Phoenix Journal · Ductwork
Carbon dioxide is the cheapest, clearest signal you have that a space is under-ventilated. Read it properly and a CO₂ monitor becomes an early-warning system that flags a struggling extraction setup before the air ever feels wrong.
Every person in a room is quietly breathing out carbon dioxide, and that simple fact is one of the most useful diagnostic tools you have.
When ventilation keeps pace with the people and the cooking, CO₂ stays low and steady. When it does not, the number climbs. That climb is a warning - not because the carbon dioxide itself is dangerous at these levels, but because it tells you fresh air is not arriving fast enough to dilute everything else building up alongside it: heat, humidity, cooking odours, grease-laden vapour and the general staleness that makes a busy kitchen or dining room feel oppressive.
A CO₂ monitor turns that invisible process into a number on a wall. Used well, it is an early-warning system. It flags a ventilation shortfall while there is still time to act - a blocked intake, an extract fan running slow, a make-up air unit that never switched on - rather than after the complaints, the condensation and the lingering smell have already told the story for you. This is about reading the signal correctly and knowing what to do when it moves.
What the numbers actually mean
It helps to anchor those figures. Outdoor air in the UK now sits at roughly 420 ppm, so that is the floor you are working up from - no indoor space beats fresh outside air. The banding most commonly used here follows public health advice popularised during the pandemic and echoed by bodies such as CIBSE: under about 800 ppm suggests a space is well ventilated, 800 to 1,500 ppm is a grey zone that warrants a closer look, and readings that sit stubbornly at or above 1,500 ppm point to poor ventilation that should be addressed.
Those are ventilation-quality thresholds, and they matter long before you get anywhere near a health limit. The HSE workplace exposure limit set out in EH40/2005 is 5,000 ppm averaged over an eight-hour day, with a short-term limit of 15,000 ppm over fifteen minutes. You will almost never approach those numbers in a normally occupied room. That gap is the whole point: CO₂ becomes useful as a proxy for airflow at a few hundred ppm, not as a toxin at several thousand. The European standard BS EN 16798-1, meanwhile, puts around 950 ppm above outdoor levels in the range for a high standard of indoor air quality, which lines up neatly with the 1,000 ppm figure many people use as a mental line in the sand.
The single most important thing to understand is that a CO₂ monitor does not measure danger. It measures dilution. In an occupied space, people are a steady, predictable source of carbon dioxide, so the concentration in the room reflects how much fresh air is being supplied per person. CIBSE Guide A recommends a fresh air supply of roughly 8 to 10 litres per second per person for comfort; supply air at around 8 litres per second per occupant and CO₂ settles near 1,000 ppm, while cutting that rate lets the number rise. Read the reading backwards and you can infer the airflow.
That is why a rising trend is such a good early warning. If your monitor normally sits at 700 ppm through a lunch service and one week it drifts to 1,300 ppm with the same covers on the books, something in your ventilation has changed. A belt has slipped, a damper has closed, a filter bank has loaded up, an intake louvre is blocked, or a fan is no longer moving what it used to. You have caught a mechanical fault through its effect on the air, days or weeks before it would have announced itself through misery on the floor.
There is one distinction worth nailing down, because it causes genuine confusion in kitchens. Carbon dioxide, CO₂, is not carbon monoxide, CO. Carbon monoxide is a toxic product of incomplete combustion from gas ranges, ovens and any flued appliance, and its limits are far stricter - the HSE long-term exposure limit is 30 ppm, with a short-term limit of 200 ppm. A CO₂ monitor will not protect anyone from a faulty gas appliance, and a CO alarm will not tell you whether the room is ventilated. In a commercial kitchen you want both, doing different jobs. Treat the CO₂ reading as your ventilation gauge and the CO alarm as your life-safety device, and never let one stand in for the other.
A quick word on the instrument itself. The sensors worth having use NDIR technology - a small infrared source that measures how much CO₂ is actually in the sample - rather than cheaper estimating sensors that infer the value from other gases and drift badly. Look for a device that logs a trend over time, not just a spot value, because it is the shape of the curve across a service that tells you the real story. A monitor that quietly recalibrates against fresh air, or that lets you set it manually, will stay honest for years rather than months.
A number on a wall only earns its keep if it changes what you do. The most reliable approach is to learn your own baseline first. Put a monitor where people actually are - the pass, the dining area, a break room - not tucked in a corner by an extract grille where it will read artificially clean. Watch it across a normal week and note where it settles at quiet times and where it peaks under full load. That baseline is your reference. Early warning is really about deviation from it.
From there, a simple response ladder keeps things calm and consistent. While readings stay below roughly 800 ppm, you are comfortable and no action is needed. As the number moves through the 800 to 1,500 ppm band, treat it as a prompt to check the obvious - is the extraction on its correct speed, are intakes clear, is make-up air actually being supplied - and to increase ventilation where you can. If the monitor holds at or above 1,500 ppm during normal occupancy, that is a genuine flag: your system is not delivering the air the space needs, and it warrants investigation rather than a shrug.
What that investigation finds is usually mechanical, and often about balance. Extract and supply have to move together; pull air out of a sealed kitchen without letting fresh air in and the whole system chokes, which is exactly the condition a rising CO₂ trend exposes. This is where monitoring joins up with how a system was set up in the first place. If a ductwork run was undersized or a fan poorly matched, the shortfall was baked in before day one, which is why the design documents your ventilation system needs matter so much. And a system that read well at handover but drifts over time is telling you that upkeep has lapsed - the distinction between commissioning and maintaining a ventilation system is precisely the difference between a good first-day reading and a good reading two years on.
Grease is the reason that drift so often shows up in kitchen extraction specifically. As deposits build inside ductwork and across filters, they narrow the airway and add resistance, and the fan simply cannot pull what it was designed to. CO₂ is a blunt instrument for spotting that - it reacts to the whole room, not to the duct alone - but a slow, creeping rise in your baseline over months is a fair sign that airflow is being throttled somewhere, and buildup is a prime suspect. The monitor points; a proper inspection confirms.
Questions
No, and this is an important distinction. A CO₂ monitor measures carbon dioxide as a proxy for how well a space is ventilated, whereas a carbon monoxide (CO) alarm is a life-safety device that detects a toxic gas produced by faulty or incompletely burning gas appliances. Their exposure limits are worlds apart - 5,000 ppm for CO₂ over an eight-hour day versus just 30 ppm for CO. You need both, doing their separate jobs.
Most UK guidance treats readings below around 800 ppm as a well-ventilated space needing no action. The 800 to 1,500 ppm band is a prompt to check your ventilation and increase fresh air where you can. Readings that sit at or above 1,500 ppm during normal occupancy indicate poor ventilation that warrants investigation. Just as useful is watching for deviation from your own baseline - a rising trend under the same load signals a change worth chasing down.
Not directly, but it can flag the symptom. As grease builds inside ductwork and across filters, it restricts airflow, so the extraction cannot move the volume it was designed to. That often shows up as a slow, creeping rise in your CO₂ baseline over weeks or months. The monitor points you toward a ventilation shortfall; a physical inspection of the ducting confirms whether buildup is the cause.
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