Hello again, thank you for reading. My Worcester condensing boiler has a history of the condensate pipe blocking. It was ‘boiling’ earlier which apparently is diagnostic. 
 

So far so good: I grapple under the sink to clear it, except this time it doesn’t seem easy/the same. There seems to be lots of water ensuing when I unscrew the pipe. 

 

So I flushed  loads of hot water soda   (sodium carbonate decahydrate) crystal solution down the sink which hasn’t helped.  

Do I really need to let it drain when I unscrew the condensate pipe? It’s in a pig of a location, and will flood under the cupboard. 
 

Do I need to get the pipe flushed from the boiler end? (Still under warranty).

 

Ultimately relocating the condensate pipe would be good, but easier said than done…

 

Thank you! ☺️ 

Couple of issues I've come across with the outlet pipes and draining off of the condensation.

 

The pipe exiting the boiler has had a slight upwards incline and so trapping water in the first section of pipe below the boiler. Mass builder's don't you love them.

 

The sink drain (U bend) has had a partial blockage and so backs up water to the condenser pipe.

 

With an external venting condenser pipe this has been sat on a gully cover and frozen upwards with a large ice core.

 

All of the above have caused the boiler to shut down

@twice round the blockThank you. It definitely seemed like a partial blockage. It seems to have resolved now we’ve thawed so maybe I need to lag some more. It is also possible there is a hidden dip somewhere in the pipe.