Flux residue inside new copper pipes

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I’m remodelling my bathroom, which involves a fair bit of new copper pipework and a layout change.

To do this, I brought the hot and cold supplies to a central point and temporarily capped them with JG Speedfit end stops. Over the past 1–2 weeks I’ve been slowly soldering all the new pipe runs and end points. I haven’t connected the new pipework to the supply yet because I wanted to finish everything properly first.

When soldering, I wiped away excess flux beforehand, and after each joint cooled I cleaned the outside of the pipes thoroughly. All the pipework looks clean and shiny externally.

However, I’ve just noticed on a small offcut of copper pipe that there’s a stubborn green residue on the inside of the pipe. I’m assuming this is flux residue, similar to what you sometimes see on the outside. It’s made me realise that because the pipes haven’t been flushed yet, there’s probably flux residue sitting inside the new pipework for the last week or two.

I tried running the offcut under a tap for a few minutes, but the green residue didn’t really clear. Maybe that’s because it wasn’t pressurised.

My questions are:
  • Is this something to worry about?
  • Once I connect the hot and cold supplies, will flushing the system clear this out?
  • Should I just run water through each outlet for a good while, or is there something else I should be doing?

Sorry for the long post, and thanks in advance for any advice.

If it's relevant info, i've been using Fernox Powerflow Flux

Thank you.
 

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