Phoenix Journal · Ductwork
Smart buildings have stopped ventilating on a fixed timer and started reading the room in real time, moving only the air that occupancy and cooking actually call for. Here is how demand-based control works - and why it changes maintenance rather than removing it.
A modern building no longer ventilates on a fixed timer and hopes for the best - it reads the room, minute by minute, and moves only the air it genuinely needs to.
For a long time, mechanical ventilation was a blunt instrument. A fan came on when the building opened, ran flat out all day, and switched off at night, whether the space held two hundred people or none at all. It was simple, it was reliable, and it was extraordinarily wasteful - both of energy and of the equipment being run into the ground for no reason.
Smart buildings have quietly rewritten that logic. Instead of guessing at demand, they measure it directly, then match airflow to what is actually happening in each zone. The result is a system that breathes with the building rather than against it: quieter, cheaper to run, and easier on the fans and ductwork. Understanding how that control loop works matters to anyone responsible for a commercial kitchen, because the same intelligence now sits over extraction systems too - and it changes how you plan your maintenance, though not whether you need it.
Demand-controlled ventilation rests on a simple idea: let the building tell you how much fresh air it needs, rather than deciding in advance. The most common signal is carbon dioxide. People exhale CO₂ as they breathe, so a rising concentration in a room is a reliable proxy for how many bodies are in it. A sensor on the wall reads that level continuously and asks the ventilation system to bring in more outdoor air as the room fills, and to ease back as it empties.
The thresholds are not arbitrary. BS EN 16798-1, the current European standard adopted in the UK for indoor environmental quality, sets four categories of air quality expressed as CO₂ above the outdoor background - around 550 ppm for the highest category, 800 ppm for the middle, and 1350 ppm at the more relaxed end. CIBSE TM40 covers the same territory from a health and wellbeing angle. A smart building is, in essence, a machine for holding the room inside your chosen category using the least air movement possible.
CO₂ is only the headline. A well-designed system reads several signals at once - temperature, humidity, occupancy from motion detectors, and in some cases particulate or VOC levels - and blends them into a single instruction for each zone. An empty meeting room is left alone; a packed one gets a surge of fresh air the moment it needs it. Nothing is ventilated on faith.
The signal has to become movement, and that is where the building management system, or BMS, earns its keep. The BMS gathers readings from every zone, compares them against the targets it has been given, and translates the gap into a command for the fans. Crucially, modern fans are driven through variable speed drives, so instead of a crude on or off, the system can dial a fan to any point between idle and full tilt.
This is where the real money is saved, and it comes down to physics. Fan power does not fall in a straight line as you slow a fan down - it falls with the cube of the speed. Drop a fan to 40% of its rated speed and it moves less air, yes, but it draws only around 6% of the power it used at full pelt. Even modest, near-constant reductions in speed compound into very large savings over a year, which is why the specific fan power figures that Part L of the Building Regulations scrutinises fall so sharply once demand control is in play. Approved Document F, updated in 2021 and in force since June 2022, now also insists that mechanical ventilation is properly commissioned and tested, so the intelligence has to be proven, not just installed.
In a commercial kitchen the same principle appears as demand control kitchen ventilation, or DCKV. Here the trigger is cooking rather than breathing. Temperature probes and optical or infrared sensors in the canopy watch for heat and smoke from the appliances below, and the extract and supply fans ramp up only when the line is busy. During prep, quiet spells and wind-down the fans sit at a fraction of full speed. Manufacturers report fan energy reductions of anywhere from a quarter to two-thirds against constant-volume running, with knock-on savings on the heating and cooling of all that replacement air, and paybacks commonly landing inside one to three years. If you want the detail on how these systems are specified and where they pay off, our guide to demand-controlled kitchen ventilation covers it in full.
It is tempting to assume that a cleverer system is a lower-maintenance one. For grease extraction, the opposite can quietly be true. DW/172, the BESA specification for kitchen ventilation, now explicitly addresses DCKV and recirculation, and it is clear that demand control changes how a system behaves rather than what accumulates inside it. Every gram of fat, oil and grease that leaves your appliances still ends up on the internal surfaces of the canopy, filters and ductwork - smart controls simply decide how fast the air carrying it is travelling.
That last point deserves care. When a system spends much of its day running at reduced speed, air velocities inside the duct fall. Lower velocity gives airborne grease more time to settle and cling rather than being carried through to the discharge, so deposits can build in stretches of ductwork that a constant, high-flow system might have kept scoured. The fire risk that grease represents is unchanged by how intelligent the fan controller is. This is why TR19 Grease, the industry standard for cleanliness of kitchen extract systems, applies to a demand-controlled kitchen exactly as it does to a traditional one - measured by deposit thickness, verified, and recorded for your insurer.
The same tension shows up when smart controls are retrofitted into older stock, where access and duct routing were never designed with modern maintenance in mind; our note on managing ventilation in listed and heritage buildings looks at that balancing act. Wherever the building sits, the lesson is consistent: intelligent ventilation earns its savings on energy, and a disciplined cleaning regime protects everything underneath it. The two work together, not instead of one another.
Questions
No. Demand control changes fan speed to match cooking activity, but the same grease still leaves your appliances and settles inside the canopy, filters and ductwork. If anything, the lower air velocities during quiet periods can let grease cling rather than being carried away. TR19 Grease cleaning frequency is driven by how much deposit builds up, not by how the fans are controlled.
The most common is carbon dioxide, because people exhale it and rising levels reveal how busy a space is. Systems typically also read temperature, humidity and occupancy, and in kitchens they use heat and smoke sensors in the canopy to detect cooking. A building management system blends these signals and adjusts fan speed for each zone against targets such as the categories in BS EN 16798-1.
Because fan power falls with the cube of fan speed, running a fan at 40% of its rated speed uses only around 6% of full power. In practice, DCKV systems report fan energy reductions from roughly a quarter to two-thirds against constant-volume running, plus savings on heating and cooling the replacement air. Paybacks commonly fall within one to three years, though the exact figure depends on your kitchen's hours and cooking pattern.
Phoenix Duct Clean · by the numbers
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