Q: I’m going away for two months. Is there anything I should do to my solar thermal system while the house is empty? 

A: It’s always important to look at the relevant part in your solar thermal user manual, but the most important advice is to leave your system turned on. It has been designed to endure long periods of low to zero hot water ‘draw off’.

Ideally you don’t want the collector temperature to go above 120-140 degrees as the fluid within will vaporise and ‘stagnate’, which puts the system under additional stresses and strains and reduces the effective life of the solar fluid.

When an installer designs a solar thermal system for a property it is imperative that they take measures to keep ‘stagnation’ to an absolute minimum.  Stagnation occurs when excessive temperature in the collector causes the fluid to evaporate, as the system is under pressure (this usually takes place at 120 degrees C). We know that this is going to occur occasionally during the summer months and holiday periods.  Each time stagnation occurs it reduces the effective lifespan of the solar fluid and causes additional stresses and strains on the system components.

Most solar thermal systems are set to achieve a maximum cylinder temperature of 60 degrees C, so if they generate more heat than this (usually on a very hot summer day or when there is little or no hot water usage), something known as a ‘heat dump bypass’ could be put in place to ‘dump’ the system’s excess heat into a radiator and prevent stagnation ever occurring.

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

If you have a question about anything in the above blog, please ask it in the comments section below.