Hi gents, 

I moved to the new house and brought my old solar geyser panel with me. Which silicon to use to insert the coper vacuum pipes into a manifold ? Original silicon I used doesn't harden ( witch it shouldn't ) so I wander is any similar silicon is on SA market?
thanks for an advice 

That is the difference between silicon grease and silicone.

Silicon grease will also eventually dry out because it's silicon particles of different sizes suspended in silicon oil (Polydimethylsiloxane/PDMS) that eventually evaporates at higher temperatures.

Any silicon thermal grease should be acceptable, but I think the cheapest is if you can get some from a plumbing shop.

https://za.rs-online.com/web/p/thermal-grease/2173835

Thanks P1000, this is very helpful