Sounds as though it's starving for air or have a blockage on the exhaust side. Ok, you cleaned the heat exchanger tubes, did you clean out the base under the tubes (mine has small access panels on the side of the boiler, near the bottom toward the rear). Is your chimney clear? Does it have an elbow where fly ash or creosote might build up?Ok i did that and still not getting a huge flame, its heating house but flames then it smolders. Flames up when i open door.