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Sheet M-103
PPDPDD PACE

HVACconcept

HVAC distribution systems: all-air / all-water / air-water selection

One-line orientation

HVAC systems are categorized by the medium that carries heating/cooling to the space — the exam tests which type to choose under given constraints and each type’s control logic.

Key points

  • All-air: single-zone, CAV, VAV, dual-duct, reheat. Ducted, centrally treated air is delivered to the space; the duct itself is the thermal medium, so it needs the most vertical shaft and ceiling space.
  • All-water (hydronic): fan-coil units, water-source heat pumps. Water carries the thermal energy and air is conditioned locally at the terminal; pipes are small, so it saves duct space but needs local ventilation handling.
  • Air-water: induction units, etc.; a small central primary air supply (for ventilation) plus a local water coil at the terminal (for load) — balances the two.
  • VAV vs CAV: VAV varies air volume at a constant supply temperature (efficient, good for variable loads); CAV holds constant volume and varies the supply temperature (simpler, less efficient).

Confusions / comparison

MediumTerminal handlingTypical useWhat it saves
All-airaircentrallarge spaces, high ventilationcentral control, easy maintenance
All-waterwaterlocaltight floor-to-floor, perimeter loadsvertical duct space
Air-waterbothlocal + some airhigh-rise perimeter zonesbalances ventilation and space

All-air control logic — VAV vs CAV (the most-tested air-side distinction, otherwise only in the drills):

CriterionVAV (variable air volume)CAV (constant air volume)
What variesAir volume, at a constant supply temperatureSupply temperature, at a constant volume
EfficiencyHigher — throttles airflow to match the loadLower — moves full airflow regardless of load
Best fitVariable or multi-zone loadsSimple, steady single-zone loads

HVAC distribution: the carrying medium sets how much building it eats

SCHEMATIC

Named by what carries the heat — air, water, or both — and where it is conditioned. Air needs the deepest shaft and ceiling plenum; water the least.

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HVAC distribution modes — all-air, air-water and all-water compared on one building section One continuous two-story building section, drawn in cut structure, is divided into three equal bays that share the same roof slab, two floor slabs, grade line, and a single fixed floor-to-floor height dimensioned outside the building at the right. Each bay runs a different distribution system, and only the mechanical run and its terminal change, so the bays are a fair comparison rig. Left bay, all-air: a wide blue duct — the widest run — rises up the shaft and runs along the deepest dropped-ceiling plenum on each floor to a ceiling diffuser that throws supply air down into the room. An amber band fills that deep all-air plenum as the trap, and a callout tucked under it reads: tight floor-to-floor, never all-air — because the duct is itself the thermal medium it needs the most shaft and ceiling depth and eats the most of the fixed floor-to-floor. Middle bay, air-water: a narrower run — a slim blue primary-air duct plus a thin teal supply-and-return pipe pair — rises up a mid-width shaft into a medium-depth plenum, and both converge into a perimeter induction unit drawn ink-neutral because it takes both media, the primary air jetting past a local water coil with no fan. Right bay, all-water: the narrowest run, a thin teal pipe pair, rises with almost no dropped ceiling and branches low into a floor-standing fan-coil unit that has its own fan. Below the grade line each run is witnessed by extension lines and dimensioned on its own line clear of the ground hatch, schematically about thirty-one, twenty, and ten inches, narrowing left to right; identical plant boxes sit at the base of every shaft. Run width and plenum depth both shrink from all-air to air-water to all-water, so the ranking of how much building each medium eats reads from either the widths or the plenums alone. L2L1FLOOR-TO-FLOORFIXEDtight floor-to-floor→ never all-airPlant≈31 inAll-airPlant≈20 inAir-waterPlant≈10 inAll-water
  • air — duct / primary air
  • water — supply / return pipe pair
  • induction — air nozzle + water coil, no fan
  • fan-coil — fan + water coil

The duct is itself the thermal medium, so all-air's shaft and ceiling plenum run deepest — they eat the most of a fixed floor-to-floor. A tight floor-to-floor selects water-based distribution (all-water or air-water), which needs far less vertical space.

→ economizer cycle · Construction: Enclosure (envelope loads affect selection) · Code: energy code.

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