Heat, energy & performance
Extraction is only half of the airflow story. For the extract to work, replacement air has to come back in - and when the make-up air is wrong, the symptoms look like an extract fault even when the extract is fine. DW/172 treats supply and extract as one balanced system for exactly this reason.
The short answer
Extraction is only half of the airflow story. For the extract to work, replacement air has to come back in - and when the make-up air is wrong, the symptoms look like an extract fault even when the extract is fine. DW/172 treats supply and extract as one balanced system for exactly this reason.
The detail
Air removed from a kitchen has to be replaced from somewhere. DW/172 sets the dedicated make-up, or supply, air at around 85 percent of the extract flow rate, so the room stays close to neutral pressure. That balance is what lets the extract pull its design volume out cleanly.
Starve the room of make-up air and it goes negative. The extract fan now has to fight the vacuum it is creating, so it moves less air than its rating suggests - the extract underperforms not because it is greased, but because there is nothing to replace what it removes. The fault looks like the fan; it is really the supply.
Negative pressure has visible tells. Doors become hard to open or slam shut, air whistles in through gaps, and gas appliances can misbehave because combustion needs a proper air supply. In the worst case, negative pressure can pull products of combustion back rather than letting them vent, which is a safety issue.
So diagnosing a hot or under-ventilated kitchen means checking both sides. Grease raising extract resistance is one common cause; blocked, switched-off or under-sized make-up air is another. Often both are present, and treating only one leaves the kitchen still struggling.
What it means for you
If your kitchen shows negative-pressure symptoms - pulling doors, whistling gaps, gas appliances struggling - the make-up air is worth checking before you blame the extract fan. A perfectly clean extract will still under-deliver in a room that cannot replace the air it removes.
The right approach treats airflow as the two-sided system DW/172 describes. That means confirming the extract is clean and pulling its design volume, and confirming the supply is delivering its share, so the two are balanced and the kitchen sits near neutral pressure.
Phoenix Duct Clean restores the extract side to TR19 condition and can flag make-up air imbalance found during the visit - so you fix the real problem rather than chasing the wrong half of the system.
The service behind the guide
Sibling guides
Smoke and odour control · Cleaning before a heatwave · Duct and filter cleaning in one visit
Questions
Make-up air, or supply air, is the fresh air brought in to replace what the extract removes. DW/172 sets it at around 85 percent of the extract rate so the kitchen stays near neutral pressure and the extract can pull its design volume.
Look for negative-pressure symptoms: doors hard to open or slamming, air whistling in through gaps, and gas appliances misbehaving. These suggest the room cannot replace the air being extracted, so the supply side is at fault.
Yes. A perfectly clean extract will still under-deliver if the room is starved of make-up air, because it has to fight the vacuum it creates. The fault looks like the fan but is really the supply.
Combustion appliances need a proper air supply, and severe negative pressure can pull products of combustion back into the room rather than letting them vent. Balanced make-up air at around 85 percent of extract prevents this.
Yes. DW/172 treats them as one balanced system. Grease raising extract resistance and poor make-up air are both common causes of a struggling kitchen, and often both are present, so checking only one can miss the problem.
Phoenix Duct Clean · by the numbers
It may be make-up air, not just the extract. Book a TR19 clean and a full airflow check.