I have installed thousands of evacuated tubes on solar hot water systems. If installed properly they will lasts for many years but sometimes for no apparent reason evacuated tubes will lose their vacum. In my experience of the many thousands I have installed I’ve only had to replace approximately 12 tubes over a period of 6 years.
The easiest way to tell if your evacuated tubes or tube has lost it’s vacum is by the colour. In most cases the whole of the tube will go a milky colour or part of the tube will go a milky colour.
Remember there is no water in the evacuated tube only in the heat pipe inside the tube. Replacement is relatively simple if your evcuated tube collector is on a frame located on a flat roof. Not so easy if on a 35 degree pitched roof. It is important to note no replacement should be undertaken without the applcation of heat paste to the heat pipe bulb.
In most cases your evacuated tubes should be under warranty ie Apricus has a 15 year warranty but you will have to pay for labour.
If you have bought a thermosyphon evacuated tube solar hot water system then the task of replacement is more difficult as most of these systems have water in the tubes. The trick here is not to get burnt so you will need to drain the system down before removing the tube. Also do not refill the system without allowing the evacuated tubes to cool down. You can do this by covering the tubes from the sun or waiting til after the sun has set but again make sure the evacuated tubes are cool or you run the risk of fracturing the glass of the tubes.
Sunworks Eco Plumbing