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replacing a steam boiler on Wednesday and have a question with the main vents. the house has only 1 main that goes completely around the perimeter of the basement. I estimate it to be about 100 feet of 3 inch iron. there are a total of 9 radiators attached all with 1-1/4 iron. it is a 1 pipe system. we always replace the main and radiator vents but i'm sure they are undersized every time. the only vent my company seems to be able to get easily is a hoffman #75. this forum seems to recommend Barnes and jones and gorton more often than not but my question is how many Hoffman #75's should i expect to use at the end of this 3" main? and just for knowlege, how many big mouths? the boiler will be a burnham megasteam 396, 2 risers, dropped header configuration tied into the main with a 3 x 2 eccentric coupling. p404F with subtractive differential. main set for 1.5 or 2 and differential set for 1.