Ventilation vs air conditioning: why cooling isn't fresh air

This is one of the most common — and most expensive — misunderstandings in commercial buildings. Air conditioning and ventilation do two different jobs. Air conditioning conditions the air already in a room (cooling it, heating it, and removing some moisture); it does not bring in fresh air from outside. Ventilation supplies that fresh air. A building usually needs both — and in workplaces, adequate fresh air is a legal requirement.

Two different jobs

• Air conditioning recirculates the air in a space and changes its temperature. It makes a room cooler or warmer, but it doesn’t replace stale air with fresh.
• Ventilation brings outdoor air in and stale air out, keeping oxygen up and carbon dioxide, odours, moisture and pollutants down.

You can have a perfectly cool room that’s also stuffy and stale — which is exactly what happens when a space is air-conditioned but under-ventilated.

What the law expects (workplaces and buildings)

Fresh air isn’t optional in occupied commercial buildings. Two sets of rules apply:

• Workplace health and safety: employers must provide adequate fresh air. HSE guidance is that the fresh-air supply rate should not normally fall below 5–8 litres per second per person.
• Building Regulations (Approved Document F): for offices and similar non-domestic spaces, the standard is to supply outdoor air at 10 litres per second per person, or 1 litre per second per square metre of floor area — whichever is greater.

Importantly, where a mechanical system (including air conditioning) recirculates air, the guidance is that it must be filtered and have fresh air added before recirculation, with fresh-air inlets kept open — another reminder that cooling and fresh-air supply are designed together, not interchangeably.

Kitchens and special cases

Some spaces need dedicated extraction on top of general ventilation. Commercial kitchens are the obvious one: cooking needs extract rates well above ordinary occupancy ventilation, plus a matching supply of make-up air to replace what’s extracted — a specialist design (see restaurants and commercial kitchens). Toilets, shower rooms and similar areas also have their own extract requirements.

What this means for your project: when you plan comfort, plan air quality alongside it. Fit air conditioning alone and you may meet your cooling need but fall short on fresh air — and potentially on your legal duties. Larger systems can combine the two: air handling units on a chilled-water system, for example, can condition and ventilate. The right answer is a system designed around both temperature and fresh air, sized from a proper commercial heat-load assessment.

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