Solar Thermalsyphon water heater

I just got back from a trip to my mom's house in Hawaii. When I was out there I built her a solar thermalsyphon water heater for her. I built a 2x4 frame with a 3/4"plywood backing. Laid down some insulation and covered that with some corrugated metal roofing. Made a grid out of copper tubing with an input at the bottom and output on the top. We got a discounted electric water heater from Lowe's from their scratch and dent. Covered it with a sheet of tempered glass and sealed it all up. I set the angle at a premium angle for leaving it the same angle all year. Ideally we should change the angle twice a year for maximum efficiency, but since we weren't planning on changing it, I decided to use the average angle.

She told me that it got so hot that if she didn't turn on the cold water it would burn her. It is designed to be able to hook up to electricity for back up. But we didn't have access close by so right now we don't have electric backup. It should be good for most of the year though. I will be trying to do an updated version on her rental house on my next trip later in this year.

That's awesome!

I'm going to be building a much smaller one for my RV sometime pretty soon. I want to use a glycol loop so it can't freeze though.

Go DIY!

There is a general rule of thumb for sizing the solar heat collector of 1 sq/ft of collector for every 2 gallons of storage tank size. I had a 30 gal tank, so I built a 15 sq/ft collector. Too small of a collector and you might not get the stored water hot enough, too big and the water can boil and turn to steam.

Nicely done! I love it!

If it works that well in Hawaii, it ought to work great here in Phoenix, too.

Will this work with larger diameter steel pipe instead of copper? (I don't have copper pipe...but i do have some steel pipe...I am guessing it is probably not usable for drinking water piping, but I only need this for showers and possibly washing clothes or other things needing hot water).

I also don't have glass in large enough single pieces...but I have a few smaller pieces from the lids of scanners and copiers and such, and I have several large pieces of acrylic (including some kinda scratched-up 1/2" to 3/4" stuff in full sheet size somewhere).


I do have a 4'x4' 1cm-thick mirror, if this can be placed to help reflect sunlight onto the collector (if this would be useful towards end-of-day).

AM, I don't know the different properties of water heated in steel to be able to say how safe it would be, but I see no reason why it wouldn't work to heat the water. To be on the safe side, I wouldn't drink from it, like you mentioned. I think it might depend on how much larger a diameter you are talking about. If it is too much larger, it might not heat all the water as efficiently and you might have to scale up the heat collector size compared to the formula that I used. In essence you will have less surface area on the water compared to the volume of water for heating. But I would bet if you put a steel pipe painted black out in the sun at your house it would get hot as hell by the end of the day. So you would get solar heating with it. You will need a bunch of T connectors and 2 90 degree elbow connectors for whatever pipe you decide on using. I put the glass sheet on after testing it for a day. I wanted to check for leaks before I sealed on the glass. It still got really hot without the glass sheet. I couldn't hold my hand on the copper pipes for more than a second before having to yank it away from the heat. The glass just intensifies the heat, keeps it from escaping back out. I think that you could build a system without the glass, but again, you might need to up size it a bit to compensate for a little less efficiency. One of my goals for building this was to try and upcycle as much used material as possible for this project. Unfortunately, I didn't have a lot of time on this trip, and she didn't gather as much supplies as I had hoped. I wanted her to start sourcing glass panels, used sliding glass doors, or large windows. She said that her friend had a bunch, but he flaked out on me and we ended up having to buy because I didn't have time to wait. I'm sure YOU will have no problem finding a used piece of glass. One thing that I did find from this project is the glass is almost the most critical piece when it comes to sizing. If you can find a piece for free, try to size your collector to that piece. To buy the glass and have it cut was over $100. That was a killing point for me in this project since I was trying to keep it low cost with upcycled materials. The original design was to use 2 pieces of glass, make it double pane. I think that would make it even more efficient, but I had decided that this would not be necessary in Hawaii and went with the single pane. I think for most the year this would also be the case in Arizona, but maybe the double might be worth it in the winter time. In other areas of the country, I would definitely suggest the double pane. It just wasn't worth the extra work to make it double in Hawaii with my limited time. I personally think the acrylic should work, just don't know about the scratched up pieces. I guess it depends how scratched up they are. The scratches have a potential of blocking some of the sun from coming through. But I have a friend who lives next to a golf course who wants me to help him get some alternative energy stuff going at his house. And my idea was to build one of these with an acrylic top to protect it from errant golf balls that constantly fly into his yard. A mirror would definitely help to concentrate the sun rays onto the collector, but you would just have to figure out an angle that reflected onto the collector without blocking the sun at some other point of the day.

I just realized that I didn't give too much details of how I actually did it. I've been posting in a lot of areas where everyone just thought it was cool as hell, but realistically they weren't really going to try to replicate it.....I definitely have to change the mindset while posting here. :lol: :lol:
The thermalsyphon aspect of this build makes it that I didn't need any pump to circulate the water as it gets heated. To me this was a very important part of the build, because I didn't need to supply electricity to make the thing run and that is less things to fail in the long term. It is based on thermal dynamics, where hot water rises. So, I used the bottom "drain" outlet from the water heater to be the input supply for the heat collector. As the water heats up in the collector, the hot water exits the top of the collector and goes back into the water heater at the side pressure relief port. I had put in a T connector so that the water could go into that port, but there was still a pressure relief valve at the top of the T. Then I just used the regular cold inlet and hot outlet in the same way you normally do.
I originally designed the system to use CPVC (regular PVC can't handle hot water, it will break over time) but found that in Maui, it isn't legal to use CPVC for house plumbing, gotta thank the strong Plumber's Union for that one. So there aren't a lot of CPVC parts available down there. In fact, I was at the store and the person working there was trying to tell me that I didn't know what I was talking about. He was a plumber, and they didn't even MAKE the part I was looking for. I explained to him that maybe his store didn't carry them, but considering the fact that I actually OWN those parts in Florida, I do know that they make them. :lol: I had to improvise with a few parts with what they had. I think I will try to use PEX on the next build down there, just to simplify things. But I don't feel that PEX is as insulating as CPVC, so I would recommend the latter if it is readily available in your area.

PEX is better, and a small 30W circulating pump would not take much power. it would be cheaper to buy some used panels that people discard on CL. that is how i got mine, from CL and the want ads in the newspaper.

that was 15 years ago i bot the last one, now the copper in those panels is worth 10 times what i paid for the panels i bot.

i paid less then for a 4x8 kennametal panel, $80, than that copper tubing alone.

True, a 30w pump doesn't take much power, but no pump takes even less. :lol: PEX is better at insulating than CPVC? Do you have data to support that opinion?

no, easier to use and the insulating value is essentially the same imo, and not important at all in the design. there is no cold there to insulate against anyway. biggest loss will be the solar admittance through the glass. and the collector color should be the flat black solar spray stuff. i found the paint burned off with time anyway. at 8500' in the colorado mountains the UV index is different than in the tropics.

i would never use threaded plastic fittings again and am cautious about using plastic connectors to anything. i use brass and crimp the PEX, or soldered fittings, but in pressurized city water plumbing.

There is quite a run from where we put the solar collector to where it enters the building. Next to the building there was too much shade. I did bury the CPVC along that run, for extra insulation. I think that any temp below the heated temp should be insulated against. I don't feel comfortable burying PEX. The interior of the solar collector is painted flat black with high temp grill paint. The outside of the wood was painted the same green as the trim on her house for aesthetic reasons. I don't think green wood or black wood is going to make much heat difference when we are talking about 2x4s.
I have used PVC and CPVC for more than 30 years. If done right, there has never been a problem. The only problems we ever had were accidently hitting a line and cracking it. The PVC/CPVC work that I did at a very young age far outlasted the copper work done by a professional in my mom's rental house. I have used both, and have found that both are good if done correctly, the plastic stuff just doesn't corrode like the copper.
But, like I said, I will look into the PEX for the next build out there since it is hard to find the correct CPVC fittings in Maui. It will come down to a matter of price at that point. I know what is available there and what changes to the design I need, but if PEX works out to be cheaper, I will go that route. But PEX does require more tools than CPVC. But once I buy the tools, the expense will already be spent. One major reason for doing this build was to show people that they can build a hot water source without spending tons of money or having a bunch of tools that they don't already have.

This is the run of pipe towards the house in the shallow trench that I dug for it.

they once built them into the roofs of houses in LA. before oil...

I rekon pex would be fine buried.

I'd protect any plastic tubing from sunlight and certainly I'd insulate the hot runs.

i would insulate the cold runs too. burying the service to the tank when the air temps is likely 50o hotter doesn't make sense. i can see just huge air leaks around the donkey dick insulation from the outlet of the tank. that has to be air tight, insulated and sealed to the max to prevent loss of heat, worrying about the insulating value of the tubing but not using effective insulation is penny wise, pounds wasted. that location could be change to one with very short runs of the plumbing and maybe to defend the plumbing from accidents. that's why they used to build them into the roofs of houses, and the tank was up in the attic. something no architect would ever recommend now. but the pipes should be insulated on both sides, running it through the ground is part of it.

but it is so neat to see her finally get hot water, and for free. that is what is so great about solar, the heat is free. all the hot water you can waste! you have my respect for doing this too.

I ran out of time to build this. I pressure tested it on my last day. I had to leave her to finish up the insulation of all the piping. She got it better insulated after I left, but she doesn't know how to send pics, so I had to post what I had when I left. The inlet to the tank isn't buried. In fact, since we still needed to make sure it would all work, I didn't even hard line the input. If you look, there is just a hose connected to the inlet. The hose is out in the sun, so it is actually probably getting hot just sitting in that hose before going into the tank.
She also has a big outdoor hot tub that they stopped running because it costs so much to keep it hot. I think I'm going to build a collector for that on my next trip. I'm planning on just using the hot tub itself as the "storage tank" so that I don't need to have a water heater storage tank.

My mom finally got around to getting some data from the water heater for me. She said it was overcast all morning and at noon she tested the temp coming out of the water heater. 128 degrees. Not bad for a cloudy day.

I also installed 55 watts of solar panels and a 400 watt windmill.
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I put the solar panels on the side of her house because she is just learning about this stuff and it helps her to be more active with seeing how they are connected, and keeping them clean etc. They are only 55w, so I wasn't too worried about having maximum efficiency out of them. Even at max it won't be that much. But they do produce power even in the shade, and they get great morning sun.

The windmill is whipping and will set off the charge controller ebrake when it really gets going. So now I'm having her run her Koi pond water pump off that system to use the power she is making.

dnmun said:

PEX is better, and a small 30W circulating pump would not take much power.

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Do you know if those circulating pumps can handle very hot water?

I've been trying to find circulating pumps and they all are about $200. Is there something I'm missing? I think this setup might actually work pretty good for one of her houses that it doesn't have much yard space with sunlight. If I get a 12v circulating pump that can handle hot water, I can put a little 30w solar panel with it. That way it will always be circulating the water when the water is being heated in the collector. When the collector is no longer making hot water, the pump will stop circulating it because it will no longer be getting the solar PV power.

you would put the circulating pump in the cold side, and turn it on when the collectors reach a temperature set point, then turn off when the temp drops.

that kinda stuff is expensive. i used taco pumps, the 6C and 8C, and back then they were $75 new, but the small ones are very low volume, but still significant since i used them for circulating water through the hydronic heating of the house, but it was passive solar so that was backup.

if it is working now, just go with that, imo. a good start.

i buy stuff like that used off CL. i bot 2 grundfos pumps type UP 15-18 SU and a thermostat in a nipple on a 1/2" plug for $75 with some copper flow splitting valves and wire. he wanted $90.

Well, it is working for her house. But she also has 3 rental units that she pays utilities for, and the estate hot tub that she pays the electricity for. So I still need to build more for her. And since they are charging 38 cents/kwh for electricity on Maui, I have a BUNCH of friends that I need to help out as well. I have proven that this system works, but I am looking for ways to make it cheaper or more efficient. This one will pay for itself in less than a year at those prices, but faster would be even better. The problem with CL is that not that much stuff is available on the islands on CL. And if I were to buy it up here and ship it down there, it would eat up most of the savings anyways. I will keep an eye on it there, but haven't seen too much pop up yet.
It seems like having a temp on/off control would add cost and complexity to the design. I think the only reason you would need a circulating pump is just to keep the hot water in the tank and keep a collector that is at a higher point than the storage from getting too hot and exploding from steam build up. I guess the electric water heater already has a built in thermostat for when it would kick on the heating element. I suppose you could tie the water pump into that, so instead of the heating element turning on, it would activate the pump to turn on. But that would only be for the temp in the storage tank, so it wouldn't necessarily address the collector over heating problem.

I'll have to look into this more. Like I said, the current setup works for the current situation, but I want to have an answer for all situations.

Well, you might be able to get a really cheap pump, if you're willing to "fix" it.

I just got a "broken" aquarium pump (for nothing) for a several-hundred-gallon tank, that is based around a 145W induction motor requiring a run capacitor. Guess what was broken? The little 9uF run capacitor--everything else works fine! (I borrowed a 10uF one out of my cieling fan for a few minutes to test the theory, since it appeared mechanically sound and had no winding problems/shorts/opens). Label says its' 52Liters/minute, "max height" of 6.5meters, which I assume to be the max height it can pump water up to, so it'd probably work fine for a circulation pump, at a guess.

I expect if you put an ad on CL or even Freecycle you might find people with something like this.

To use one, would you just use a thermostat to turn it on and off, so it only runs when the water in the tank is not as hot as the water in the solar panel, with some significant amount of hysteresis so it doesn't engage constantly but also doesn't run when the water in the panel is colder than that in the tank?

yes, that is the principal behind the pump controller. he has a small system, small tank and panel so he could just use some kinda poppet button switch they use for the over temp protection. but if it working now, just leave it.

you will build bigger better next time. once you get the solar kick, the free energy part, you never go back.

the insulating part in the northern climates is important too. i am thinking i should do a tutorial on how to seal the air leaks in these old houses. i finally tore out the last of the plaster i had to remove to get to a wall cavity that communicates with the outside through holes along the perimeter of where roof meets the dormer.

this double insulating i am up to now will reduce the energy consumption about 40-50% i think because of the amount of air movement i am blocking. more comfy and the big thermopane windows make it totally light. something i really need here in the winter.

Just got another update from my mom. She took a temp reading on the water output at 2pm on a sunny day. 150 degrees. Not bad considering that it is in the shade for most of the morning. She went for almost a year without having a hot water heater, and even before that she would rarely take baths, just showers, because of the energy usage. She said that she has been taking really hot baths almost every other day. She has a well and a solar water heater now so she decided to live it up and enjoy one of the things that makes her really happy. We also recycle the grey water for non-edible plants.