In floor heat. Thermosyphon

I'm trying to use the diversion energy from my solar/wind to heat water and run PEX tubing in loops just under the surface of the floor. I have a 48v heater element to put in a water heater and feed a manifold.
I'm not sure if this will work with a smaller water heater tank, 10 gallons? Water heaters sure are pricey now. I was also hoping to make this all thermosiphon. Has anyone done this? any advice would be appreciated.

Use a pump to cycle the water, the electricity that the pump turns into mechanically energy turns into heat energy, so don't be afraid to get 2 uses from that energy. You dont really need a tank to store your heat. You can store it in your floor/mass.

Thermo siphon would be extremely difficult to get to work well.  It relies on a large temperature change to drive the flow and radiant heat relies on a small temperature change to keep the heat even throughout the space.

You can do it like Benji suggests or you can build/buy a LARGE insulated tank to store heat and then draw it as needed.  There are several variations at http://www.builditsolar.com

Thanks for the replies. I am trying to avoid using more electricity and a pump.

Well......
Assuming you are working with a static quantity of water, as this advise is valueless with a pressurized system.

To establish a thermosyphon a temperature and elevation gradient are required.
The temperature gradient can be established by putting your heat source two foot lower than your heat sink. this will require a sump to situate your heating element, This can be as simple as a 18" piece of 4" stainless with a pex connection on the bottom, and a cross fitting at the top to accommodate a pex connection a temperature and pressure valve, and a long standing vent pipe at least 2' higher than anything else in your system, open at the top to atmosphere to keep this from becoming a bomb,(the T&P valve is backup in case this pipe gets plugged.).
The pundits say each gallon of water equals 1 stick of TNT at 70lbs steam pressure and reviewing water heater explosions (Google is your friend!) I can believe it.
Obviously overall temps must be kept below 212 Fahrenheit or you have just created (an inadequate!) pressure vessel, in the case of a blockage of the vent pipe, a button thermostat at the high point of the tank and the high point of the source to turn power off to the element would be in order
Using anything other than stainless or possibly brass, will introduce rust into your system. plastic will not hold up to high temps reliably, galvanized iron is just a slower (and not by very much!) source of rust, black iron starts rusting immediately.
The only thing that mitigates rust is a lack of oxygen...make up water provides a constant source of new oxygen.
Since your floor is (optimally) flat a holding tank offering a little elevation may be helpful. (My bet is it will circulate as you are depending on a hydraulic lock......but it may be extraordinarily sluggish!)

Think of your heat source as a pump, it draws cold water into the bottom of itself and forces the hottest water out of its top, the direction of flow depends on your connection to the sink, and storage tank, and keep in mind any air entrapment will break hydraulic lock, and stop thermosyphon.
My bet is best performance will be achieved by routing the hottest water into the top of the storage tank and pulling from the concrete sink for your source water...kind of heating the concrete as a secondary action to the pumping of water.

I was thinking of using a closed-loop system with some sort of Glycol. This is separate from my taps, its for floor heat only. I'll admit that I am trying to learn more about thermosyphon use. I have some experiences with heating water in an alternate way. one was my dad in the seventies bought some kind of curved panel mounted on the roof that directed sunlight onto a tube and this was connected to a large water heater in our garage. I don't remember a pump. since this was in Phoenix we had plenty of really hot water whenever we needed it.  
 Years later I had a tractor-trailer rig with a generator for running an AC unit while parked. I hooked up a block heater to the lower radiator hose and could keep the truck's coolant hot and warm the cab. This also preheated the engine and helped with starting in cold weather. I remember how the instructions said to have the heater lower than the radiator.
  There was another one, my dad hooked up in concrete pipes in his new driveway to an instant heater with a pump in his garage. I wasn't around for the install but he had problems with the pump. When my folks moved out it didn't work anymore.
   Funny, Now that I have read what I just wrote, The idea of a heater like the one I had on my truck. Basically a large water heater element in a tubular housing mounted lower than the pex hoses in my new system might just work really well. This would work with any excess diversion power. I can see using a cold water crossover and adjusting the hot water output to around 100 degrees. This setup eliminates a boiler or water heater tank. Maybe.   Universal engine heaters.   engineblockheater.net/products-and-services/electric-engine-heaters/        Kind of a simpler version of most tankless water heaters. Remember I am trying to put excess solar/ wind power to use. 48v.
 The crazy ideas we come up with.
 

This would have an air bleed off and a pressure relief valve. A backflow preventer and an expansion tank.

was going to make a big post here, decided to search the web first, then found a link to this thread.  so reviving, I hope.

I've been thinking about this for a number of years.  I have radiant tubes in my earthen floor.  I'd plumbed it all into my tankless water heater and a circulation pump.   Turns out, I don't like running that much electricity through the pump in the cold months (short days) when I don't get that much solar.   And I don't like paying the propane man so much.  Oh, and I have a firewood/brush *problem*, I need to constantly get rid of brush and wood.  

So, I don't want to blow myself up, so I'm thinking of an OPEN system - as in, open to the sky.  I'm on a hill, so I can set up some sort of burn place like a patio fire pit below the grade of the floor I'm trying to heat.  

I'm thinking of a copper (vertical) coil, maybe 4" ID, with the top, after some fireproof distance, going to an insulated PEX tube that connects to my floor tube.  This whole distance from coil to start of floor might be 10 meters let's say.   Then the return line would go to a small pot, for lack of a better word, near the fire pit, with a low outlet that goes to the low end of the coil.    The wood fire quickly, powerfully heats the water in the coil, creating the pressure up the coil and through the coil in my floor... heating the floor, cooling the water as it comes out and returns to that open pot relief valve, so there's minimal boil off.   All this happens on the patio or nearby, downhill from the house, probably as I hang out and enjoy the fire, adding water if it boils off.  

I *think* the only possible flaws in my plan are having the level of the water in the pot be just a touch higher than the floor, so that there's always water in the whole system.  And then the question of the water pressure created by the thermosyphon  - will it be enough to push the water all the way through the floor coil and back?

My woodfired hot tub does this all the time, without the detour through a floor.

Very interested to hear this crowds' thoughts.