A dual-duct HVAC system delivers both a hot deck (typically 40–45°C) and a cold deck (12–14°C) to each zone, mixing them at a local terminal to hit any setpoint without reheat waste; it suits buildings with strongly opposite perimeter and core loads, handling 0.3–1.5 m³/s per zone at 300–600 Pa. The cost is two full ducts and higher ceiling space, which only pays back where reheat would otherwise be large.
How Dual Duct Works
Two separate air paths leave the AHU: one heated, one cooled. At each zone a mixing box blends them in real time—mostly hot on a cold north wall, mostly cold in a sun-baked core. Because you never cool-then-reheat the same air, you avoid the 15–30% energy penalty of reheat VAV. The trade is double duct surface, double insulation, and a larger mechanical schedule; first cost runs 15–30% above single-duct VAV, so it must earn its keep through genuine simultaneous loads.
Comfort is the other payoff. A perimeter office on a winter morning can draw 80% hot and 20% cold while the south core draws the reverse, all without a single reheat coil. Occupants stop complaining about one half of the floor being too warm, which is why dual duct persists in prestige towers despite the duct cost.
Hot and Cold Deck Setpoints
Standard design holds the cold deck at 12–14°C and hot deck at 40–45°C, giving a mixed supply of 18–24°C at the zone. Deck temperature reset by season trims pumping and reheat: in winter the cold deck may rise to 16°C, in summer the hot deck may fall to 35°C. Keeping the mixed-air spread within 6–10°C of setpoint limits box modulation and noise, and reduces the chance of a box slamming to one extreme.
Comparing Terminal Strategies
| Strategy | Ducts needed | Deck temps (°C) | Reheat penalty | Best use |
|---|---|---|---|---|
| Dual-duct mixing | 2 | 12–14 / 40–45 | None | Opposite loads |
| Single-duct + reheat | 1 | 12–14 / reheat | 15–30% | Mixed, budget |
| Dual-duct VAV | 2 (variable) | 12–14 / 40–45 | None | Variable + opposite |
| Multizone constant | 2 | fixed mix | Low | Steady zones |
Key Design Numbers
Size each deck for the larger of the zone’s heating or cooling peak, so the mixing box can reach either extreme; a 40 m² perimeter office may need 0.4 m³/s of hot and 0.3 m³/s of cold. Duct static pressure commonly sits 300–600 Pa. Allow 20–30% diversity when summing decks for the AHU, and keep mixed supply velocity at terminals 4–7 m/s to stay quiet and avoid dumping cold air onto occupants.
Worked Example: A Glass Tower Floor
A 30 m floor has a north perimeter needing 0.5 m³/s heat at −10°C outdoor and a south core needing 0.4 m³/s cooling on a sunny 30°C afternoon. Dual duct sizes both decks to 0.5 m³/s; a mixing box serves north with 90% hot / 10% cold in winter and the core with 10% hot / 90% cold in summer. No reheat coil runs. Compared with single-duct reheat, this avoids roughly 25% thermal waste across the day, paying back the extra duct in about six years.
Decision Checklist: Choose Dual Duct When
- Choose dual duct when perimeter zones need heat while the core needs cooling on the same day—common in glass towers with high glazing ratio.
- Choose dual-duct VAV when those opposite loads also vary hour to hour and you want both mixing and volume savings together.
- Choose single-duct plus reheat instead when budget forbids two ducts and loads are mild enough that 15–30% reheat is acceptable.
- Choose constant multizone when zones are steady and you can fix the mix per bank, simplifying controls and cutting first cost.
Controls and Pitfalls
The failure mode is a box stuck near one deck, starving the other and causing drift or short-cycling. Commission each mixing box to its design min/max, verify deck temps within ±1°C, and reset decks by outdoor air to save 5–10% pumping. Seal both ducts well—leaks here waste conditioned air on both decks at once, easily 5–10% of total volume, and are harder to find than a single-duct leak.
Standards and Commissioning
Follow ASHRAE 62.1 for ventilation on the mixed stream and ASHRAE 90.1 for fan power. Because two decks mean two sets of measurements, balancing takes roughly twice the time of single-duct work; budget for it. Document deck temperatures and box minimums so the controls contractor can set reset schedules that keep both decks within ±1°C of design through the season.
Air Handler Sourcing
Keruite manufactures HVAC fans and air-handling equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment. A dual-deck AHU pairs well with stable 4-79 centrifugal supply fans rated to the 300–600 Pa duty so both decks hold setpoint under diversity, and the factory balance means both decks leave aligned rather than fighting each other from day one.
Hydronic Coordination and Pump Energy
A dual-deck system is only as good as the hydronics feeding it. The hot deck coil wants 40–45°C water, often from a low-temperature heating loop or a heat-recovery source, while the cold deck wants 7–12°C from the chiller. Sizing the pumps for the larger of the two deck peaks, then allowing 20–30% diversity, keeps both decks supplied without oversizing the circulators; a pump run 6,000 hours a year at 5 kW burns 30,000 kWh, so trimming its flow 20% through correct valve authority saves a meaningful slice of the plant energy.
Control the coil valves to ±1°C around setpoint and avoid three-way bypass valves that simply dump unused hot water back to the return, wasting pump power heating water that goes nowhere. A two-way valve closing with load is the efficient choice, paired with a variable-speed pump that follows the summed coil demand. Done well, the hydronic side adds almost nothing to the energy bill; done poorly, a lazy bypass loop can quietly consume 10–15% of the plant’s pumping energy and undermine the very reheat-saving argument that justified the dual duct in the first place.
FAQ
What is a dual-duct system?
A dual-duct system sends a hot deck (40–45°C) and a cold deck (12–14°C) to each zone, where a mixing box blends them to setpoint. It avoids reheat by mixing instead of cooling-then-heating. The price is two ducts, more insulation, and 15–30% higher first cost than single-duct VAV, earned back only by genuine simultaneous loads.
When is dual duct worth the cost?
It pays in glass towers and labs where perimeter zones need heat while cores need cooling on the same day. There it removes the 15–30% reheat penalty of single-duct systems. If loads are mild or steady, single-duct plus reheat or constant multizone is cheaper and good enough, and you avoid the doubling of duct and insulation cost.
What deck temperatures are typical?
Cold deck 12–14°C, hot deck 40–45°C, mixed supply 18–24°C at the zone. Reset decks by season—cold deck up to 16°C in winter, hot deck down to 35°C in summer—to cut pumping 5–10%. Keep mixed spread within 6–10°C of setpoint so boxes modulate smoothly without noise or drift.
How do I avoid dual-duct energy waste?
Commission every mixing box to its design min/max, verify deck temps within ±1°C, and seal both ducts because leaks waste air on two decks at once (5–10% of volume). Reset deck temperatures by outdoor air and keep terminal velocity 4–7 m/s to limit noise and modulation. A box stuck on one deck is the most common and most wasteful fault.