Solar Expansion Tank

Stabilizing System Pressure

When a solar heating system operates there is a build-up of pressure that can cause significant strain to the entire system, leading to inefficiency or even damage. This is especially true for closed loop systems that use fluid – glycol and distilled water – to transfer heat. A solar expansion vessel helps to stabilize pressure within the system.

Expansion VesselImage: Jaho via Wikimedia CommonsSolar expansion tanks have chambers where air is locked within diaphragms or chambers. They are generally screwed into standard 0.5- or 0.75-inch plumbing fittings with threads. Mounting brackets are available for expansion vessels ranging from 5 to 50 liters.

The heating fluid expands as the sun heats it. Without a solar expansion vessel, the increasing pressure can cause an explosion within the system. The solar expansion tank makes it possible for the fluid to expand safely as it compresses the air within the chamber. A solar expansion vessel is also designed to withstand very high temperatures.

Important Considerations When Choosing Solar Expansion Vessels


The size of the recommended solar expansion tank will depend on the total volume of the heating fluid. The factors used to determine the volume are the diameter and length of pipes used in the loop together with the size and number of collectors used in a system. As a general guide, about 5 gallons of fluid requires a #15 solar expansion tank. However, there is no harm in using a larger solar expansion vessel.

The desired purpose of the solar energy system will also help determine which solar expansion tank is recommended. For instance, the required solar expansion vessel for a system used to heat a swimming pool is different from the one used for heating a home. Additionally, the solar expansion tank required for potable water is different from the one required for glycol-based non-potable fluids. Potable water systems require a corrosion-resistant solar expansion tank, while the non-potable types use chemical-resistant chambers.

The air side of the solar expansion vessel should be pressurized to between 2 - 5 psi below the expected system fill pressure prior to connecting it to the charged system. A bicycle pump can easily be used to pressurize it to 5 psi.

Installing the right solar expansion tank will allow the fluid’s temperature to rise from zero to 200 degrees Fahrenheit while the pressure remains relatively constant.