The hydronic loop
The boiler adds sensible heat to water. The distribution system moves that heated water through terminal units, where heat transfers to the building, and returns cooler water to the boiler.
Flow and temperature both matter
Heat delivery depends on water flow and the temperature difference through the circuit. A firing boiler does not guarantee adequate heat delivery if circulation or distribution is impaired.
Circulators and zone valves
Multi-zone systems may use a circulator for each zone or one circulator with motorized zone valves. Controls coordinate a thermostat call with flow and boiler operation.
Expansion and air management
Heating water changes volume and dissolved/entrained air can interfere with circulation. Expansion tanks, air separators and vents are therefore fundamental hydronic components.