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for a wee bit.

 

 

uses include:
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Thermosiphon Direction of travel  RSS feed

 
Jeff Watt
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As my woodstove will be lit probably starting this week and will remain lit and loaded likely until late April, every year I contemplate a hot water loop to take some of the demand off my propane hot water heater. My stove sits about 9 Feet from the hot water tank which is in a closet opposite the stove off the living room. The hieght of the coil in the stove would end up ~32" high and the inlet on the top of the water heater is ~58" high. Seems ideal for a thermosiphon set up, but I have a question as to the logistics of that. For a Thermosiphon to operate properly does the rise need to be continuous and singular. By this I mean do I need to go in a straight line from the stove outlet to waterheater inlet. This poses a serious problem as it would put a diagonal pipe across the center of my living room space from roughly thigh to shoulder height. Obviously this is not an option. However a hewn beams runs this direction across the cieling. Can I go from the stove, up to cieling height along and across the beam, then drop down to the top inlet of the tank (again ~58")??

The return would go from the bottom drain of the tank to basement and across then back up to the stove coil which could be anywhere from maybe 20"-32" high on the stove depending how I set up the coil.

So will a thermosiphon operate set up like this? I could sketch a picture if my ramblings are too hard to follow let me know.

I would rather not go with a pump system as the woodstove runs continuously regardless of if we have power it also complicates things to go this route.

 
Roberto pokachinni
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Bump.  I can't answer your question definitively, but the thermosiphon works on the principle of the hot and cold cycling, so as long as your height differences are not that extreme, I think you should be fine.  I hope someone else with more experience pipes up (pun intended) for a better answer for you; there likely are some restrictions on height.  The way I see it, the hot water is going to rise out of your heating system and be replaced by your colder water from the bottom of your hot water tank; there's your thermosiphon, no matter what; but you need efficiency too.  The biggest issue that I can see from your description, if you have a continuous hot fire going, is that at some point you may overheat your system.  So be sure whatever pressure release valves you have are working well.   
 
Mike Jay
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I subscribed to this thread to see the answer an expert gives...  My belief is that you can pipe it up to the ceiling and then over.  I might try to have it continue a bit of an upward slope as it goes along the ceiling beam. 

I always get confused with siphons though when you connect up to the water heater tank.  Since the hot water in the tank is at the top of the tank, will the hot water from the wood stove want to push that hot water down or should you connect up to the bottom of the water heater?  My real hunch is that the hot water tank should be one story above the heat source.  Then the hot water can rise and go in the top or middle of the tank and then the coldest water in the tank can sink down to the heat source.

I have a similar situation except that my water heater is 2' away from my wood stove

Be sure to be safe with how you do the heat coil.  Don't create a boom-squish type of situation with steam.  When/if I try this, I'll want to rig up a water heating system that doesn't change the UL listing of my stove.
 
Travis Johnson
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Generally thermosiphons are overstated by nature. They are simple enough in theory, but seldom really work well. In your case, even as simple as it is, you would have to put a high point vent in and a check valve or two. Even then, consider this, the first combustion engines had thermosipon cooling systems and quickly abandoned them for pumps.


It might just be easier to connect a circulator pump with a relay that comes on when the hot water has a call for heat. Put another overtemp relay on going to a heat dump like a cast iron steam radiator and you would be much more safe from a squish-bang situation. All would be easy to do do with a relay system with priority, 24 volt zone valves, and a 120/24 volt transformer.

 
Joseph Lofthouse
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As a wine-maker, I do a lot of siphoning... I'm pretty sure that the proposed system would work poorly if at all. You'd have hot water trying to rise out of the water heater, and hot water flowing in the opposite direction trying to leave the stove. They would create a hot-pocket near the ceiling that would tend to prevent flow in either direction. Then, when the inevitable air bubbles formed in the system, they would likewise form a plug, near the ceiling, that would stop the flow. (The force exerted by a bubble is much greater than that exerted by thermo-siphoning). I highly commend Travis's suggestion of a pumped arrangement.
 
James Freyr
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I like Travis' idea of the pump. I tend to think though, that the pump might need to run continuously, even if it's at a very slow rate of flow, to prevent the coil of pipe at the wood stove from becoming a boiler.
 
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