Back Boiler removal & room air vent sealed

[Pagwa]

Recently had plumber & builder to remove back boiler & seal up chimney breast along with room air vent. Replaced with a condensor boiler in the loft. The chimney was sealed with breeze blocks & plastered. Since having heavy rain/strong winds condensation has formed on the inside wall where the room air vent was. Builder put a brick into the vent space & plastered over & didn't remove external vent, what do you recommend?

Should we seal chimney top, now have concerns about rain or bird debris, however we never had any damp or bird debris when the back boiler was in working?

Appreciate any thoughts or recommendations.

[htg engineer]

I removed my fire last year, bricked up and plastered over. I also removed the chimney pot and sealed the top of the chimney.

It's best to do both - not one. If you seal the bottom but not the top you will get rain entering the chimney, and with not having the bottom open there's no air flow - so it wont dry out.