Dual-duct HVAC system: how mixing boxes work and why it is rarely specified

One-line orientation

A dual-duct system runs separate hot and cold air mains to every zone and blends them at a mixing box — powerful temperature flexibility, but the simultaneous heating-and-cooling waste makes it rare in new construction.

Key points

• Two supply ducts: the central AHU conditions two separate air streams — one heated and one cooled — and delivers both through parallel duct mains to every zone in the building.
• Mixing box: at each zone a mixing box (also called a mixing terminal unit) blends the two streams. The zone thermostat modulates dampers in the mixing box to proportion hot vs cold air to hit the setpoint. More cold air = cooling; more hot air = heating.
• Thermostats: each zone has its own thermostat that independently controls its mixing box dampers — giving precise per-zone temperature control.
• Applications: historically used in hospitals, large laboratories, and complex office buildings requiring flexible simultaneous heating and cooling across many zones.
• Energy tradeoff:
  • A classic constant-volume dual-duct system maintains hot and cold air streams and mixes them at each zone. Variable-volume dual-duct versions also exist.
  • At every mixing box, the final supply temperature is achieved by partially canceling out energy already spent on heating and on cooling — similar to VAV reheat but potentially at larger scale.
  • Dual-duct systems are recognized as high-energy-use systems.
• Space requirement: dual-duct systems require substantially more duct space than comparable single-duct systems. Do not memorize an exact multiplier.
• Why rarely specified today: VAV with terminal reheat offers similar per-zone precision at lower duct space cost; energy codes have become more restrictive; first cost and maintenance complexity are high.

Confusions / comparison

	Standard VAV	VAV with reheat	Dual-duct
Supply ducts	1 (cool)	1 (cool)	2 (hot + cold)
Terminal unit	Damper only	Damper + heating coil	Mixing box (two dampers)
Per-zone heat + cool	Cooling only	Yes — reheat coil	Yes — mixing box
Energy efficiency	High	Moderate	Low — continuous H+C
Duct space	Standard	Standard	~2× duct volume
Use today	Common	Common (hospitals, labs)	Rare (legacy / retrofit)

Related

→ HVAC: VAV with reheat (contemporary alternative for precision multi-zone control) · single vs multi-zone (dual-duct is a multi-zone all-air system) · HVAC distribution systems (all-air family context).