The mixing box is where an AHU blends outdoor, return, and exhaust air, and its damper control sets both the ventilation rate and the energy bill, so actuator sizing and sequences must be right.
What the Mixing Box Does
A mixing box holds three linked damper blades: outdoor-air, return-air, and exhaust (relief). As one opens the others close in a coordinated motion, holding total flow roughly constant while shifting the blend. The outdoor-air damper enforces the minimum ventilation; the return damper recovers conditioned air; the exhaust damper balances building pressure so the space does not go strongly positive or negative. The three are a mechanical sum game, and the control logic is what keeps that sum sensible as conditions change.
Getting the balance wrong has visible consequences: too much exhaust and the building goes negative, pulling humid outdoor air through cracks and raising the cooling load; too little and the space pressurizes, leaking conditioned air out and stressing the envelope. Both waste energy in opposite directions, which is why the bias is a setpoint, not an accident.
Blending is not automatic, and poor mixing is the hidden fault in this section. Cold outdoor air is denser than return air, so in winter the two streams can stratify instead of mixing, leaving a cold band across the bottom of the coil face while a sensor higher up reads an acceptable average. Air only a few degrees below freezing in that band is enough to split a water coil tube, and the discharge sensor never sees it. Remedies include opposed-blade dampers that promote turbulence, a mixing baffle or air blender, and low-limit protection using an averaging capillary stat run across the full coil face rather than a single-point sensor.
Actuator Sizing and Sequences
Dampers need torque to seat against static pressure, so the actuator is sized from the blade area and the section pressure, often with a 1.5ร safety factor. A common control sequence modulates the outdoor and return dampers together for economy, adds enthalpy economizer logic in swing seasons, and never lets the outdoor-air blade drop below the code minimum, because the ventilation floor is a safety limit that energy savings must never erode.
| Item | Specification | Note |
|---|---|---|
| OA minimum | 10โ15% | Code ventilation floor |
| Actuator torque | 1.5ร blade load | Seats against static |
| Blade area | Per section | Drives torque size |
| Signal | 0โ10 V / 4โ20 mA | Modulating control |
| Pressure bias | Slightly positive | Building pressure target |
| Freeze stat type | Averaging capillary | Run across the full coil face |
| Coil face velocity | Typically 2.0–2.5 m/s | Supports even mixing at the coil |
The exhaust damper is the often-forgotten member: without it, opening the outdoor-air damper simply pressurizes the building. A relief path, either a barometric or motorized damper, is what lets the ratio actually change, and the actuator on it deserves the same sizing care as the others rather than being treated as a passive grille.
Actuator behaviour deserves as much specification as torque. Stroke time matters where a sequence has to respond quickly: a 90 second actuator is fine for economizer trimming but too slow for a smoke-mode changeover, which usually needs a spring-return unit closing in a few seconds. Fail position must be decided deliberately, and in a cold climate the outdoor damper normally fails closed while the return damper fails open, so a power loss cannot freeze a coil. Position feedback is worth the extra conductor, because a sequence that assumes a blade reached 20% while linkage wear leaves it at 40% will quietly over-ventilate for years without raising any alarm.
Control Checklist
- Size the actuator: Match torque to blade area and section pressure, not to the link size or a catalogue default.
- Hold the OA floor: Interlock so minimum outdoor air is never traded away for savings, even in deep winter.
- Add relief: Provide an exhaust path or the mix cannot shift and the building pressurizes.
- Pick the signal: Use a modulating 0โ10 V or 4โ20 mA loop with feedback, not just open/close.
- Set the bias: Choose slightly positive so the building leaks out, not in, with humid air.
- Specify stroke and fail position: Match actuator speed to the sequence and fail the outdoor blade closed in cold climates.
- Guard against stratification: Use an averaging freeze stat and a mixing baffle where cold outdoor air meets return air.
Why Mixing Control Matters
A correctly sequenced mixing box delivers code ventilation while capturing every available free-cooling hour, and that combination is where HVAC energy savings live. Keruite builds AHU mixing boxes with matched damper drives and control-ready linkages so the sequence the designer draws is the sequence that runs, instead of one the actuator cannot physically deliver.
The mixing section is a simple assembly that governs both ventilation compliance and a large share of the energy bill, so it deserves engineered hardware rather than an improvised box. Sections produced in an 80,000 m² plant by a manufacturer with more than 30 years of unit fabrication behind them are supplied with blade area, leakage class, and torque data so the actuator is chosen on numbers. ISO 9001:2015 covers the linkage and casing build, CCC certification covers the electrical items, and with units delivered into more than 50 countries the same section is arranged for both relief-damper and ducted-exhaust practice.
Our engineers respond to selection enquiries within 24 hours, including mixing-box damper, actuator, and control-sequence selection for AHU projects.