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Q & A: thermosiphon systems

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Most recent answer: 08/28/2012
I'm looking to build a water heater off a wood stove and wanted to know if a thermosiphon system required a closed system to function. For example, if I have a holding tank with water with a line going to a heat source and a return line going back to the water tank, does the tank need to be airtight for the convection of heat by thermosiphoning to work? Or can the tank be open on top and still allow for cooler water to flow toward the heat source and the hot water to travel back up to the tank? It will be a very simple system of a tank at a certain height, a line going down to the heat source and then looping back up to the tank.
- Travis (age 26)
Hopkins, MN, USA
There's a nice wikipedia article on thermosiphon systems:

I don't see any particular reason why the system needs to be closed, except that an open system will gradually lose water via evaporation. You do want to be very careful if you have a system in which the heater could overheat when the water is gone. One advantage of having at least a small vent opening in the system is that you don't have to worry about it somehow over-pressurizing and exploding.

You probably have the basic design well planned out, but in case some readers may not follow it from that short description, we should add a few words. The outlet pipe from the heater will head upwards toward the tank. The inlet pipe to the heater will come in either horizontally or perhaps from somewhat below the heater. That way the heated water will know which way to flow.

The hot water heating system in my house used to work this way, but we got better results with electrical pumps.

Mike W.

(published on 08/28/2012)

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