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Old 01-20-2010, 12:14 AM   #1
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Passive solar hot water temps?

I'm thinking about mounting schedule 40 black PVC pipes on top of the bus to store water which would be heated by the sun and could gravity feed to sink and shower. Does anyone have any idea how hot the water would get by the end of a 100 degree day driving in full sun? Would it get too hot to pour directly on your skin without mixing with cold water from another source? Maybe I could have one black and one white, and I could blend them at the faucet to get the right temperature... I'm wanting to avoid getting a water heater, and possibly avoid the need for a pump to pump water from a tank underneath.
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Old 01-20-2010, 01:06 PM   #2
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Re: Passive solar hot water temps?

The water heater is broke in my bus and I was going to try the same thing. I was going to run black PVC around the perimeter of the roof to heat the water but I was going to have a cold tank under the bed and mix the water at the faucets.
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Old 01-20-2010, 01:09 PM   #3
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Re: Passive solar hot water temps?

Even a white pipe is going to absorb a lot of heat. A problem that is just as important as avoiding getting scalded is making sure the water has somewhere to go when it heats up. You will need an expansion tank. Otherwise, as it heats up the pressure will increase, with potentially harmful effect.

I'm planning to put a system like this on my bus: http://www.beyondoilsolar.com/solar_water.htm. The smaller version is under a grand, and has up to 44,000 BTU of output. I plan to have a circulation setup just for that heat source, with an expansion tank, and a water to water heat exchanger between it and my water system. I want the water to water heat exchanger to be a specific temperature mixing type, that will mix cold source with hot output to bring the temperature down to what I want before dumping the water on my skin. This will of course increase the complexity, but will be quite safe, and will let me keep the weight of the water down low, reducing the risk of flipping my bus.

hope this helps,
jim
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Old 01-20-2010, 07:21 PM   #4
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Re: Passive solar hot water temps?

I'm following this thread with interest. I bought a well built roof rack (off a former church activity bus) at the truck salvage yard I got my side door conversion parts. It doesn't seem to me it would take much to incorporate some type of passive hot water heater up there. I was thinking of using it as a pre-heater for my 12 gallon Suburban propane/electric water heater but it sounds like a passive system would heat well enough as a stand alone.

Take care,
Den
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Old 01-21-2010, 12:27 AM   #5
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Re: Passive solar hot water temps?

This is a quote from one of the DIY sites. They used plastic pipe (painted black) sleeved in Cleaned out flourescent tubes (don't ask me why they did that, I just included it for the temperature info... I included the link).
Quote:
...On sunny days when the outside temperature is in the mid 80's the water heats up to about 135 or more. Even on cloudy days the water heats up to the 90's... http://www.teaters.com/modules.php?name ... le&sid=217

Some good DIY stuff on solar water heaters, etc can be found at:
http://www.motherearthnews.com
http://builditsolar.com/

Build it Solar had some really good resources including calculators and free download of a very good book.
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Old 01-21-2010, 10:23 AM   #6
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Re: Passive solar hot water temps?

Lots of great ideas - thanks everyone! I will probably end up just doing the hot water on the roof and pumping cold water from a freshwater tank underneath. Maybe I will install a remote temperature gauge so I know how hot the water is on top, since it will vary depending on weather conditions. I love gauges!
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Old 01-21-2010, 12:32 PM   #7
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Re: Passive solar hot water temps?

Quote:
Originally Posted by lornaschinske
This is a quote from one of the DIY sites. They used plastic pipe (painted black) sleeved in Cleaned out flourescent tubes (don't ask me why they did that, I just included it for the temperature info... I included the link).
.
a pipe inside a glass enclosure will get much much hotter than a pipe that is in the open. kind of like a greenhouse effect. I like the idea of florescent tubes...i had not previously thought about that.

for solar hot water, i prefer a batch solar water heater. Very very cheap, simple, and works quite well.
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Old 01-22-2010, 12:11 AM   #8
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Re: Passive solar hot water temps?

Wow! Thanks a lot. You guys just reminded me of why I salvaged and kept that old , strange contraption called an "Explosion Proof Light Fixture". I don't know how readily available they are, but this one was pulled from the trash. I'd never seen one before, but it was so cool looking that I couldn't bear the thought of it rotting away in the landfill. I guess the coolest thing about it is that it has two tempered glass tubes that are 2 1/2" diam. that are meant for enclosing two florescent lights. Now it will serve an experimental purpose even cooler than the one it was intended for! I've been tapping all of your seemingly boundless sources of cool ideas for some time now, and I suppose a "thanks again and again" is in order. Cheers!
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Old 01-22-2010, 05:08 PM   #9
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Re: Passive solar hot water temps?

Quote:
Originally Posted by lapeer20m
a pipe inside a glass enclosure will get much much hotter than a pipe that is in the open. kind of like a greenhouse effect. I like the idea of florescent tubes...i had not previously thought about that...
I know WHY the pipe was encased in glass. It's the use of the florescent tubes that I don't get. They must have access to lots of extra tubes. These were on a fixed house. I'm not sure how well the glass tubes would hold up bouncing down the road. I think TX had the worst roads we've been on.
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Old 01-26-2010, 01:21 PM   #10
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Re: Passive solar hot water temps?

They make clear plastic safety sleeves that go over flourescent tubes. Maybe that's what they are talking about. I am skeptical that anything made with actual flourescent tubes would take any amount of handling. The clear plastic sleeves might work, but it would probably be more efficient to build an insulated, glass faced box to run the pipes in.

I'm going to commit to solar water heating in the future, so I'm interested in this as well.
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