Heat, energy & performance
Fan energy does not rise in a straight line. The affinity laws tie power to the cube of fan speed, so a system fighting extra resistance either moves far less air or, if speed-controlled, spends sharply more energy to hold its airflow. A grease-clogged extract quietly costs money every hour it runs.
The short answer
Fan energy does not rise in a straight line. The affinity laws tie power to the cube of fan speed, so a system fighting extra resistance either moves far less air or, if speed-controlled, spends sharply more energy to hold its airflow. A grease-clogged extract quietly costs money every hour it runs.
The detail
Fan behaviour follows three affinity laws. Air delivery is proportional to fan speed, static pressure to the square of speed, and power to the cube of speed. That cube relationship is the one that matters for bills: small changes in what the fan has to do produce large changes in what it consumes.
Grease loads the system with resistance. On a fixed-speed fan, the operating point slides up the pressure curve and the fan simply moves less air - you lose ventilation rather than gain a bill. On a variable-speed drive told to hold a target airflow, the drive must raise speed to overcome the resistance, and the cube law turns that into a steep energy penalty.
Independent modelling shows how severe the restriction can get. A filter going from clean to heavily clogged in one study cut volumetric flow by around 52 percent, with a static-vacuum spike downstream where drive energy was dissipated generating suction rather than useful airflow. In a kitchen, grease is that clog.
The tell-tale signs are a fan running hotter, drawing more current at the same setting, or a speed drive creeping upward to keep airflow. Filter and fan monitoring systems detect clogging exactly this way - by a rising pressure differential or a rising current draw. A greased extract shows the same symptoms.
What it means for you
A clogged extract is a standing charge you did not agree to. Either you are under-ventilating, which shows up as heat and poor capture rather than a bill, or your drive is spending cube-law energy to hold airflow through a restriction that should not be there. Both are waste, and both grow as grease builds.
Cleaning resets the resistance. With filters, plenum, duct and fan back to TR19 condition, the fan meets its design airflow at its design power, and any speed drive can wind back down. The energy saved is real and continuous, not a one-off.
Phoenix Duct Clean cleans the whole path the air travels, so the fan is no longer working against grease it should never have had to. The clean addresses fire risk and running cost in the same visit.
The service behind the guide
Sibling guides
Clean extraction and lower bills · Why a kitchen turns stuffy mid-service · Care home extraction cleaning
Questions
Grease raises system resistance. A speed-controlled fan must work harder to hold its airflow, and because fan power rises with the cube of speed, that costs sharply more energy. A fixed-speed fan instead loses airflow, costing you ventilation.
One of the affinity laws: fan power is proportional to the cube of fan speed, while airflow is proportional to speed and pressure to its square. It means the energy needed to push air rises steeply as resistance forces higher speeds.
Modelling of a filter going from clean to heavily clogged showed flow dropping by around 52 percent, with energy wasted generating suction instead of airflow. In a kitchen, accumulated grease is the equivalent restriction.
Watch for a fan running hotter, drawing more current at the same setting, or a speed drive creeping up to hold airflow. These are the same signals clogged-filter monitors use, and grease produces them.
If the system is speed-controlled, yes - resetting the resistance lets the drive wind back down to design power. On a fixed-speed fan the gain shows as recovered airflow and heat removal rather than a lower bill.
Phoenix Duct Clean · by the numbers
A clean resets the resistance and the running cost. Book a TR19 extraction clean.