The Walden Effect: Farming, simple living, permaculture, and invention.

Low-tech, sustainable hot water heaters

Thermosyphon water heaterMark has been lobbying for hot water for years, and I thought now would be a good time to finally give long as we choose a sustainable option.  At the moment, we just heat up gallon pots of hot water on the stove, one or two of which are generally plenty for dishes or bathing. 

We definitely want to make (or possibly buy) a solar hot water heater as part of our upgrade, but we're thinking of doing something low-tech (an open-loop system) that will be drained for the winter.  As such, it would be handy if we had a traditional hot-water-heater tank to thermosyphon the solar-heated water into.  In a perfect world, we could leave that tank turned off all summer, then would turn it on once or twice a day, half an hour before using it, in the winter.  The primary uses would be dishes and bathing, and we're quite happy to plan both around the weather.

Here's where I'm getting stuck --- household hot water heaters are huge!  The smallest one I saw on the Lowes website was 28 gallons, but I figure we only need a capacity of one to three gallons.  Keeping the water heater turned off until we need it wouldn't be very efficient if we're heating ten times as much water as we use each time.

Point of use mini tankI had originally decided against point-of-use (aka demand) water heaters because I wanted a tank for our solar system to feed into.  However,  a bit of research shows that there are point-of-use mini-tanks of 1.3 to 6 gallons that might work for us.  However, these mini-tanks probably aren't very well insulated since they're intended to provide hot water much more frequently than we'd use it, and since they're meant to be left on all the time.  Do you think a point-of-use mini-tank would operate the way I want it to --- as a reservoir for solar hot water and as a winter heater than can just be turned on right before use?

We're very much still in the planning stages, so suggestions are welcome.  Perhaps you've bought a certain brand of any of the options mentioned above and loved it (or hated it)?  Or maybe you built your own?  Please do chime in and make Mark's life a little warmer.

Our chicken waterer kits come with tips for building heated waterers so your flock can drink clean water without effort on your part all winter.

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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Why not get a used water heater for storage of what is solar heated and then between it and the faucett have a point of use, which store no water--the most efficient kind. So the solar preheats and if needed the point of use kicks in. Ace hardware used to sell the containerless ones for $200. Back in the early days, we used a discarded water heater tank inside the solar collector, which was placed on the south side of the house or on the south roof. The box was glazed and well insulated where there was no solar gain. In a traditionally plumbed house the water pressure would fill and empty the collector when water was used.
Comment by Errol Fri Jun 28 08:41:01 2013
First commenter: Your place wasn't traditionally plumbed so how did you fill your tank?
Comment by Cheyenne Fri Jun 28 09:06:55 2013

Interim solution: solar camping shower. Advanced Elements 3 gallon one is more than enough for a shower with my long hair. Put it out in the morning, wash-up in the evening when 'yar covered in dirt and garden stuff.

I am on traditional city plumbing and use this as my shower in the summer. It's my challenge to myself to reduce my water use. If I'm really careful I can get by using half of it for a shower so 1.5 gallons and then re-fill and heat again the next day in case I want the full 3 gallon luxury. They cost around 25 dollars for one. So not cheap but I'm on my second summer with this so I'm thinking it will pay for itself in reduced water costs in another... oh 50 years ;)

I could probably get away with using it in the winter if I were willing to put it inside my solar cooker or something insulated with reflectors.

Comment by c. Fri Jun 28 10:10:05 2013

Hi Anna,

I notice that no longer connects as a simple http:/ website.

Only connects as https:/ .

Kinda annoying for those of us using simple web browsers.


Comment by john Fri Jun 28 10:39:31 2013

I used to sell these from Sunward Solar but I am no longer affiliated with the company.

For your farm I see three big advantages, First - the system is entirely solar powered giving you hot water when the power goes out and keeps your outside energy low or non existent.

Secondly, one of the mounting options allows you to use a steel structure ground mount or "timber frame" option which can be turned into a shed.

Third, the system is hot water tank agnostic. With the larger system, I suspect with one 80gallon electric tank you would probably not need a backup power source even in the winter. So you might not need to flip the electric breaker.

Some people will probably say that you will never use all that hot water because you conserve so much... Thats the point! I believe a tank that size would allow for enough storage for you guys (not the typical family of 4) in the winter months.

This system would be designed for the winter months, so I you find you are creating too much hot water in the summer take more outdoor baths or cover one of the collectors.



Comment by Patrick Fri Jun 28 11:07:00 2013

Hey folks. My family and I live in a dry cabin up here in alaska. We have a 40 gallon fresh water tank that we fill by jugs from a local spring. This runs to a 115 V on-demand water pump designed for RVs. From there, it is split and piped into a mini water heater much like the one you have pictured. It holds 2.5 gallons. I must say, I was skeptical of the performance at first, but have been sold on it for a while now.

It is insulated quite well, and rarely turns on until we drain it of water or use it excessively. The biggest cost is definitely the electricity, but in all honesty, I am not sure if the cost of the electricity would be less if we went back to heating water on the stove every time we need it. Granted, when we would heat water on the stove, we would end up using less hot water due to the sheer trouble and time it took to get hot water. So the cost is somewhat relegated. The only other problem we've had with it is that if we unplug it for a while, it seems to drain itself - we have it in a rubbermaid bin for this reason. I have not figured this one out yet - probably related to the pressure relief valve on top.

This unit has been in place now for about 4 years, with little to no maintenance. It has been very reliable, and incredibly convenient. I don't think you could go wrong with a similar device.


Comment by Colin Fri Jun 28 11:28:26 2013

Here is a link to a solar dealer in Pennsylvania. He is located halfway through PA and was easy for me to get to. With no tax and shipping cost the price of two SB20 evacuated tubes units was very reasonable. Because there are individual tubes that mount to the manifold, installation can be done by one person and transporting in a small truck or van was easy. especially if you share cost with another individual who wants a set also.

My solar heater is a active system using a pump but I don't see why you couldn't make it a thermosyphon with drain back capabilities.

My hot water is, 80 gal. elec hot water tank, or "Nyle" heat pump, or wood boiler, or solar hot water depending on what time of the year it is and what is the most cost efficient at the time by setting the thermostats a little lower than the next unit.

Comment by zimmy Fri Jun 28 12:19:13 2013

I just spent a month in Mexico. My apartment was typical and had a small water heater mounted outside the bathroom. My instructions were to turn it on 5 minutes before I wanted to shower and off immediately. It was entirely satisfactory.

If I were you, I would rig up a solar preheater and the small heater inline (with a bypass) near the point of use. There are plans on line for a reflector type solar heater. An old heater tank on it's side with parabolic reflectors of polished metal. Or the sliding glass door covered box with a hose coiled around inside style. KISS "keep is simple sister!!"

Comment by Tom Blum Fri Jun 28 12:55:59 2013

What time of day do you guys usually shower? If it's not morning then I'd bet you could figure out something very simple with solar considering how little hot water you use. For example a garden hose that is 3/4" diameter interior and 100 feet long holds ~2 gallons of water. I'm sure you know how hot water inside a hose gets when left in the sun. The smaller size of the hose is what allows it to heat up quickly.
If you get showers in the morning you could plumb this to something with a little more volume so you could temper that hot water and have the benefit of thermal mass to keep the water hot inside longer(possibly this could be within your green house addition?) An RV pump like a previous commenter mentioned will allow you to get a shower even if you just had it set up to pull water out of the reservoir and pump it through a shower head no mechanical connection would be required you could just drop the hose in take into the reservoir to begin pumping and a diaphram pump (RV type pump) won't get damaged if it runs dry.

If possible in the winter it may be nice to be able to locate the reservoir near the wood stove to allow some warm water.

Here's a link to the RV pump I am referring to.

I hope some of these help in your final design.

Comment by Brian Fri Jun 28 13:46:45 2013

If you want to go pumpless (thermosiphon), the water reservoir should preferably be above the collector. This also prevents heat loss at night. That doesn't fit well with using an in-line heater/storage unit. Additionally, such in-line heaters are not fitted with the extra in-and outlets you need. Also the driving power from the thermosiphon effect is not very large, so you need relatively large diameter piping to reduce resistance. (I've seen old pumpless central heating systems using 2" pipe). In-line water heaters are meant to be used with a mains water supply that has 1-2 atmospheres of pressure, so they built for good heat transfer rather than a small flow resistance.

You want the hot water from the collector to heat up all the water in the tank. If you use a small tank (where the water in- and outlet will by definition be relatively close together) the water might flow relatively straight from the tank inlet to the outlet without mixing well with the rest of the tank.

If your water is "hard", using an open system might not be a good idea. Limescale would clog and eventually block the relatively fine channels in the collector. A closed system with a heat exchanger is probably a better solution.

A collector can get pretty hot, so you should use materials that can deal with hot water. E.g. copper pipes with silver soldered connections. But those shouldn't be used directly in contact with a aluminium collector because of galvanic corrosion.

In short there are some design/engineering issues with solar hot water heater that aren't immediately apparent to the untrained eye.

So if you can afford it, buy a ready made (secondhand?) collector and tank combo. Those are usually made to be installed on the roof (the tank is on top of the collector as shown here) so it doesn't take up space in the house.

Comment by Roland_Smith Fri Jun 28 14:53:46 2013

I came across the compost pile hot water idea a while back. My idea is that you can run the compost pile system through the dead of winter into the solar hot water systems you are contemplating to add resiliency to the system. You could route the compost hot out through the sun room/ heated portion of the living space to help keep the citrus and seedlings from freezing and then into the solar. The video I found on youtube stated the hot compost pile produced 160 degree water declining to 90 degree water 2 months later. This would get you through the worst of January-March. link

Sorry for any errors, it my first post.

Comment by nojones Fri Jun 28 14:54:49 2013

Hi Anna, hi Mark,

When designing your system, do mind the legionaire's disease safety precautions. See for a summary.

The big risk is if you store water for several hours or up to a few days. In that case, make sure to heat it to 62°C for a few minutes, two times every day.

Comment by Mark Fri Jun 28 17:33:53 2013
I think the shallow box with a coiled black hose is a great option.
Comment by Eric in Japan Fri Jun 28 20:52:15 2013

Hi guys, I looked at this same problem last winter. The floor underneath my water heater had rotted thru. I took Pan baths for the winter until it got warm enough so i could fix the problem myself. Anyway, I searched for answers to this kind of problem. The best that i could find was on youtube. The guy had an insulated 40gallon gas water tank outside of his home. He built a small enclosure around it to protect it from the weather. He took out the thermocouple and anything else in the way. He built a small fire in the opening at the bottom. He said he didn't need to build a big fire. It acted like a rocket stove. The smoke and heat went up the exhaust pipe in the center of the water tank. In 20 minutes the water would burn his hand. He would then snuff out the fire and use up the hot water. My question to myself is what if it was winter, how do i drain it so it doesn't freeze. My answer, install it upside down on a brick fire pit and set up your pipes the way you want. The pipes at the top of a water heater if upside down will let it drain out completely, no so when right side up. Putting it on a brick fire pit, allows you to make an enclosed fire outside. Of course i haven't seen anyone turn one upside down to do this but it was the only answer i could come up with myself. i would try it too but that would be one more thing to put on a long to-do-list of your blogging, keep it up.

Comment by John York Sat Jun 29 01:49:38 2013
We are currently renovating a delapidated farmhouse. We bought a small (5 gallon) water tank that uses a regular plug to run when we first started renovations. We hooked up an outside shower with the tank in a shed attached to the house. When we needed a shower we would plug it in a half hour or so beforehand. It provided enough water for my husband and I to each have a shower and get cleaned up. Now we have the plumbing installed in the house, we have hooked the small tank up in line with a large one. We will let the large one heat the well water to basement temperature and just heat 5 gallons at a time. Should we have company or decide we need a luxurious bath, we will then turn on the big tank. Sounds good in principal..... At our last home we bought an expensive electric on-demand heater. In less than a year, it blew a hole in a pipe. We then heard that another problem with them is hard water buildup. We have a broken small propane on demand heater that hopefully can be repaired and used at our cottage that has only a little solar power.
Comment by lynn mueller Sun Jun 30 12:32:03 2013

Reliability is a problem with solar powered systems, which also require either an electric pump or a sometimes difficult to coordinate thermosiphon mechanism.

Old fashioned wood burning water heating systems like many of us old-timers remember are cheap, reliable and efficient. As a kid I remember waking up each winter AM and first thing going down to the basement to stoke the coal burning furnace and then throw a few sticks of scrap wood into the smaller water heater. It took only a few minutes to warm up the 30 (?40) gal tank and we had enough hot water to waste all day.

Comment by doc Mon Jul 1 19:24:51 2013

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