Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 34·2.43
Openings for inspection and cleaning
Inspection and cleaning openings must use purpose-made factory components compatible with the flue system, with access covers meeting the same gas-tightness and thermal insulation standards as the flue itself.
Openings for inspection and cleaning should be formed using purpose factory-made components that are compatible with the flue system. The access cover to the opening should have the same level of gas-tightness as the flue system and an equal level of thermal insulation.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet L - Combustion Appliances and Fuel Storage Systems.pdf