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Are you going to put cross tubes in the flue? If not think about putting in a spiral sheet of stainless steel. It will force the hot gases to make contact with the sides of the flue. It really increased the efficiency of my donkey and the flues are only 1/2 inch. In heat transfer turbulence is your friend.
It is as simple as this
Hi ChrisGreat work It was me that posted some drawings of the hand pump I made using flat valves and O rings which works really well.When testing my boilers I also use a one way valve in the pipe from pump to boilerCheersRich