Extraction economics
The extract fan runs longer than any appliance in the building and its condition is invisible from the floor. Grease rewrites the resistance it works against - and whether the system is fixed-speed or controlled, somebody pays. Here is where the money goes, and the rhythm that keeps it.
Extraction economics
The extract fan switches on before the first burner and off after the last one. It runs through prep, service and cleandown, every trading day, moving air against whatever resistance the ductwork puts in its way. And in most kitchens nobody has the faintest idea what it costs, because its consumption is buried in the electricity bill and its condition is hidden above the canopy. That combination - always on, never seen - is exactly where quiet money lives.
The physics of the bill
A duct system has a resistance curve: pressure loss rises with the square of airflow, set by the geometry of the ductwork, its fittings and its internal surface. Grease deposits change that geometry. They narrow the free area and roughen every surface, steepening the curve, so the same fan speed moves less air against more pressure. From there one of two things happens, and both cost money.
Two ways to pay
A fixed-speed fan on a fouled system simply delivers less air. Capture at the canopy fades, heat and moisture stay in the room, the interlock is proving a fan that is running but no longer doing its job, and the kitchen gets hotter and slower every month. The energy bill barely moves; the cost lands on the people and the food instead, and eventually on the fire risk assessment.
A fan on a controller that chases an airflow or pressure setpoint responds to fouling by speeding up, and the affinity laws make that expensive fast: power rises with the cube of speed. The system quietly ramps toward its limits, drawing more current week by week to deliver the same air a clean system moved at a fraction of the power. The bill creeps, the motor runs hot, and the drive spends its life working against dirt.
The compounding costs
Across typical UK restaurant metering, ventilation accounts for around 20% of total energy use, with cooking near 30% and refrigeration around 25%. Those three lines are not independent. Extraction that underperforms leaves heat in the room, so refrigeration works harder against a hotter ambient and the brigade works slower in it. A well-maintained system earns its keep three times over: the fan moves its design airflow at design power, the room stays workable, and every heat-sensitive machine in it gets an easier life. The same inspection that protects the energy line also feeds the compliance one - cleaning frequency is set by usage under TR19 Grease, and the cost of ignoring it shows up everywhere except the line marked extraction.
Making it routine
The rhythm is simple: clean to the usage band, check filters weekly, and treat any change in canopy capture or fan noise as data rather than background. Baffle filters are the cheapest lever in the whole system - they are the first surface the grease meets, and a filter left to load up passes the problem straight into the ductwork where removing it costs far more. Wash them on a fixed day rather than when they look bad, and pair the duct clean with a mechanical once-over: belt tension, bearing condition and damper positions all change the resistance the fan works against, and all three drift silently. A system maintained on that cycle runs at the power it was designed for, and the savings arrive the quietest way possible - as bills that simply stop creeping.
Questions
Ventilation typically accounts for around 20% of a UK restaurant's total energy consumption, and the extract fan is the largest single part of it because it runs from first prep to final cleandown. Its exact draw depends on motor size, speed and the resistance of the ductwork it works against.
Yes, one of two ways. A fixed-speed fan on a fouled system delivers less air, so heat stays in the kitchen and every other system works harder. A speed-controlled fan ramps up to compensate, and because fan power rises with the cube of speed, the extra kilowatts add up quickly.
Airflow scales with fan speed, pressure with speed squared, and power with speed cubed. Slowing a fan 10% saves roughly 27% of its energy; forcing it 10% faster to overcome grease costs about a third more. Small changes in resistance become large changes on the bill.
TR19 Grease sets the intervals by usage: heavy use of 12 to 16 hours a day points to roughly 3-monthly cleaning, moderate use to 6-monthly, and light use to around 12-monthly. Actual grease measurement on the day can shorten or confirm the interval.
A grease-loaded system can be down 30 to 50% on its design airflow, and removing the deposits removes the added resistance that caused the loss. If performance stays low after a compliant clean, the survey usually finds a mechanical cause - slipped belts, damaged dampers or a fan due for service.
Phoenix Duct Clean · by the numbers
TR19 Grease extraction cleaning to the usage band your system actually runs at - assessed, cleaned and certificated.