Solar Powered Preheater for Tankless Water Heater





I wanted to preheat water with solar energy before it went to my electric tankless water heater. The higher the temperature of the water going into the heater, the less electricity will be used to bring the water up to the preset temperature on the heater = money saved and it's Green!

First I re-installed my old 40 gallon gas-fired tank-type water heater. It is not even hooked to the gas line, it will just be a holding tank for the preheated water. I plumbed it so that the supply water comes into it first and then goes to the tankless heater. I'll get some more use out of it and it won't have to go see Mr. Recycler yet.

I installed a new pressure relief valve on it for safety. I popped off the plastic drain valve at the bottom of the tank and installed a steel nipple which then adapts to cpvc. That will be the cold water feed to the solar collector. Then I installed a "T" in the copper pipe between the output of the holding tank and input of the tankless heater. That is the warm water return from the collector. Don't worry about envisioning those connections right now... they will be obvious when you get to the "step 6" page.

Then, I threw together a "hot box" out of 2"x4"s and a piece of particle board. I made it to the dimensions of an aluminum framed window pane I had. I just used 3" drywall screws to hold the 2"x4"s together and 1-1/4" drywall screws to put the back on. I used a lot of silicon caulk to make it as air and water tight as possible. Painted it flat black and plumbed it with 1/2" cpvc. Then I painted the cpvc black too.

I don't know why I used this zig-zag configuration. I just started building and this was what I ended up with. I sometimes do things like that. It didn't occur to me until later that most of the solar collection boxes I see use a manifold across the bottom with several vertical tubes going up to another manifold at the top. Probably more efficient. But I'm sure this box will not be the one I keep. It was just a proof-of-concept quick-and-dirty.

Sorry about the fuzzy picture... the rest are better.

Step 1: Up Goes the Temperature!


Quick and dirty or not, it does a fine job of capturing the heat. I lifted the glass and took a reading and was surprised to find a 100 degree temperature rise!

( Nit pickers... notice the Max reading in the second )

Step 2: Let's See If It Will Move Water...


So I plumbed it up to a (clean, new) plastic garbage can full of water and thermal convection started moving the water through the pipes.

(Just like hot air rises above cooler air, hot water will also rise above cooler water because the hot water is less dense thus its specific gravity is less. This is called thermal convection and we can use it to eliminate the need for a pump to circulate the water.)

The water in the garbage can will quickly stratify with the warmer water at the top and the coldest water at the bottom. The heated water in the pipes will rise, going through the hose to the top of the can. The coldest water at the bottom of the can will be drawn into the pipes to replace the rising heated water.

And the convection was stronger than I expected with only 1/2" tubing. Bits of pollen in the water that drifted in front of the return pipe suddenly shot to the other side of the can at surprising speed.

I drilled a small hole in the top of the collection box and inserted a probe thermometer to be able to read the interior air temperature at any time.

Step 3: Where to Put It?


The best place to harvest sunlight on my property would be in the middle of the front yard. Nope... eyesore. The city Yard Nazi would have a stroke when he saw it. So I decided to build a small outbuilding on the edge of the driveway and mount the solar collection box on the "roof". But... if the collector was located on the roof... then thermal convection would not circulate the water. I'd have to use a pump and I didn't want that.

While making observations with the collector hooked up to the garbage can, it got later in the afternoon and a pesky tree shaded my project. I went in the house and grabbed a largish mirror off the wall and brought it out and reflected the sun onto the collector. Hey... it worked pretty good! So I grabbed another narrower mirror I had and watched the temperature rise higher.

That's when I decided where to mount the collector. Right on the corner of the house where the holding tank was inside, only about 3 feet away! Sure, that spot is in the shade for half the day... but I've got mirrors!

At first I just propped the mirrors against a concrete block. It was a pain to adjust the mirrors as the sun moved. They were top heavy and the edge resting on the ground was constantly on the verge of slipping. I eyed an old shipping crate that was scheduled to be converted into kindling and I chopped it up it to make two holders for the mirrors. I did say "quick and dirty" didn't I?

Step 4: Mirror Manipulation


The narrow mirror is still held up by a block, but I reassigned a Lazy Susan to hold the bigger mirror up.

The crate is held to the Lazy Susan with a regular door hinge. The mirror is held up by a piece of 1/2" cpvc that pivots on a single drywall screw. There is a cpvc straight adapter with a washer inside so the long bolt will pivot inside when it is turned. The bolt is threaded through a large nut welded onto a steel plate which is held by the wooden blocks. Thus turning the bolt adjusts the vertical tilt of the mirror and the Lazy Susan handles the horizontal pan.

This, of course, will be improved upon, using stepper motors and control electronics, but right now I just want to see the thing move hot water to my holding tank!

Step 5: Mounted and Working Solar Collector


Here is the collector mounted and plumbed to my holding tank which is just inside the wall, right beside where the pipes go in. Both mirrors are reflecting on the collector and the probe thermometer went as high as 170 degrees F even though the box itself is in the shade of the house!

Kinda close to the 180 degree rating of the cpvc though! Of course, 180 degree air temperature in the box does not mean the pipe is that hot. The water is sucking the heat out of the pipe. If I get the pipe itself up to 180 degrees, then I'm doing something awfully right!

The original glass pane had an unfortunate accident and got replaced by another pane that was not quite the same size. The lady at the local recycling center is on the lookout for more glass panes and mirrors for me.

A one inch thick piece of styrofoam has been glued to the rear of the box to help insulate. Double-paned glass would no doubt be an improvement too.

It is a pleasure to put my hand on the feed pipe and then on the return pipe and feel the temperature difference. From initial tests, the water appears to get a temperature rise of 20 degrees F on each trip through.

I realize that copper would transfer the heat better than cpvc. But copper is expensive and this is just a test box. If the cpvc holds up to the heat of 3 large mirrors, I may stay with it. Otherwise, I will probably go to copper with a future collector.

Step 6:


Here is a diagram which shows a general overview of what I built.

It might make the system a little easier to understand.

Arrows indicate the normal convection flow. The coolest water at the bottom of the holding tank sinks by gravity to the collector where it is heated up. Then it is lighter so it gets replaced by more cool (heavier) water. When no water is being run in the house, no water is going through the tankless, so the warmed water takes a right turn in the "T" and goes back to the top of the holding tank. It needed to go there anyway to replace the water leaving at the bottom.
Not shown is the dip tube which deposits incoming water in a heater to the bottom of the tank so it won't mix with and cool off the hotter water at the top on its way down.

Just remember that in order for convection to work for you, the collection box must be lower than the holding tank. The tankless heater can be low or high... it doesn't matter because the water pressure is pushing the water through it.

When hot water is turned on in the house, water is pushed through the tankless heater from both the top of the holding tank and from the bottom through the collection box. No matter. The water the tankless heater receives will be much warmer than if it came straight from the street.

Step 7: Mana From Heaven Aka: Money From Sunlight


Now go build one. It's not going to build itself! You can preheat water for any type of water heating system, gas or electric, tankless or with a tank, and it will translate into savings for you. Just be safe and get professional help if you have to in order to follow local codes and so forth. Yes, that was a disclaimer.

Remember, even if what you build is not enough to supply all your needs, that doesn't matter. If it just supplements what you have, then every bit of energy you harvest is money saved! It amounts to turning sunlight into money!

Oh, one other thing --- If the ambient outdoor temperature is higher than the temperature of the water coming out of your supply pipe, this thing will work even without sunlight. That's right... on a cloudy day or even at night! If your water comes out of the supply pipe at 55 degrees and it is a cloudy, but warm 80 degree day (or night), the water will still circulate until the holding tank is at 80 degrees. And feeding 80 degree water to your heater is better than feeding it 55 degree water!

Money saved - day AND night!

Q - "What if it gets cold at night? Won't that cool off the water in my holding tank?"

A- No. If the water in the collector is cooler than the water in your holding tank, then the cooler water will just sit there because it is heavier. Circulation will stop until the sun comes up and heats the water above the holding tank temperature. Perfect automatic control!

Just remember that hot water rises. If your regular heater has a tank, then it needs to be at the highest point in the system. The solar collector needs to be at the lowest point and the holding tank between the two.

If you use a tankless heater, then you only have to worry about having the collector lower than the holding tank and you don't have to worry about the height of the tankless. It could even be in your basement. Don't forget to insulate all your connecting pipes!

Now what...

Well, I need automatic control of the mirrors.

I'm learning about heliostats. Heliostats are different from solar trackers. A solar tracker keeps something (like a pv panel) pointed at the sun. But we don't want to point a mirror at the sun.

A heliostat follows the sun and positions a mirror in such a way that it keeps the sun's reflection on one spot... your collection box. It is a bit more complicated than a solar tracking system because the mirror has to be pointed at an angle that exactly bisects the angle between the sun and the collection box and that angle is always changing. Also, PV panels are forgiving about the vertical tilt angle but a heliostat has to be accurate in the Z axis as well as the X in order to keep the reflection on one point.

I don't want a M$ Windows controlled system. I don't really want to have to learn to use a Pic controller if I don't have too. I want it to be powered by a small lead acid battery charged with a small pv panel.

I'm leaning towards a system that uses GaP (green) LED's as sensors which control a Full Bridge Motor Driver. But I'm still in the initial stages of that. One thing I have learned is that thinking about heliostats makes my brain hurt.

For those not familiar, LED's can convert light into electricity, similar to a photovoltaic cell. Gallium Phosphate (green) LED's do it best. Three green LED's wired in series and exposed to bright sunlight will produce a voltage high enough to switch TTL logic circuitry. Before you get any ideas, LED's are not a practical replacement for photovoltaic panels. They will produce a voltage but practically no current.

I just ordered a bag of 100 water clear green LED's and spent last night destroying/salvaging two defunct printers for motors, gears, etc.

I hope you liked my first Instructable. Visit my project site: to see other diy projects I have documented. Also documented is my Electric Tankless Water Heater with performance observations if you are interested. Hey, it's free.

UPDATE August 8, 2009 - I'm still working on the heliostat but have been slowed down by the need to make a relatively complex circuit board for the controller. I got sidetracked building a tabletop cnc machine (a la Tom's Easy Mill) to make the pc board with. I'm almost to the point of making the board, waiting for engraving bits to arrive from the other side of the world.

My build of the mill and pretty much day-to-day progress is being documented at:

If you don't mind, click a star up there and give us a rating. Thanks!




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    44 Discussions


    8 years ago on Step 7

    A most excellent presentation!

    I've been toying with the idea of augmenting my current heating system with a solar hot water heater for some time now and have been wondering how I might make it happen.

    I have an indirect system where a loop from my boiler goes into tank to heat the water. My boiler is about 85% efficient, and I'd love to improve that efficiency using solar.

    Your project confirms several ideas I've been thinking about, so given time, money and materials, I just may be able to implement something.

    Keep up the good work!


    9 years ago on Step 7

    Hi Dave,

    Thanks for this instructable. Really inspiring :)

    One thought: you might want to stress the fact that it is really important for anyone who wants to build a similar setup to keep the 'old' heating ON (either tank or tankless) and not be tempted to switch it OFF (or set it to a low temperature) and only use solar heated water. The warm water that reaches you shower must always have been heated to at least 60°C/140°F (!) to prevent the possible build-up of Legionella in the holding tank getting into your longs while showering.

    And one question: what are your thoughts on leaving this collection box outside and connected during the winter in areas where it can freeze up to -20°C? Will ice in the pipes rupture them and cause big leakage or do you think the mass of the relatively warm water in the holding tank inside, by way of natural heat exchange through the water pipes (read: heat loss) prevent the still water in the pipes outside from freezing up?

    Thanks for your thoughts!

    11 replies

    Reply 3 years ago

    Dave, legionela is fake.
    It explained here (but only in spanish)

    Hundreds of Spanish towns take over 100 years using a tank on the roof, is the cold water, and is using as emergency water because in past the water was very unstable.

    At present, the water supply is never the fault, but they still used by tradition.
    There never cases of legionella.

    Cases of legionella is never unnoticed or hidden presence, everyone knows that when there is an outbreak or spread of legionella many people die in a few days. But that does not happen there with standing water on the roof and they drink, plus I have seen many of these deposits have mud, mud, extremelly dirt and not any type of mantenance.
    The Spanish government uses harmful products in water for do possible drinking with no risks (is no risk in short time but other big problems in long time) to ensure that no one will acquire any bacteries in first 48 hours... after this hours is off warranty. But nothing happens there with the backwaters.

    Legionella is now an excuse to sell the industry standards and promote evil energy expenditure.


    Reply 7 years ago on Introduction

    Disabuse me if I'm wrong, but I though that I had read that Legionella resulted from hotels keeping the water at such high temperatures benevolent bacteria was killed off leaving the heat-tolerant Legionella bacteria without any competitors.

    In my travels through Asia I saw water heated everywhere solely by solar, and Legionella was never an issue.


    Any bacteria that can live at the human body temperature, cannot usually live at temperatures higher than that... Pasteurizing, for example, doesn't require boiling, but a slow method temperature around 65 F for 45 minutes complies with pasteurization requirements. So, if milk is safe for by pasteurizing, so should water!


    Reply 7 years ago on Introduction

    Laughingcoyote, I have no idea how the Legionella bacteria holds out in an environment populated with plenty of other (benevolent) kinds but the idea here is to use this pre-heater for normal tap water, which in most 'modern' countries, usually is relatively void bacteria. Not completely, but still.

    So then the question is in what environment, in terms of temperature only (since there will be hardly any competition), the Legionella bacteria will thrive. The article on Wikipedia I referred to, confirms what I have been told about the subject: they thrive between 25 and 50 degrees Celcius but will be killed when the water reaches a temperature of about 60 degrees.

    All cases of Legionella poisoning I have heard/read about involve three things: (1) water with a temperature within the correct range, that (2) has been still for long enough to let the bacteria propagate plus (3) a shower, garden sprinkler, fire hose, or fountain that sprays that water around so the bacteria can get airborne (living in tiny droplets) and reach fragile long tissue. Given these conditions, infection is likely.

    Maybe the solar heated water in India is never used in showers with shower heads that spray fine enough droplets, or the water usually reaches above 60 degrees C, or the tanks are never so big that they keep water long enough to let Legionella build up...


    Reply 7 years ago on Introduction

    Having been an engineer on a ship in Alaska for many years I can tell you. You don't want any water siting in a pipe at -20 C. Having had many pipes rupture even those that were valved off, because I didn't get them drained. If you live in an area that gets cold you need a draindown valve that empties the water out. And your system needs be designed to not hold pockets of water, like a manifold system rather than a single loop. A manifold system looks like a bunch of capital H's and water enters the bottom and is drawn from the top.

    Or else use a closed loop that runs water with anti-freeze or other liquid through a heat exchanger in your water tank.


    Reply 9 years ago on Introduction

    Hi Ravan, Two good points. Actually, I had not even thought of the possibility of problems from bacterial buildup in the holding tank. I'm on chlorinated city supplied water. But someone getting their water from a well or using rain water would probably do best to read the Wikipedia link you posted and take appropriate precautions. In my Instructable, I only touched on the problem of freezing weather. In the picture on Step 5, there is a comment box that says I plan to convert over to a heat exchange system and use some kind of antifreeze solution when winter gets here. I got a couple of new but broken tank-type water heaters for free from my local plumbing supplier. They just had defective gas valves and I guess it was not worth the cost and hassle to return them to the manufacturer. They were just set out back and were free for the taking, otherwise they were destined for the recycler yard. I plan to take the tank out of one and put it in a 55 gallon drum. The drum would be well insulated and contain the antifreeze solution that circulates to the outside collection box while the heater tank is plumbed into the tankless heater supply line. I might use isopropyl alcohol/water as the antifreeze. To answer your question, no, the warm water in the holding tank will not protect the pipes in the collection box in the system I have built. It might be okay during the day with sunlight falling on the pipes, but on a very cold night or a very cold cloudy day, the circulation will stop and the temperature of the pipes will approach the outside ambient temperature. The pipes would need to be drained or some other measures taken to protect the pipes from freezing and bursting.


    Reply 9 years ago on Introduction

    Hi Dave, thanks for your response.

    About Legionella and chlorine: I understand (but am no expert on the subject) that the little critters are more resistant to chlorine than most other contaminants. Found a study on the subject on that might tell a more learned mind what the risk are. Obviously, the level of chlorination of your city tap water needs to be considered. Apart from that, it looks like they found that chlorinated water of a higher temperature (in the study 35 C which would normally be perfect for survival) Legionella dies quicker than in chlorinated water of lower temp (4C and 21C in the study)... So by your preheating method you seem to be actually reducing the risk of Legionella! If that is a correct in deduction of course... ;)

    Sadly (in this respect) I am using non chlorinated water so by preheating, I am increasing the risk. By ensuring that my electric boiler tank always raises the water temperature well above 60C, I should be fine... I suppose this last stage heating (reaching over 60C) on night current only will be enough. Electric heating is relatively expensive so I would like to keep it limited to the reduced price of night time consumption.

    About the freezing risk problem: I found a company in the UK, SolarTwin, that actually sells a somewhat similar system. The company claims their collectors do not need draining in winter since the pipes are made of some silicone rubber that can withstand expansion from freezing water. As soon as the sun comes back, the water will melt again and the system will be back functioning like before. As far as I can tell, their collector is designed for low pressure water systems, not high pressure city water... Maybe that allows for the use of more flexible tubes in the collector. I wonder how rigorous tubes must be for high water pressure and still be flexible enough to withstand 4% expansion when the water inside it freezes up. And if at all possible, what kind of tubes/material would suit the job.

    Any thoughts on this last missing piece in the puzzle? If that is tackled, there would be no need for any heat exchange / antifreeze type solutions which would make it so much more expensive and difficult to install




    3 years ago

    Great fun project I might try this to heat the greenhouse a bit above freezing in Winter

    Probably by using manifolds rather than zig-zag you could heat more water faster. The zig-zag probably provides more surface area than you need.

    I just read about one design that uses a heat exchanger from a discarded refrigerator (My refrigerator manual calls it "condenser"). It was advised to get a professional to first remove the freon or whatever is inside nowadays.


    7 years ago on Step 7

    Hi Dave good project i am about to start something like this one thing i found is old fluro tubes around the pipes increase heat when you have the pipe in the middle of the tube and sealed with foil duck tape on each end you get like a 360 degree heating of the pipe plus a nice green house effect so it keeps more heat in on those cloudy days, from what i seen of your design it would not be too hard to add this.

    This idea is on this site as well but he used pvc pipe so it would work alot better with those nice copper pipes.

    1 reply

    Engineers in Gaviotas in Brazil (described in "Gaviotas: A Village to Change the World") used discarded fluro tubes to do the same thing. The engineers used some sort of reaction to oxidize the surface of the copper pipes to make them the very darkest black possible. They then found that the temperatures became so hot that the fluro tubes burst, so they created a vacuum inside of them. The result was that they could get the temperature much higher using their recycled rubbish than Japanese engineers using the latest and most expensive technology. Besides water heaters, they heated oil which they ran through copper-coil stoves used first to sterilize hospital equipment and later to process pine resin.


    7 years ago on Introduction

    I have an oil fired furnace that has no separate holding tank. It keeps turning on to maintain the temp. in the furnace hot water core. I want to shut it down in the warm months. Can I add some sort of solar heating unit to this system. Maybe to preheat the water that goes to the furnace from my well input. A tee type add on or what? My back south facing deck gets pretty good sun. I would put it there. Any suggestions? I just got hit with a more than $1,000.00 cost of 300 gallons of home heating oil when my tank ran below 7 1/2 inches on a measure stick. The furnace shut down. The strainer must be at that height. It happens every year. Please help, these oil prices are killing me!


    8 years ago on Step 6

    This is fantastic! I'm looking to do something similar.


    9 years ago on Step 7

    Hey Davetech, Nice instructable. I'm looking to build a similar solar water heater for the tankless gas heater for my pool. I was wondering how the copper t-junction worked to feed the hot water into the tankless heater. I'm concerned about plumbing the system so that it doesn't pop leaks and/or back up. Thanks!