Don't know if it was you or not Roy but someone had this problem because they had the flue going at a 90 degree angle. If you need the flue to go somewhere except straight up then have it going over gradually instead of straight up and then a 90 degree corner. What happens is the exhaust will build up in the flue instead of going round the corner, and this in turn will trip the exhaust safety device. Peter Fogwillwww.window-tools.com