This hot tub of Dutch design uses a coil around a fire to heat and circulate water from the wooden tub. No electrical hookup required!
After filling the tub with water, a fire can be started in the attached basket to kick start the heating process. The hot water starts to rise in the spiral, which automatically starts pouring in through the spiral, and begins circulating from the bottom to the top. The temperature is adjustable and to warm 700 liters of cold water the tub takes around two and a half hours.
In the 70's my dad designed something like this to heat hot water when on Boy Scout camping trips. He used a small trash can, plumbing fittings, and a coper coil, in which a fire was started. The lid on the can kept the heat in, and a spigot on the bottom allowed you to get water out without dipping.
The main difference, and one these designers should have thought of, was that the top line from the coil came into the can and then had a pipe inside that forced the hot water down toward the bottom of the can (not too closely to the intake). In this way the new hot water perked up (heat rises) through the colder water and helped to heat it all faster.
In the hot tub design shown here, the top layer of water will be much warmer than the bottom. My fathers design created a more homogeneous temperature zone.
Ken : I think the temperature gradient is a feature, not a defect, this allows water pumping and circulation through convection. Maybe the tub itself plays the role of the additional pipe you describe.
Posted by: Thinkerer on September 19, 2008 at 5:22 PM
This design is some years old -- the tubs shown are ferociously expensive for what you get (typical for designer stuff) and somehow look like they should have vegetables and a giant spoon in with the bathers (which was the designers' intent, I think).
That said, naturally convective ("thermosiphon") heaters have been around for as long as people have been able to fabricate pipe and offer a great deal of simplicity and flexibility with regard to the heat source such as combustion, geothermal loops and even solar with many home-made projects available on the web.
A good general description of construction techniques can be found at
The tank inlet system that @Ken describes will homogenize the temperature more quickly but consequently reduces convective circulation speed....not an issue with a small tank and a blazing fire (our version is portable using an old milk can and has a spigot as well).
I know quite a bit about solar panel construction. Solar PV panels make little financial or ecological sense because of the processing of silicon. The cost is high both in dollars and toxic waste. I was referring to solar heating panels which cost very little, are simple to make (pipe, black paint, perhaps a sheet of glass, and if you want to get fancy some aluminum reflectors to concentrate the light) and are ecologically friendly. If you get sun in your state, it makes little sense NOT to use them.
Posted by: Anonymous on September 20, 2008 at 12:24 PM
burning trees is pretty much carbon neutral. the carbon released in burning is taken up by living plants. carbon being in a cycle like that breaks even. the burning of fossil fuels (plants that lived a long time ago) puts carbon into the atmosphere that has been trapped under ground which is a gain in carbon. if the plants were not burnt the carbon would be released by bacteria when the plant rotted anyway.
always good to use as much solar thermal as you can. though small scale wood burning is not a bad thing.
Posted by: Craig, on September 25, 2008 at 7:36 PM
Hey Mr YAY,
Unless you are living in a house made from recycled tires and adobe, shut your mouth. Did you burn a fossil fuel today that wasn't ABSOLUTELY 100% nessesary to life or death?
Self-righteous naysayers like you are a waste of oxygen and blogspace ALWAYS!
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Ha!
In the 70's my dad designed something like this to heat hot water when on Boy Scout camping trips. He used a small trash can, plumbing fittings, and a coper coil, in which a fire was started. The lid on the can kept the heat in, and a spigot on the bottom allowed you to get water out without dipping.
The main difference, and one these designers should have thought of, was that the top line from the coil came into the can and then had a pipe inside that forced the hot water down toward the bottom of the can (not too closely to the intake). In this way the new hot water perked up (heat rises) through the colder water and helped to heat it all faster.
In the hot tub design shown here, the top layer of water will be much warmer than the bottom. My fathers design created a more homogeneous temperature zone.
Reply to this comment
Ken : I think the temperature gradient is a feature, not a defect, this allows water pumping and circulation through convection. Maybe the tub itself plays the role of the additional pipe you describe.
Reply to this comment
Yay for burning trees and dumping carbon into the atmosphere. Go with solar collectors for the win! Recycle old panels or make your own.
Reply to this comment
Once again people forget about all the harmfull
chemicals used to construct these wonder solar panels --
I guess if you dont know about it -- that means it is so much better
Reply to this comment
This was in one of those gadget catalogs, I think it was The Sharper Image but it might have been Hammacher Schlemmer (or both) several years ago.
Reply to this comment
This design is some years old -- the tubs shown are ferociously expensive for what you get (typical for designer stuff) and somehow look like they should have vegetables and a giant spoon in with the bathers (which was the designers' intent, I think).
That said, naturally convective ("thermosiphon") heaters have been around for as long as people have been able to fabricate pipe and offer a great deal of simplicity and flexibility with regard to the heat source such as combustion, geothermal loops and even solar with many home-made projects available on the web.
A good general description of construction techniques can be found at
http://www.green-trust.org/2003/fireandwater.htm
The tank inlet system that @Ken describes will homogenize the temperature more quickly but consequently reduces convective circulation speed....not an issue with a small tank and a blazing fire (our version is portable using an old milk can and has a spigot as well).
Reply to this comment
I know quite a bit about solar panel construction. Solar PV panels make little financial or ecological sense because of the processing of silicon. The cost is high both in dollars and toxic waste. I was referring to solar heating panels which cost very little, are simple to make (pipe, black paint, perhaps a sheet of glass, and if you want to get fancy some aluminum reflectors to concentrate the light) and are ecologically friendly. If you get sun in your state, it makes little sense NOT to use them.
Reply to this comment
burning trees is pretty much carbon neutral. the carbon released in burning is taken up by living plants. carbon being in a cycle like that breaks even. the burning of fossil fuels (plants that lived a long time ago) puts carbon into the atmosphere that has been trapped under ground which is a gain in carbon. if the plants were not burnt the carbon would be released by bacteria when the plant rotted anyway.
always good to use as much solar thermal as you can. though small scale wood burning is not a bad thing.
Reply to this comment
Another big plus is the grill you can add to use the wood burning portion as a barbeque. Brilliant.
j.
Reply to this comment
Hey Mr YAY,
Unless you are living in a house made from recycled tires and adobe, shut your mouth. Did you burn a fossil fuel today that wasn't ABSOLUTELY 100% nessesary to life or death?
Self-righteous naysayers like you are a waste of oxygen and blogspace ALWAYS!
Reply to this comment