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Some do, some don’t. Older systems were steam, newer ones are hot water. Steam (really water vapor - it’s not at high enough pressure to be real steam) systems have one pipe, hot water have two pipes. Hot water also has a circulator pump, almost always the old rumbling B&G 100. They made that pump for ages, then the wet rotor can pumps became popular. Maybe they still make the 100. What a maintenance joy they were…
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