OK I had a look at my charcoal forge just now after it had cooled off and I have noticed a serious design flaw. Namely no way to clean ash that drops in the air pipe. On the plus side the charcoal I use isn't leaving huge clinkers. Only a handful after a full days work. Any tips on a work around for the ash problem would be appreciated. So far it seems to mostly be filling the gaps between the hebel bricks and acting as extra refractory.