I have the greatest respect for you guys. Unsung heroes.
Can you send us a picture of how the condense joins the waste
I am enclosing the photo of the area under the sink. The condensate is coming in from the left hand inlet (not shown). The washing machine is on the right hand side inlet in grey pipe. The the trap, then connects to a white flexi type pipe (cant be seen). It then connects to the cream painted copper waste pipe. Builders have found it a challenge, as the copper waste pipe is high and does not provide a good slope. Not sure how it can be improved.
When boiler failed. One boiler engineer suggested to pour drain cleaner from time to time, which might not have helped with the copper pipe corrosion. I never felt comfortable when such advice was given. My friend has been pouring drain cleaner ever since. It has been rodded and the sink water runs really well now. Physical solution is better then chemical solution....