Understanding Thermosiphon

Thermosiphon is the dynamic principle that allows a Chofu wood-fired heater to circulate water into a soaking tub without a pump. It’s important to understand this concept when setting up a Chofu.

Wikipedia: Thermosiphon (or thermosyphon) is a method of passive heat exchange, based on natural convection, which circulates a fluid without the necessity of a pump… Convective movement that starts when liquid in the loop is heated, causing it to expand and become less dense, and thus more buoyant than the cooler liquid in the bottom of the loop. Convection moves the heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity….. https://en.wikipedia.org/wiki/Thermosiphon

Thermosiphon Circulation With a Chofu Heater

When a fire is built in the Chofu heater, heat is transferred into the water inside the water jacket surrounding the firebox. The heated water expands and becomes more buoyant, rising up and out of the top circulating tube and into the tub. Simultaneously, cold water from the bottom of the tub is pulled into the water jacket through the bottom circulating tube. This circular pumping action continues as long as there is heat in the firebox.

A Critical Factor

To produce thermosiphon circulation, the top circulating tube must rise upward to the tub at a rate of 2-inches per foot or greater for the horizontal distance. The steeper the slope, the faster water circulates. Note: Faster circulation does not produce a faster heating rate.

Stirring the Tub

Because thermosiphon circulation moves water slowly through the heater and tub, the temperature in a tub becomes stratified, with a hot water layer on the surface and cold water on the bottom. The water must, therefore, be stirred with a paddle to mix the temperatures.