Humidification, Ventilation and Air Conditioning of theatres is a major source of energy consumption in hospitals and turning it ‘down’ (not off) is called ‘setback’. Turning on setbacks after-hours can save more than $10,000/year/ theatre in electricity spend without affecting theatre safety.
HVAC Standards: when theatre in use
| Standard | Air Changes Per Hour | Fresh Air (ACH) |
| WA Health HFG Engineering | 20 | 10 |
| AS1668 2 Ventilation of buildings | 25 | 10 |
Pressure differentials between spaces is described the below figure from AS1668 2
The below table is found in the New South Wales (Australia) Risk Assessment on HVAC variation and summarises humidity and temperature criteria.
HVAC Standards: in Setback (Standby)
Victorian (Australia) Guidelines state that
The system will be capable of operation in a set-back mode when the operating theatres are not
in use. In set-back mode the operating suite will:
• set to minimum outside air
• widen temperature dead band to 14°C to 30°C
• always maintain full humidity control within the limits set in Reference table 1 (which is 30-60% for most OR types).
4.147 In set-back mode, the following system hierarchy will apply:
1. air flow regimes and filtration (maintaining a positive pressure theatre relative to adjacent areas)
2. humidity control
3. temperature control
AusHFG guidelines state that OR temperatures can range 16-28˚C and references
AS 1668.2:2002 The Use of Ventilation and Air-conditioning in Buildings, Part 2 describes general concepts around turning off ventilation when not used (not directly referencing healthcare on this point).
Western Australian guidelines state “Space temperature and humidity control can be set-back to conserve energy. Set-back conditions shall not compromise condensation control.”
Canadian and US regulations allow for setbacks, with minimum ACH less than 6 provided humidity and temperature are controlled.
The American Society of Healthcare Engineering has detailed advice set back strategies.
Indian Authorities allow air changes to reduce to 25% (from 20 ACH) and turn off temperature control. Positive pressure must be maintained to prevent contamination of the theatre.
Taiwanese study (Lee ST et al) found that a minimum of 6 ACH was required to maintain acceptable air quality and positive pressure.
Support from Clinical Bodies
The British Orthopaedic Association, Royal College of Surgeons, American Society of Anaesthesiologists are some of the clinical organisations which have explicitly encouraged implementation of setbacks.
Scheduling
The American Society of Anaesthesiologists recommends reduction in airflow during setback but cautions that time must be allowed for theatre to return to standard rate of air changes per hour when theatre becomes in use. Scheduling the setback is critical. For purely elective theatres a good model is a timed ‘setback’ which only becomes active if there is a lack of motion detected at the time setback is planned. Return to normal ACH occurs on schedule or by manual intervention by theatre staff. Emergency theatres need careful planning and motion sensors may be critical (see below).
Complex scheduling has been used in Spain where individual patient tracking allows preparation of theatre HVAC when patient is in ‘preop’ area and then standby mode once patient leaves theatre.
Movement +20mins only: A simpler scheduling system, used in a large European hospital is where the HVAC runs at ‘active’ levels for 20minutes after any movement (thus including cleaning periods), then turns to setback. They argue there is no risk of contamination during a non-occupied period, provided positive pressure is preserved relative to adjacent spaces. The system returns to active level whenever motion is detected and they estimate the activation of active ventilation only takes seven seconds from when motion is detected. This method would work for elective and emergency theatres and is supported in the American advice
Further information
ANZCA ESN Seminar September 2025 – “HVAC – Sustainability issue hidden in plain sight”