Worcester 240 RSF fan replacement?

Hi all, need to replace my fan, no worries there, 3 screw a little bit of jiggery pokery and bingo. However, can anyone tell me if I get paste/ cement with the new fan to, I think, seal the flu and the pipe that, I think, goes to the air flow pressure switch. Also can anyone confirm that without sealing the flu and pipe my boiler will take longer to fire up, if at all.

Thanks.