Quickly before I dash off to the Crucible.
Its a good product and maybe worth considering if you get extreams of cold, But, 5 ltrs will be a waste of time.
We used a similar procuct for the heat pumps and the minimum was 25% solution and that hasn't changed, therefore depending on the size of the system 25ltrs + would be required, Check out the fernox website there's some good info.
Buy your own and put it in after the guy has tested and gone.
https://www.plumbnation.co.uk/site/fernox-alphi-11-protector---25-litre/