Overheating can be a problem in solar thermal installations. We can suggest a variety of measures to ease or prevent overheating.

Common signs of solar overheating

If a system regularly overheats you may often experience some of the following problems:

  • the system fails to heat the water effectively
  • leaks from joints and seals
  • failure of the expansion vessel
  • activation of the pressure relief valve, releasing high temperature steam (a possible  safety issue)
  • a reduction in the life and efficiency of the antifreeze
  • degradation of the antifreeze, leading to acidity and subsequent corrosion of the pipe work, cylinder or panel
  • the need for a solar chemical power flush
  • a drop in pressure and air trapped in the system
  • pumps and components failure
  • damage to solar panel surfaces (leading to a permanent reduction in efficiency)
  • invalidation of warranties

Some basics to prevent overheating in solar thermal systems

Even a well designed and installed system will overheat occasionally. It is important that the solar system can do so in a safe and stable fashion. In order to achieve this, it may be advisable to carry out some of the following work:

  • increase the size of the expansion vessel
  • change the location of the expansion vessel
  • fit a pre-cooling vessel
  • re-configure the pipework connections to the expansion vessel

Other reasons why a solar system might continue to overheat

If the system does continue to overheat on a regular basis it is advisable to investigate the possible causes, which may be:

  • the panel area is too great for the size of the cylinder.
  • the cylinder is being heated by another heat source so the solar system has little work to do
  • little or no hot water is being used during sunny periods.

Further ways to reduce solar overheating of solar systems

There are a number of ways to avoid overheating even if the panel area is too great for the cylinder:

    • Fit a radiator heat dump. A three-port valve diverts the flow from the solar panel to a radiator when the cylinder has reached design temperature. The excess heat is given off to the atmosphere around the radiator, whether inside or external.


  • Fit a fan assisted heat dump. Works as above but is suitable in locations where there is not sufficient space for a radiator. This system also has the ability to dissipate larger quantities of heat.
  • Cover up / partially cover the solar panel(s).
  • When the cylinder reaches temperature operate the central heating system circulation pump in order to dissipate the excess heat around radiators throughout the house.
  • Increase the storage temperature of the water. In hard water areas, care must be taken if there is no water softener fitted, to avoid scaling. A temperature mixing valve may need to be fitted to protect householders from excessive water temperatures. Not all types of pipework are able to withstand very high temperatures.
  • Fit a larger  or second cylinder
  • Divert the excess energy when the cylinder has achieved maximum temperature to a swimming pool or hot tub
  • Divert the excess heat to a cylinder in another building / property
  • Run off hot water to a safe store to be used for other purposes later
  • Growing a vine that’s foliage partially covers the panel in summer and dies off in winter, leaving it clear. The greenest, albeit impractical approach!

Some of the above solutions to avoid overheating are expensive but are usually worthwhile in the longer term.

Once we have surveyed your system, we will propose the most economical and practical solution to resolve the problem.