5. How to improve mechanical ventilation (including air conditioning)

Mechanical ventilation brings fresh air from outside into a building.

You should speak to the people who manage the day-to-day operations of your workplace ventilation systems to:

  • understand how they operate
  • make sure they’re supplying fresh air into an area and how much
  • make sure they’re maintained in line with manufacturers’ instructions

You shouldn’t lower mechanical ventilation rates if the number of people reduces in an area temporarily. 

You should base ventilation rates on the maximum ‘normal’ occupancy of an area.

Maximising fresh air

These systems will provide adequate ventilation if they are set to maximise fresh air and minimise recirculation. 

If your system draws in fresh air, it can continue to operate. You need to know how much fresh air it draws in and if this provides adequate ventilation. You may need to increase the rate or supplement with natural ventilation (opening doors, windows and air vents) where possible. 

You can also consider extending the operating times of mechanical ventilation systems to before and after people use work areas.

Recirculating air

It is preferable not to recirculate air from one space to another.

Recirculation units for heating and cooling that do not draw in a supply of fresh air can remain in operation provided there is a supply of outdoor air, for example windows and doors left open.

Recirculation units (including air conditioning) can mask poor ventilation as they only make an area feel more comfortable.

Find out more

More information about different ventilation systems is provided by the Chartered Institution of Building Services Engineers (CIBSE).

This page is reviewed regularly and updated to reflect any changes in the guidance.

Page last reviewed: 30 April 2021

Next review due: 31 May 2021

Updated 2021-03-31