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Solar Thermal in Thermosiphon- Load Side HX

jeffbuilds Member Posts: 6
I've been using the Simple Drainback solar thermal system plumbed in thermosiphon with standard backup systems with excellent success.  Attached are images, below is a brief description.

The goal of this method of plumbing is to produce maximum solar fraction with minimal cost associated with addition of solar.  Whether using this for DHW, or for radiant, or with solar alone or solar with wood stove auxiliary, this system works, and works well.

The Simple Drainback system is a load side heat exchange system.  That means that water in the tank is collector fluid, and potable water is inside the 100' spiral heat exchange coil.  The tank has an air space at the top for expansion and drainback function.  The load side exchanger is, by design, a very high temperature tank.

By placing the backup tank cleanout slightly above the inlet of the heat exchange coil, colder water at the bottom of the backup tank slips down into the HX, where it's heated and moved back up to the regular tank.

Placing a "T" in that short line provides cold water feed.  A check-valve stops backflow.  That's it.

Thermosiphon works well with wood stove auxiliary heat as well.  Here, the trick is to keep lines short, with ease of fluid movement.  Try www.hilkoil for a good stainless heat exchanger in the wood stove.


  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588

    Hi Jeff,

    Welcome to the Wall. This is still my favorite factory-built drainback system design.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
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