even work without pump.
We originally planned on using an old water heater, but couldn't get any of the upper plugs open. Ultimately, we want to preheat the water going into our propane water heater.

Why is the panel INSIDE?

An obvious disadvantage of the solar panel inside is that you get less sunshine for your plants. However, if we'd had more sliding glass door panels and time, we would have built a greenhouse with much more glass.
The advantage of having the panel INSIDE is that it didn't freeze until it got down to 9 degrees the second night in a row. If you leave a solar water panel outside during freezing temps you either have to build a drainback system (no water in the panel when it freezes) or use antifreeze. We wanted to keep it simple.
ANOTHER advantage of keeping the panel inside is that the heat doesn't dissipate outside (you can barely touch the glass when it's sunny) and it helps heat the greenhouse.  The panel and copper of course also get warmer inside.
In spring that will no longer be desirable because it will get too hot during the day.  But it's extremely easy to move the panel outside right in front of the barrel, just have to drill a hole for the garden hose into the plastic.
Construction:  SIMPLICITY rules
Tools required: copper cutter and bender.  Those tools are rather expensive (about $10 or so each) considering our low budget and I hope we'll have some use for them in the future.
If we'd known that we'd make only one cut, we would have just used the hacksaw.  However, the BENDER is a MUST HAVE unless you're very skilled at bending copper without kinking it.  One kink and the water won't flow anymore.
copper bender and plastic tee package

We actually had a torch kit with solder and all that, bought copper fittings and got ready to solder when we realized the kit didn't come with butane. DUH!  They didn't have any butane at the local hardware store, but they DID have plastic barbed tees.
From the garden hose to 1/2

So here's how this is put together:
The barrel had a basic garden hose connection.   We got one of the many bad garden hoses we save for an occasion just like this one and attached a short piece.  It so happens that leftover 1/2″ clear plastic tubing we had laying around from some other project perfectly fit inside the water hose.
We bought some 3/8″ clear plastic tubing which happens to fit perfectly over the 3/8″ copper tubing and barbed 1/2″ plastic tees reducing to 3/8″.

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The upper tee, joining the two 3/8

Unfortunately lots of reflection from the glass, but I hope you can see the upper connection.  We stuck the 3/8″ copper in a short piece of 3/8″ plastic tubing and that goes on the barbed tee.
And of course we spray painted everything black.
Because this system is NOT pressurized, it does not leak.
After the 2nd night of extended temps in the teens (to 9 degrees), it froze.  I noticed that the tank was EMPTY the next day.  It took about 5 minutes max. to fix it:
Take out the screws to take the OSB off, stick the 1/2″ tubing back on the barbed tee, screw the OSB back on.
Wish everything in life was so simple.

Here it is from the inside.  I forgot to mention, we screwed an old 1 x something to the OSB and that rests against a chair and is how we keep it upright or at whatever angle we want.
The hose to the tank sags a little, but it still works.   We keep getting air bubbles as the water heats and since we have two copper tubes, you apparently don't get all the air out when you flush the panel with water.
On sunny days I check occasionally to see that it still works (never know!)  and I let the air bubbles out.  I take the plastic tube out of the barrel, lower it so that water flushes the air out, put a finger on the tubing end and put it back in the barrel.  It has to be rather full to make that work without air getting back in the tube. And while I do this, I can tell that it IS working because after a few sunny days the water at the top of the barrel is definitely warmer.
So this is the simplest solar water heater I've seen yet!
We looked at many youTube videos, but they all required soldering and a lot more copper.  This is nothing to write home about, but it works and it's cheap except for the copper, barbed tees and tools we had to buy.  I think it might work almost as well with black tubing of any kind, have to try that on a slow day.
I cover the tomato plants with a blanket every night when it freezes and that's shade cloth on the outside.  Certainly not award winning tomato plants (the good ones are in the adobe greenhouse), but I didn't want to kill them off and needed something to TEST the greenhouse.  3 or 4 plants towards the center of the greenhouse finally froze during the 3rd night of single digit temps.  To my surprise, many plants not only flower, but they also set fruit.
I don't know how many more of these extreme freezes to expect this season, it's very unusual to get single digit temps or even temps in the teens in our area.
We had the largest heat gain during the coldest temperatures.   9 degrees outside, 26 degrees in the greenhouse.  Then the greenhouse thermometer broke and it must have been colder when the panel and a few tomato plants froze.
Last night the outside low was 20 with 28 in the greenhouse.   We'll be transplanting chards, beets and broccoli into a greenhouse salad bed soon and we'll see how that goes.
An even simpler, yet more efficient system:
Get a cheap used solar water heating panel.  Sometimes you can get them for $20 or so.  Just make sure it's not leaking.
If we hadn't found the barbed tee and plastic tubing, I would have used duct tape and hot glue.  Since the system isn't pressurized, almost anything would work for the connections. Who cares about a little drip in a greenhouse?
Even simpler:
Just get 1/2″ or 3/4″ tubing and spray it black.  Attach it directly to the window wiggling your way up from side to side.  Why didn't we just do that?  Watched too many videos on how to build a solar water heater 🙂
You'd still get sunshine inside for the plants and it would be much cheaper and faster to build.  We might try that!  Last winter we did our dishes with water heater in black one gallon jugs and they sure got HOT just sitting on the adobe greenhouse shelves.  Don't know what happened to that practice, we got lazy.
And you might have noticed the old gallon milk and water jugs filled with water throughout the greenhouse.  We have probably about 40 or so in the greenhouse and two water barrels.