Recirculation in ventilation became a widely discussed topic after the COVID-19 pandemic. Until then, engineers had been quietly designing it into buildings and property owners quietly operating it. A common practice in colder months was to shut off fresh air intake entirely — a simple way to cut heating costs in a shopping centre or office block. Recirculation was sold as an energy-saving measure. After the pandemic, it started being criticised as a health risk. Both claims contain truth. Both are also oversimplified.
This article looks honestly at what recirculation actually is, when it works and when it doesn't — and why the energy-saving argument deserves closer scrutiny.
What is recirculation?
Recirculation means that a portion of the air extracted from a space is redirected back in, rather than being fully expelled and replaced with fresh air. In a typical centralised system, return air is mixed with fresh air in a set ratio — for example, 80% return air and 20% fresh air.
In Europe, recirculation is used primarily in large buildings with a central air handling unit (AHU) — shopping centres, airports, large office complexes. In hotels, recirculation is relatively uncommon in Europe, unlike in the United States, where HVAC systems are typically designed to handle ventilation, heating and cooling in a single integrated loop.
The energy-saving argument — and its limits
The main selling point of recirculation is energy efficiency: why heat or cool fresh outdoor air from scratch when you can reuse already-conditioned return air?
That argument is valid — but only under certain conditions.
It holds when the alternative is constant full-volume supply ventilation. In that comparison, recirculation is genuinely cheaper, because less energy is needed to bring incoming air to the desired temperature.
It stops holding when the alternative is demand-controlled ventilation with heat recovery. In that case:
- Heat recovery captures the thermal energy of outgoing air and transfers it to incoming air — energy loss is minimised at the limits of the technology
- Demand-controlled operation means ventilation is reduced to a minimum in unoccupied spaces — the central system runs at a fraction of full capacity
- When occupancy increases, airflow is increased precisely as needed
In the European climate, where heating demand in winter is high and occupancy variation across a building is typically significant, this combination is often more energy-efficient than recirculation — without any compromise on air quality.
CO₂ — where is the actual control?
Recirculation does not solve the CO₂ problem. It dilutes it.
When a system operates at 80% recirculation, only 20% of the air per cycle is replaced with clean air. The equilibrium CO₂ concentration in the space stabilises at a significantly higher level than it would under pure supply ventilation. Energy is saved at the expense of air quality — even when that trade-off is not made consciously.
The question is also where — and whether — CO₂ is measured at all. In practice, three models are common:
Fixed mixing ratio — CO₂ is not measured. The recirculation proportion is constant regardless of occupancy or load. The cheapest option, but air quality control is entirely absent.
Sensor in the return duct — provides an average CO₂ reading across all served spaces. This is the equivalent of measuring the average body temperature of all patients in a hospital ward and prescribing fever medication to everyone when the average crosses a threshold.
Sensor in each room — accurate data, but more complex and more expensive. At this level of precision, the system's complexity and cost begin to approach those of a supply ventilation solution. It provides knowledge of CO₂ levels, but does not guarantee sufficient air exchange in rooms with increased occupancy.
The pattern is consistent: the more precise the CO₂ control in a recirculation system, the more complex and expensive it becomes.
The European regulatory direction
The EU Energy Performance of Buildings Directive (EPBD) and WHO indoor air quality guidelines are moving toward stricter requirements. CO₂ monitoring in commercial buildings is becoming mandatory — not advisory.
In this context, the question is no longer only about energy savings. It is about which system can simultaneously deliver measurable energy efficiency and guaranteed air quality — without requiring a trade-off between the two.
What does this mean in practice?
Recirculation is not inherently the wrong choice. In large spaces with a consistent occupancy profile — where load is relatively stable and filtration is properly maintained — it can work well.
The problem arises when recirculation is sold as a universal energy-saving solution in buildings where:
- Occupancy varies significantly between spaces and across time
- Room usage profiles differ substantially
- CO₂ control is duct-based rather than room-based
- The alternative — demand-controlled ventilation with heat recovery — is available
In those conditions, the energy-saving argument deserves careful examination. Because the same or better result can often be achieved without recirculation at all.