As everyone knows, just about everything expands when it gets hotter. With gases, they can exist in the same space and build up pressure, but liquids have difficulty doing that – when a liquid expands, it needs more space.

Solar thermal systems can experience a very wide range of temperatures in a single day, and those changes mean your installation needs to take solar fluid expansion into account or it risks bursting. Expansion tanks are built to do that.

Most expansion tanks have an internal membrane (or bladder) that contains air and which is open to the main circuit. When the liquid in the system heats up, it expands and needs more room; since the air in the bladder is a gas it can be compressed, leaving space for the liquid to grow without damaging system components.

The main down-side to this is the rubber membrane or bladder which will slowly deteriorate. Over the years, an alternative "expansion chamber" system has been tried, where an air-filled tube or chamber is kept sealed while the solar thermal system is installed, then a valve is opened, effectively adding a pocket of compressible air to the tank without the need for any rubber.

Unfortunately, this setup isn't perfect and solar fluid slowly invades the expansion chamber, the air escapes into the pipes and the whole thing becomes useless.

Commercially-available expansion tanks normally have a Schrader valve on the bottom which is used to adjust the internal air pressure. Using a standard tire gauge, you should set the pressure to 3 lbs/in² below the pressure of the system when the fluid is at 60°F (15.5°C).

Bear in mind that it's very important to choose the right sized expansion tank: if you install one that is too small, you'll get a build-up of pressure in your solar thermal system, which will damage components (and risk bursting the pipes). Obviously, you'll also need to pick a tank which has a membrane made of a material that's compatible with a glycol and water mix, too.