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Hydronic heating systems—hot water and steam—represent fundamentally different heat distribution methods requiring specialized knowledge. 

Hot water systems begin with distinguishing closed systems (modern standard) from obsolete open gravity systems. Closed systems seal water within pipes, boilers, and radiators after initial filling. System pressure runs 12 psig cold and 20-30 psig operating, with temperatures typically 160-180°F. Understanding that manufacturers discourage draining (dissolved air in fresh water causes problems) affects maintenance recommendations.

Air removal systems prevent circulation blockages. Manual air bleeder valves at radiators or central locations allow occupant purging. Automatic air scoops and separators connected to expansion tanks handle ongoing air removal. Understanding that lack of air removal isn't necessarily deficient but creates maintenance challenges helps frame recommendations.

Expansion tanks absorb water volume increases from heating. Newer bladder tanks separate water from air; older tanks without bladders can become waterlogged and fail. Tank sizing depends on system water volume, with 1-4 gallon tanks common residentially.

Aquastats control boiler firing through high limit, low limit, and differential (DIFF) settings. High limit settings work with thermostats for space heating; low limit settings maintain water temperatures for domestic hot water production via tankless coils or indirect-fired water heaters.

Piping materials (copper, steel, PEX, polyethylene) and proper installation (tee orientation, support requirements, monoflow fittings) impact system performance. Series-connected radiators create severe heat distribution problems. One-pipe systems with monoflow fittings and two-pipe systems (direct return or reverse return) solve series connection issues with varying complexity and cost.

Distribution devices include cast iron radiators (traditional), baseboard heaters with finned tubes, and convectors (enlarged baseboard versions). Radiant floor/ceiling/wall systems use S-shaped pipe patterns for even heat distribution.

Steam heating systems share boiler similarities but operate fundamentally differently. Steam rises by convection (no pumps needed), operating pressures remain very low (½-2 psig), and temperatures stay around 212°F. Regular maintenance demands—weekly sludge draining, frequent water level monitoring—require active occupant involvement unlike "set and forget" forced-air systems.

Steam system types (one-pipe counterflow, one-pipe parallel flow with Hartford Loop, two-pipe) each solve the water-versus-steam fight differently. Understanding this fight—water returning to boilers while steam moves toward radiators through the same pipes—explains the infamous steam heating banging (water hammer).

Hartford Loops prevent boiler damage by raising wet return connections 2-4 inches below normal water levels, preventing complete drainage from leaks or pressure imbalances. Equalizer pipes ensure pressure equilibrium between steam supply and wet returns.

Vents and traps control air release and water drainage. Main vents (ball float or inverted bucket traps) should be located downstream from final distribution devices. Air vents on one-pipe system radiators and thermostatic traps on two-pipe systems require regular cleaning and replacement (3-6 year service life).

Steam piping must be steel (copper discouraged), properly sloped (1 inch in 10-20 feet depending on system type), and insulated (lack of insulation causes performance problems and water hammer even if originally uninstalled). Older

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