HVAC · AHU
AHU Control — Supply Air, Dampers and Ventilation
AHU control sequences for supply air temperature, fan volume, economiser operation, frost protection and demand-controlled ventilation.
Quick answer
AHU control modulates fans, dampers, valves and coils to deliver the required supply air temperature and volume to each zone. Sequences must coordinate economiser operation, frost protection, filter monitoring and demand-controlled ventilation, and define safe positions when a sensor or fan fails.
What an AHU sequence controls
An AHU sequence controls the whole air path: outdoor, return and exhaust dampers, filters, heating and cooling coils, humidifier, fans and the terminal boxes downstream. Constant-volume units hold a fixed supply volume; variable-air-volume units modulate fan speed and box positions. Supply air temperature may be fixed or reset based on zone demand. The sequence must define startup and shutdown order, damper positions, fan proof and coil enable. It must also define what happens when the building is occupied but the AHU is off. That transition is where comfort complaints usually begin. A unit that starts with dampers in the wrong position will trip on frost or high static pressure. The sequence should prove each actuator before it enables the fan.
Dampers, valves and coil control
Damper and valve control looks simple until authority and changeover are considered. The economiser sequence compares outdoor and return conditions and decides when free cooling is beneficial; the changeover needs hysteresis to avoid hunting. Minimum outdoor air must be maintained for ventilation even when the economiser is closed. Coil valves should be selected for the actual pressure difference, and the control signal should be mapped to flow rather than to a nominal percentage. Frost protection is not optional in cold climates: a duct thermostat, a low-limit switch and a valve position must work together, independent of the normal control loop. A valve that is oversized for the coil will control in a narrow band and then step. Authority is a selection issue, not something the controller can fix.
Ventilation on demand
Demand-controlled ventilation adjusts outdoor air to the number of people actually present. CO₂ is a useful proxy in densely occupied spaces; occupancy counters or schedules work better where CO₂ lags. The sequence should define a minimum fresh-air rate, a maximum rate, a deadband and a response time. It should also coordinate exhaust in toilets, kitchens and car parks, because unbalanced extract can pull air through the wrong path. Over-ventilation is a real energy cost, but under-ventilation is a health and comfort problem. The control should fail towards the safe rate, not towards the energy-saving rate. The sensor should be in the occupied zone, not in the return duct, when the goal is occupant exposure. Return-air sensing is still useful for system control.
Safety, faults and smoke control
An AHU is part of the building's safety systems as well as its comfort systems. Fan proof, filter differential pressure, frost protection and smoke damper interfaces must be hard-wired or otherwise independent of the normal sequence. The smoke control strategy should be agreed with the fire engineer, not invented by the controls contractor. On a fault, the AHU should move to a defined position: fans off, dampers closed or open as required, valves closed. Alarms should distinguish a filter that needs changing from a fan that has failed. Test these behaviours during commissioning with the fire system, not only in the graphics. A damper that closes on a fan that is still running can damage the duct. Interlock the fan proof with the damper command.
AHU functions and failure modes
6 rows| Function | Controlled by | Failure to design for |
|---|---|---|
| Outdoor air | Mixing dampers and economiser | Damper stuck and minimum air lost |
| Supply temperature | Coil valves and staging | Valve authority and sensor drift |
| Air volume | Fan speed or inlet guide vanes | Duct static pressure and fan surge |
| Filtration | Filter monitoring | Blocked filter and fan overload |
| Frost protection | Thermostat, low-limit switch and valve | Coil freeze during cold start |
| Smoke control | Smoke dampers and fan control | Interface not tested with fire system |
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Frequently asked
01What is an economiser cycle?
It uses cooler outdoor air instead of mechanical cooling when conditions allow. It needs a changeover sequence with hysteresis and a minimum outdoor-air position.
02Should supply air temperature be fixed?
Not necessarily. A reset can save energy, but it must respect zone humidity and the terminal boxes' ability to deliver the load.
03How is ventilation controlled?
By CO₂, occupancy or a schedule, with defined minimum and maximum outdoor-air rates and a safe failure position.
04What happens if the supply fan fails?
The sequence should stop the coils, position dampers, and alarm. A fan proof signal is the basis for that response.
05Can an AHU sequence control VAV boxes?
It can coordinate with them through the supervisory layer, but the box control loop usually runs locally at the terminal.
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