Barrie, The carbon that is coating the tubes is something that it is wise to remove, as it is a great insulator. Phill Phill, I think the hard coating of carbon in the flues will cause no problems whatsoever as hard carbon is a good conductor of heat. Not so with the soot etc, because it is not compact. I would leave it alone as long as the boiler steams well. Jesse in Tennessee USA MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.