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You do not want both boilers to be kept at the same temperature. When one boiler heats another boiler, the extra heat used to keep the other boiler up to temperature is wasted and lost from the non-operating boiler jacket and the flue.
By piping them in parallel, each boiler can contribute its own heat when it is in use. Through the proper use of flow control valves, you can have either boiler control its own circulator with little change in the existing piping.
By installing inlet and outlet valves and piping the boilers in parallel, you can repair either boiler without having to stop heating the home, as is done in commercial buildings.
