Solar expansion vessels (also known as expansion tanks) are the most common source of failure in solar thermal (solar water heating) installations. Although we can pump them up with a foot pump, it is likely that the vessel will deflate again if the pressure has been lost before.

Zilmet are a popular producer of expansion vessels. Most solar vessels are white in colour. However, other manufacturers and colours are available.

Solar expansion vessels need to be sized correctly for your system. Most commonly, they are 18l or 25l in volume.

Do I need an expansion vessel in my solar system?

Expansion vessels are an important part and safety device of your solar hot water system. The expansion vessel allows your system to expand in case of increased system temperatures or during stagnation.

Do I need to regularly replace my solar expansion vessel?

If your expansion vessel holds pressure and does not leak, there is no need to replace it. We check the condition of your expansion vessel during our service routine and will advise if we need to replace it.

Further considerations

We consider a number of issues when it comes to solar expansion vessels. These are:

  • the diaphragm inside the solar expansion vessel can fail. If this is the case, we need to replace the expansion vessel
  • certain solar expansion vessels have replaceable bladders. we may not need to replace these types of vessel as a whole.
  • the Schrader (car type) valves often fail.  We can replace the valve and can pump up the vessel with air to the correct pressure (not Nitrogen as originally supplied).
  • If the mild steel expansion vessel is corroded and pin-holed, we will need to replace it. See below for samples of failed expansion vessels.

Mounting of expansion vessels

The mounting of a solar expansion vessel is critical. We often see incorrectly mounted expansion vessels. As a result, the mounting can can contribute to problems with your solar water heating system.

How can I avoid my expansion vessel failing?

We can reduce the chance of expansion vessel failure by stopping your solar system overheating or fitting a pre-cooling vessel. There are various reasons why this may be the case. We asses your solar water heating system and advise on any changes to settings or set-up that will increase the life-span of your solar expansion vessel.