how do I vent the heat exchanger?

Simply go to www.gassaferegister.co.uk to find your local Gas Safe Registered Engineer as ony Gas Safe Registerd Engineers can repair boilers as you can put your self and others at risk by touching boilers parts that you don't have knowledge how to deal with and is very easy to cause a fault by touching parts that you don't understand how it works.
Good luck.

Fradizela.
November 2015