This is the question about my Ravenheat Boiler CSI 150 T

It is in Summer Mode (No Heating, Hot water only). It keeps tripping with fault sign 02, which according to Fault List is :

Anti-cycling of the DAS (Evacuation of smoke)

I have checked the Flue outlet etc. All seem to be OK. So what does statement mean and cause of it ?

A. Q. Awan, June 2018