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Having my steam boiler replaced. This is a one pipe, parallel flow (I believe), gravity return system. Right now there is a single system riser which connects to a tee that then feeds the mains to the left and right. Several contractors have come to look at the job. Some suggested 2 separate takeoffs to feed the left and right main separately OR a single system riser that splits into a Y. Is this correct and can someone explain what the need for this is?