Thermosiphon Loop w/ Single Water Tank

Thermosiphon loop on a coal stove.
Thermosiphon loop on a coal stove.
  1. The hot water supply to the house from the hot water outlet on the tank, for a heated tank the outlet will be marked as the hot water outlet. This pipe can easily conduct heat out of hot water tank especially if it's copper tubing, it should be insulated at least three feet from the tank.
  2. The cold water supply going to the cold water inlet on the tank. For proper operation of any hot water heater it's important that both cold water inlet and the hot water outlet are utilized accordingly. This pipe should also be insulated at least three feet from the tank.
  3. Cold water supply to a mixing valve, insulate this as well.
  4. A thermosiphon loop has no temperature control and can produce water temperatures in excess of 180 degrees especially if the hot water is underutilized or a small tank is being utilized. Water at these temperature can instantly burn someone. If you do not already have a mixing valve on your system it's critical for the safety of you and others that a mixing valve is utilized. Keep in mind even if homeowner is accustomed to and aware of really hot water guests will not be.
  5. The shut off valves are optional but can save a lot of hassle in the future if for example the need arises to drain the hot water tank to install a new one.
  6. The unions are also optional but would be installed for easy installation of a new tank. Inevitably a heated tank will need to be replaced at some point in time in the future. Unions allow replacement without having to re-plumb anything before them. While not depicted they can also be utilized on the hot and cold water side of the thermosiphon loop.(#11 and #13)
  7. The hot water outlet will clearly be marked on heated hot water tank, it's important to not mix it up with the cold water inlet.
  8. Same as above it will be marked, the difference between the two is the dip tube(#15).
  9. A temperature/pressure release valve is the most important component of a heated hot water tank. Some only limit pressure or they can limit both pressure and temperature. It will release water from the tank at a set point if the pressure or water temperature becomes dangerously high depending on the type. Because of the higher temperatures created with a thermosiphon loop it's critically important this is installed. While the image depicts a typical location that would require a T pressure relief valve it can be located anywhere on the loop or the tank. Note: Pressure release valves are preset for domestic hot water or boilers, this requires one preset for a hot water heater. Pressure relief valves used on boilers have a much lower set point for pressure.
  10. The pressure relief valve discharge pipe is another important safety feature, when a temperature/pressure relief valve opens extremely hot water is vented. If no pipe is present it will vent where the valve is and anybody nearby can be scalded. Ideally it should terminate far away from anywhere people may commonly be or vented into a closed drain.
  11. The hot side of the thermosiphon loop which should be connected to the high side of the U shaped pipe and somewhere high on the water tank on the other end. As water is heated it will rise through this pipe into the top of the water tank. Temperatures conducted into the tubing can be very high directly adjacent to the stove and products like Pex may not be rated for it, it should be metal tubing. This pipe should also be insulated.
  12. U shaped stainless steel pipe inside the burn chamber to heat the water. While other piping may be used the inside of coal stove can be a corrosive environment and a high grade stainless steel is preferable. Kits are available or it can be custom made. Holes need to be drilled into the heating unit to accommodate this. In the event it will not be used the future it can just be left in place with no water connection or removed and plug the holes.
  13. Cold side of the thermosiphon loop, this is connected to the lower side of the U shaped stainless steel pipe and typically connected to where the drain is on a tank. Heat being conducted into the tubing directly adjacent to the stove can be very hot and may exceed the ratings for products like Pex, it should be metal. For optimal performance the connection to the tank should be level or even higher than the connection to the U shaped stainless steel pipe.
  14. The existing drain valve if present needs be removed and replaced with a direct connection or a T drain valve. The location of the drain itself can be anywhere as long as it's at the lowest point possible.
  15. The dip tube is an extension of the cold water inlet, this insures the cold water from the supply first goes to the bottom of the tank. Without a dip tube connections made to the top of the tank will allow cold water to mix with the hottest water in the top of the tank resulting in lukewarm water from the faucet. If you are utilizing an unheated tank and it does not have a dip tube the cold water supply should be connected to the bottom of the tank.
  16. The elevated base is one of the most important parts of setup for optimal performance.