Safety first - In a concentric system the inner pipe is surrounded by air. In case carbon monoxide escapes from the flue pipe into the air supply pipe, the boiler shuts down. The room remains clear of CO2.
For shaft renovations, just a fire collar is really not sufficient. Cox Geelen’s wall plates are flame-resistant as well as heat insulating, a must-have in renovation! Classification EI60 and higher. Read more or download the certificate.