Phoenix Journal · Energy & Cost
An oversized fan looks like harmless insurance, but it is one of the most expensive things a kitchen can be built with. Fan power rises with the cube of speed, and the make-up air lands a second bill. Here is the fix.
An oversized extraction fan looks like harmless insurance - surely more extraction can only be a good thing? In running-cost terms it is one of the most expensive mistakes a kitchen can be built with, and the reason is a piece of physics that catches almost everyone out. The cost of over-extraction is not proportional to how much extra air you move; it climbs far faster than that, and it lands on two bills at once. Understanding why turns a hidden, permanent overspend into a fixable one.
The power a fan draws does not rise in step with the air it moves - it rises with the cube of the fan speed. Move 25 percent more air than you need and you are not using 25 percent more power; you are using close to double. Turn that around and it becomes the opportunity: slowing a fan to 80 percent of full speed cuts its energy use by roughly half. That single relationship is why an oversized fan, or a correctly sized fan simply running flat out all day regardless of what is cooking, quietly burns far more electricity than the extraction job actually requires. A small excess in airflow is a large excess on the bill, every hour, forever.
Here is the cost that stays hidden. Every cubic metre of air an extraction fan throws out of the building has to be replaced by the same volume of fresh air pulled in from outside - the make-up air. In a British winter that replacement air arrives cold and has to be heated to room temperature before it is comfortable. So an oversized fan over-extracts, and you pay a second time to condition all the extra outside air it dragged in. This make-up-air penalty is frequently the larger of the two costs, and it is almost never attributed to the fan on the bill, because it shows up as heating rather than ventilation.
Over-extraction has knock-on effects beyond energy, too. Pulling too much air out creates excessive negative pressure in the kitchen: doors become hard to open, draughts appear, flues can be affected, and conditioned air from the dining room gets dragged through and thrown away. An oversized fan is not just expensive; it can make the space unpleasant and hard to control. Getting the supply and extract air into balance is part of the same problem: if the make-up air supply cannot keep pace with an over-large extract, the fan ends up fighting the building itself, working still harder and dragging yet more conditioned air out through every gap.
The answer is almost never to leave a big fan running at full tilt. Because the appliances under a canopy are rarely all cooking flat out at once, an extraction system sized for the maximum load spends most of the day extracting far more than the actual cooking requires. Demand-controlled kitchen ventilation puts sensors over the cookline and modulates fan speed to the real cooking activity, so the fan drops back when the line is quiet - and thanks to the cube law, even a modest reduction in speed yields a large saving on both the fan and the make-up air behind it. That is the core of demand-controlled kitchen ventilation, and it is one of the strongest levers for cutting ventilation running costs. Variable-speed drives, efficient motors, a correctly sized canopy and balanced make-up air all pull in the same direction.
There is a related trap worth naming. A grease-clogged system behaves like an oversized-fan problem in reverse: the fan has to work harder and draw more power to force air through a restricted, fouled duct, moving less air for more energy. So the full picture for efficient extraction is three things together - clean, so the fan meets its design airflow at design power; correctly sized, so it is not moving more air than the kitchen needs; and demand-controlled, so it backs off when the cooking does. Get all three right and the extraction system does its safety-critical job for the least possible energy. Ignore any one of them and you are paying the cube law, and the make-up-air bill behind it, for the life of the system.
Questions
Because fan power rises with the cube of speed, not in step with airflow. Moving 25 percent more air than needed uses close to double the power, not 25 percent more. So a fan that is oversized, or simply run flat out all day regardless of cooking, burns far more electricity than the extraction actually requires - every hour it runs.
Every cubic metre of air extracted must be replaced by fresh air drawn in from outside, and in winter that air has to be heated to room temperature. An oversized fan over-extracts, so you pay a second time to heat all the extra replacement air it pulls in. This make-up-air penalty is often larger than the fan's own energy cost, and it is easy to miss because it appears as heating rather than ventilation.
Match extraction to demand instead of running flat out. Demand-controlled kitchen ventilation uses sensors over the cookline to modulate fan speed to actual cooking activity, and because of the cube law even a modest speed reduction saves a lot on both the fan and the make-up air. Variable-speed drives, a correctly sized canopy, efficient motors and balanced make-up air all help - as does keeping the system clean so the fan is not fighting grease.
Phoenix Duct Clean · by the numbers
A grease-loaded fan mimics an oversized one, working harder to move less air. We clean to TR19 Grease so your fan meets design airflow at design power - the base for any demand-controlled saving. Certificated, UK-wide, overnight.