Thermal Expansion Tanks

Expansion tanks are necessary in closed-loop solar thermal water heating systems. In these systems, the potable water supply never enters the solar collectors. Instead, a propylene glycol solution (water and glycol) is used as a heat transfer fluid. This is done to prevent water freezing in the collectors or tubing, which can do irreparable harm to the system.

Expansion tanks contain an airtight chamber and a bladder or diaphragm. The tank screws into 1/2" or 3/4" plumbing fittings along the line that the transfer fluid runs. When the system is operating, the transfer fluid is heated by the sun in the collectors. From there, it travels through tubing to the water tank inside the home, where it transfers that heat to the water supply. When the glycol mixture heats up, it expands, which is why an expansion tank is necessary.

The expansion tank allows the transfer fluid to expand safely by compressing the air within its chamber. If not for the expansion tank, something within the water heater would blow up. The size of the expansion tank depends on the total volume of fluid coursing through the "veins" of the solar thermal system. According to Homepower Magazine, a solar thermal system containing three to six gallons of fluid should have a #15, or 2 gallon, expansion tank, although it never hurts to go larger. An appropriate expansion tank will allow the transfer fluid to from 0 to 200 degrees Fahrenheit without a change in system pressure.

Thermal Energy Pumps / Thermal Actuator / Thermal Expansion Tanks