Solar powered Preheater for Tankless Water Heater

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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!

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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 picture...lol )

Comments

[Unkel]

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!
Unkel

[ravan]

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!

[laughingcoyote]

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.

[Boldhawk]

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!

[Boldhawk]

yes, you're right. I mean Centigrade/ Celsius!

[ravan]

Surely you mean C, not F ?

[ravan]

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...

[AsianSpanker]

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.

[laughingcoyote]

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

[Davetech]

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.

[ravan]

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 http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=239530 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

:)

Allard

[laughingcoyote]

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.

[Blackice504]

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.

[laughingcoyote]

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.

[triumphman]

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!

[diynot]

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

[tahoekid]

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!

[Davetech]

Hi Tahoekid, It is just a simple "T" inserted between the holding tank and the tankless heater. No valves or anything needed. The arrows show the convection flow. When water is pulled through the tankless heater, some will be pulled through the collection box as well as from the holding tank, but that should not matter. Hope this helps and good luck with your project.

[Davetech]

After some thought, there may be a better set up than that first diagram. It is the way my system is set up, for intermittent use, where the holding tank slowly warms up and only occasionally gets drawn from. But for a pool, I'm going to have to make some assumptions. Like the tankless heater will run for long periods? And its supply is not from the street, but from the pool? If so, you might not even benefit from having a holding tank. Your pool itself is your holding tank. This diagram might be a better setup for a pool. I'm not a pool man and I don't even play one on TV, but this is my best guess. If your tankless is getting its supply from the pool and just recirculating it, then it doesn't matter where the preheated water gets injected. Any calories you add to the water will make life a little easier for the tankless.

[ravan]

This seems to be a good starting point but there is one thing missing: a pump to circulate water to/from the collection box. At least for most pools because it is simply not feasible to place the collection box below the pool (usually below ground level already) so the heat collection also takes care of water flow. Two options spring to mind: 1. connect the collection box between the heater and pool so that the already installed pump does the work - or - 2. add a second pump that is driven by a small solar panel. This second option can be a simple setup since it would need no battery for solar power storage since there is only pump action needed when the sun shines anyway.

[Aldeas]

I wonder if it would be possible to mount the control circuit "eye" of a regular sun tracker in a shaded box on top of your collector, but keep the actual control motors on the reflective mirrors? Then your collector would "track" the reflection of the sun from the mirrors, keeping them turned toward your collector at all times. Just a thought, possibly a little less dizzying than the heliostat.

[redrok]

This is what I call an "In-Line" heliostat tracker.

There are other places you can place the sensors for In-Line trackers.

Very close to the mirror is the best place. It looks at the sun back through the mirror and attempts to keep the sun in the center of the mirror. When working it is stable.

However, there is another stable position. This is when the suns image is anyplace on the edge of the mirror.

There is another tracker type called the "Receiver Axis", or "Target Axis" tracking mount. It's a bit more complicated to describe. See:
http://www.redrok.com/heliolighting.htm#receiveraxis
The main advantage is it can't get confused because the tracker is always aimed directly at the sun. It is mounted on a small mechanical gear mechanism.

Duane

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[Davetech]

That sounds like a good approach, but I don't know how to re-aquire the target if clouds blocked it for a while. With more circuitry, I could stop it from searching while the cloud was blocking the sun, but then I'd have to re-aquire. I think I'm going to have to learn to use a Picaxe micro-controller. Ordered one last night and have started scaling the learning curve. Thanks for the idea!

[Davetech]

Update: I've found that the Picaxe is pretty easy to work with and is Fun! Now I'm leaning towards using your idea, by putting 4 sensors around the collector box; one above, one below, one to the right and one to the left. As the sun's reflection drifts off the box, a sensor will detect it and signal the microprocessor to run either the x axis or the z axis motor to un-light the sensor. Waiting for parts to arrive and working on permanent mirror mounts now.

[Andydog]

http://www.thenakedscientists.com/forum/index.php?topic=22494.0

This is my system been running for about 6 weeks. hottest water 78 degrees C
Introduced thermostatically controlled taps in Kitchen and Bathroom for shower. (Had to it was too hot) Takes 15 minutes sunshine to reheat water when run cold. Have showers off it and don't need to rush :) Fill washing machine up with solar heated water so saving there also. Used to have leccy shower, now have solar shower saving more leccy.

Black plastic tube coil acts as expansion tank and also primary heater for the water, plus extra reservoir. May change to larger bore tubing but it works so don't fix it for now at least.

Thanks for the comment Dave.

Andrew

[Davetech]

Hey Andy! It looks like you've "really got it goin' on there! " I don't have a way to directly measure the temp of the water in my holding tank (yet) but I have a probe stuck up against the "T" pipe under the insulation. But I know I don't get near that hot of water. It usually reads around 40 C. You definitely have a good system there and it is going to save you a lot of money. Maybe when my heliostat is finished and it keeps three mirrors on my collection box all day... maybe I'll see temperatures close to that. Thanks for the post!

[Andydog]

Hi Dave your more than welcome :) Love these money and Earth saving projects. The secret is in using corrugated steel sheets painted mat black, double the surface area in the same cabinet and the corrugations house the upright 15 mm tubes so the heat source is surrounding the pipe rather than the pipe laying flat on a back plate. No need to worry about circulation and distribution of heat either as the density changes in the water from the solar heat introduces efficient circulation making sure the water is heated all over the system. Downside of having a hot water tank is that sporadic sunlight will never get the heat required for domestic use due to the large volume of water in the storage / hot water tank. With a direct system you heat what you need and it takes 15 minutes to heat the water back up after fully emptying it. Having a shower for example requires a sufficient amount of cold water to maintain the flow of hot water for the entirety of a shower without needing to rush. Wife can take a shower and has done after just a few minutes, the time it takes for me to get dry and dressed is all that's required. The mirrors will undoubtedly help by increasing the heat and the time the panel is exposed to the sun. Great idea. Have to watch the neighbours here though and the panel as it stands has not generated any adverse interest locally so leaving well alone :)

[Davetech]

I've been documenting my automation progress on the aptly named: http://www.cerebralmeltdown.com/forum/

It has a diy heliostat thread and also a diy cnc thread.

Come join us!

[jimalee37]

Dave i really like the idea it seems easy enough for even me to try , but i am confused on how to plumb all of this together with the water heater. could you help with that. thanks jimmy

[Davetech]

Sure, just shoot me an instructable email and tell me about your heater system and I'll try to help you.

[Jalloy]

(removed by author or community request)

[Davetech]

Hey, go right ahead! I'm flattered. I've been slowed down by a few things (like my D Drive suddenly going belly up) but I have assembled a linear actuator that the micro controls nicely. It is built from dead vcr and printer parts and mounted on flattened ABS plastic pipe material. Here's a couple of pictures of it:

[Pindell]

My water heater is in the basement. I guess I'd have to keep the initial storage container (garbage can in this case) on the main floor in order to use thermal suction and gravity feed from near the top of the initial storage container to my electric water heater in the basement somehow, right? Or is there another way that you'd propose (preferably without using electric pumps, etc.)?

[Davetech]

Well, with that setup, the warmest water in your basement heater would convect up to your holding tank. Not what you want. Just remember that hot water rises. You would have to overcome the convection with some kind of pump. To take advantage of convection, your regular heater needs to be the highest item in the system, with the solar collector box at the lowest point and the holding tank in between the two.

[Davetech]

-Unless - your water heater in the basement was tankless. That would simplify things. The collector box could convect up to the holding tank on your first floor and just the water flow when you turned on a faucet would send the warm water from the holding tank down to your tankless heater in the basement.

[Davetech]

I have added that point to the instructable.

[aleutianwind]

Hi Nice setup and easy to build. Thanks for sharing. I refilled mine after the freezing weather and it has been working great, We just love almost free showers. Thanks again.

[frollard]

I want to design a turbine that uses either/both freshwater intake pressure and/or graywater output pressure (2.5 storey house = lots of energy) to help the solar heated water along. It wouldnt take much oomph to get a reasonable flowrate.

Great instructible - and keep us updated with the heliostat!!!

[ve2vfd]

Thats a pretty cool project! I think I'll probably adapt it as a warm water system for a friends fishing cabin. He is hundreds of miles off the grid, water is a gravity fed system + gas pump to refill the water "tower".

[Davetech]

The next time he needs to paint the tower tank, tell him to paint it flat black.