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Friday, January 27, 2012

Free, More Hot Water? - Part 1

A solar water heater for an RV? Is it possible? Wouldn't it be cool to unroll a small rubber collector on the roof of your RV, adhere it to the roof, plumb it into your small-ish water heater and have LOTS more hot water, while using less propane and/or electric to heat it? Sounds like a Free Lunch, doesn't it?

Nope. Not this time.

Here's the scoop. For years folks have been using Solar Thermal hot water systems. In fact, Passenger Pete has two solar collectors on his roof that feed a heat exchanger mounted on the top of his electric hot water heater. This system is filled with anti-freeze and circulates through a coil that exchanges heat with water circulating around in another coil. This is a good working system, but a bit too complex for what I'd use in an RV. It needs thermostats and pumps to maximize it's use and the heat exchanger system itself would have to fit someplace.

In a simpler system we have a collector of some kind (more on that later) on the roof and it feeds the hot water heater/tank directly. So there's no anti-freeze in the system, just water. Of course you'd have to make sure that it is drained in the winter (if you actually HAVE winters!) or have some way of draining water back into your water system and out of the collector if there was a danger of freezing. So far, so good. Since this project is really in the planning stages, I am not sure what temperatures the water will get to from the collector, so I will have to test some different configurations. I'm hoping I can set up a simple recirculating system that feeds my little Atwood 6 Gallon water heater.


I'll put bypass valves on both sides, so I can disconnect the collector from the hot water system. I mean, if it's cold, why heat all that extra water! That brings up another good point, since the collector has to have water in it to heat, then it stands to reason that you could have more gallons of hot water to use. I'm looking at adding a 6-10 gallon collection system so I should be able to have that much more hot water.

How do you get the water up to the collector? Good question....yes, I know I'm talking to myself, but that's okay. I figured a small 12v water pump near the inlet of the hot water heater running from power I get from my solar panels (or you could install a small one amp solar panel) with a manual switch or perhaps a flow switch or temperature switch or even a pressure switch wired in for automatic operation. Since the system only needs to pump water up to the collector and that will only happen when the sun is up, it won't really affect my solar charging or power consumption. I'll have to make sure the pump will lift water to the height of the roof. The one pictured will lift water 2.5 meters or 8.2 feet and will pump 350L/hr or 92.5 gallons/hr of water. It uses 450mA of current. Gravity will be used to get the newly heated water back down to the hot water heater.

Now, on to the roof! I'm looking at a flat, tubular style of rubber or polypropylene collector. These get connected to "manifolds" that if needed can be daisy chained together to form the basic collector. From the manifolds they will be connected to flexible hoses leading down into the coach, probably through the refrigerator roof vent opening.

I've got to figure out how much square footage of collector we'll need to feed the water heater. Since it's flat and will be a few square feet in size the weight of the water will be spread out. that's a good thing because it weighs 8.4 lbs/gallon. I try to be as nice to my roof as possible. Roof leaks kill so many RV's.

I am waiting for several manufacturers to get back to me with some real technical specs of the collector material. Mainly, I'll need the amount of liquid they hold per square foot and how much they will heat water at a given sun level. Then I'll be able to calculate how much area the collector will have to cover to make this work.

Of course, since it's WINTER here, I can't really do any heavy, "real world" testing, but I will assemble the test system now and plan on writing a second article as more information is available.

Be seeing you...Down the Road,

Rich "The Wanderman"
http://www.thewanderman.com

24 comments:

  1. Love Your articles always looking forward to the next. Thanks For taking the time to write about Your great ideas. Any Video??? Thanks, Chris

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    1. Chris,
      Thanks! It really means a lot to know someone is enjoying my work.

      I am happy to include some video. As soon as it warms up!

      I will begin using video to supplement photos for builds.

      Thanks again,

      Rich "The Wanderman"

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  2. Rich

    I found your article stimulating. As a mfg engineer, I am concerned about the end item market cost. Perhaps you could come up with a kit and dwgs so people like us could build like systems. I have a small 12 v. bilge pump mounted inside a 6 gal bucket that I use to refill my fresh water. I look forward to more specifics on your project since I plan to be in Quartzsite next winter.

    Ken

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    1. Ken,
      As soon as I confirm the sizes and other bits and pieces needed, I will post a list with sources. I'm also going to try and work with one of the large manufacturers to sell a working product(s) at a reasonable price.

      I agree with you, this would be a great, simple add-on.

      Thanks!

      Rich "The Wanderman"

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  3. Back in 1971 I converted a 64 panel truck into my first camper. I took an inner tube and modified the valve stem to fit a small hose using hose clamps. I could put water into it or get water out. I fastened it to the roof. I had some warm water - not much - but enough.

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    1. Anon,
      Cool! Sometimes it's the older ideas that work the best. Inner tubes always remind me of my childhood....my grandparents had a summer home on a lake. They always had lots of inner tubes of all sizes to float around/play with.

      Thanks,

      Rich "The Wanderman"

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  4. If it is part of the pressurized water system, why would you need an additional pump?

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    1. Gil,
      The main reason is to recirculate the water through both the collector and the hot water heater tank. This way the entire system gets up to max temperature for use later on. The primary water pump would circulate cold water through the system (and be a power hog)

      What I have created is a closed loop system, until you are ready to use hot water, when you use it, the regular water pump replenishes the supply in the roof collector. The secondary circulation pump will not turn on when the primary RV water pump is on (using water)

      It's a bit counter-intuitive...but should work well.

      Thanks,

      Rich "The Wanderman"

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  5. I like the idea, but not the idea of climbing onto the roof of my motor home. Can this be set up alongside one's RV when at a campground or boondocking?

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    1. James,
      I see no reason you could not set this up on the ground. You'd need to have some kind of quick disconnect to you hot water system and a place to carry the collector.

      Depending on the information I get from the manufacturers, you MAY be able to roll up the collector for storage.

      Rich "The Wanderman"

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  6. I think you would need a variable speed pump so the.water could could acquire the heat which would depend on your amount of sun.

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    1. Anon,
      The small recirculation pump is setup with a thermostat at the outlet of the panel. When the water in the collector reaches a preset point it will turn on the pump until it drops below a certain point the it will stop. If the water through the closed loop is constant the pump will not run continuously.

      Rich "The Wanderman"

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  7. Doug - TexasViewpoint@aol.comJanuary 28, 2012 at 4:00 PM

    Simplify, simplify, simplify

    I love the idea of the flat "tank" with miles of tiny tubes for the water. Be careful getting too small and too long of a tubeing as the water pressure needed to drive the thing by either city water, pump or gravity would be tremendous. Unless you are satisfied with a mere trickle you might experiment with sizes/lengths of tubing.

    I'm guessing that this water would be used primarily for showers and not for cooking since the insides would become, well, stagnant after a pretty short time. If that is the case then you might not want the water to recirculate through the whole hot water system.

    Instead, you might consider a closed system only for the shower. And let the city water pressure drive the system. Put a "Y" fitting on the park's/city's water supply. One line for the normal usage and a separate water hose from the "Y" to the ground level of the RV. Connect to a smaller hose that would go up to the "tank" on the top of the RV to reduce the weight on the hook-ups at the top of the RV. The line up the side should be the same or larger than the tubes in the flat tank which need to be the same size or larger than the tube delivering the water from the tank to the shower head. And, be sure that the incomming water supply is at the opposite end of the "tank" than the out going hot water.

    As part of your camping set-up the flat "tank" would need filling. The water would fill the flat "tank" removing the air into the opened faucet in the shower stall. Once the water started flowing in the shower stall - turn off the shower stall's hot water faucet. Leave the city water turned on. Then wait for the "tank" to heat (a good part of the day) and take your evening shower(s). The city water pressure would drive the hot water through the tubes without the need for a pump. You might have a third faucet control for the gas-heated hot water just in case someone takes a too-long shower. As an added bonus - leave the water hose as much as possible in direct sunlight to add that many more quarts of hot water to the mix.

    For the boondockers - fill the "tank" at the last stop and install maybe a switch or something that can be pushed or pulled that opens the upermost end of the "tank" so gravity can pull the hot water down to mix with the cold from the pumped cold water.

    Simply my two cents. Good luck with your good idea. Just don't overthink it.

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    1. Doug,
      WOW! That was really well thought out. My idea is really just a closed loop system for my small hot water loop. The re-circulation is closed between the roof "tank" and the regular atwood water heater tank. The pump switches on and off by temperature, and would not runn at night at all. I will make provisions for drainback, but unless I am getting up before sun-up I should be OK with the retained heat in the atwood tank. I can always switch on the burner or electric heater to get up to temp before the sun does it for me....I will be testing the time-to-temperature values based on the size of the collector. Ideally the collector will gravity feed the atwood tank at around 2 gal/minute or so. A bit over what I need for the shower with an Oxygenics shower head.

      I'll try various configuration and ten report back on my findings.

      Stay Tuned!

      Rich "The Wanderman"

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  8. Rich

    Would you not loose all your heat at night? You need something to stop the circulating when the sun isn't shinning.

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    1. Anon,
      The pump won't run when it gets dark at all. I'll just be using the regular 6 gallons in the water heater and whatever temp is left in the solar collector.

      I can always fire up the propane burner or electric element if I need more.

      Rich "The Wanderman"

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  9. If you used a small solar panel to feed the power to a small DC pump, it would only come on when the sun is shining and the water would be heating. Then when the sun goes down, the pump would also shut off... Kevin

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    1. Kevin,
      Exactly what I was thinking. Though, I will have a thermostat at the collector outlet as well. When the temperature reaches a set point it will automatically circulate until it drops below that.

      Rich "The Wanderman"

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  10. My first solar water in Florida was nothing more than a panel on the roof with reflectors on the bottom and pipes mounted above the reflector and a plastic cover on top. The water was piped from my cold water supply to the solar panel then into the electric water heater, the heater stayed on so you always had hot water. The solar panel only acted as a pre-heater. I remember that it really cut down on my electric bill. There was no pump involved

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  11. Anon,
    While that would work in a city water scenario, in an RV you need to pressurize/pump your own water. The city water has it's own pressure coming from the supply side. That's likely what got it to your collector, then gravity got it back down to your electric hot water heater.

    Rich "The Wanderman"

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  12. Hiya Rich. I'm new here but terribly interested in your projects, especially the solar ones; electric and hot water.

    I've been toying with the idea of making a solar water heater using the coil off the back of a refrigerator and putting it in a small reflector box.

    I realize you're severely limited on your roof top real estate but was wondering if you had considered a small auxiliary tank that will thermosiphon? Then, connect the hot water outlet from that tank either through your existing water heater or bypass it and go directly into the hot side of the plumbing. Of course, it would also require a cold water fill line to keep the aux. tank full.

    Really, no more plumbing than what you have planned. It would negate the need for an extra pump, since your regular pump would supply the cold inlet water to refill the tank.

    What about managing over-temp? It seems to me, under the right circumstances, you might be making some pretty hot water or even steam. It's gotten me to thinking about how to make a temperature differential controller, perhaps with some type of arduino set-up, and a way to "dump" or cool the too hot water.

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    1. Paul,
      Interesting design strategy. A thermosiphon tank is a GREAT idea. I am unsure that it would work on my end, since I am attempting to avoid the whole concept of filling up the exterior collector/tank. With a low amperage draw pump, used simply as a circulator it negates having to keep track of anything but overall temperature. This way, a simple thermocouple will take care of switching duties and no behavioral or usage changes are needed when you use hot water.

      The installed pump in most RVs can be a VERY high amp draw in comparison.

      On the off chance that too much or too hot water is being created/made, it would be a good idea to have some kind of overpressure relief valve to keep things safe. I think that idea is going into the system. Above and beyond the overpressure valve of the hot water heater itself.

      Stay tunes, when it gets warm enough to start assembly and testing it be quite interesting to see the results!

      Thank!

      Rich "The Wanderman"

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    2. Rich, of course the main pump uses power but it wouldn't be used as a circulating pump. It would be used only when the auxiliary tank needed to be refilled. No more power use than when the pump needs to refill your regular water heater.

      I am, however, considering some type of circulating pump for my over-temp set-up that will only come on when needed. Either that or just a pressure blow off and then refill the tank with cooler water. We'll see.

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    3. Paul,
      I can see setting up the system using your method. As soon as the sun comes out in force I will try it plumbed both ways and we'll see what's most livable.

      Thanks!

      Rich "The Wanderman"

      Delete

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