Airless – air handling systems criteria

Design, operation and maintenance criteria for air handling systems and components for better indoor air quality and lower energy consumption


AIRLESS was an EU project coordinated by Mrs. P. M. Bluyssen from TNO Delft now completed. Its aim was to develop strategies, principles and protocols to improve and control the performance of air handling systems and components in buildings for incorporation in codes and guidelines.

The Swiss contribution was focused on the following items:

  • The development of a diagnosis procedure(DAHU) able to quantify airflow rates, to detect and quantify leakage and shortcuts in air handling units. This procedures also allows to assess the overall ventilation efficiency in the ventilated space, and the power efficiency of fans and heat recovery units.
  • To develop a testing protocol for rotating heat exchangers, allowing to quantify the transfer of contaminants from exhaust to supply air.
  • To apply these diagnosis protocols to 10 air handling units and 5 rotating heat exchangers, and to use this experience to draft new maintenance strategies.
  • To assess the effects of improved units and improved maintenance on energy use.
  • To propose innovative solutions to improve the quality of supply air.


The DAHU diagnosis procedure includes methods to measure the following quantities:

  1. Main, secondary, and leakage airflow rates in air handling units, using multi-tracer gas dilution technique.
  2. The mean age of air in the ventilated space and global ventilation efficiency in two-way air handling units.
  3. The heat recovery efficiency by heat exchangers
  4. The fan power efficiency.
  5. The transfer rate of organic volatile compounds from exhaust to supply air through rotating heat exchangers

A PC-based computer program to help in planning and interpreting such experiments was developed. This tool helps in preparing the experiments and in interpreting the raw results to obtain the quantities mentioned in items 1 to 4 above.

Twelve units were measured according to this procedure. In one such unit, the CO2 generated by occupants was used, in parallel with tracer gas, to check this simple method.

The transfer rate of various volatile organic compounds by rotating heat exchangers was measured in two units and in different cases (with and without purging sector, at various temperatures, with and without filters). These measurements show that this transfer can be significant in some cases, in particular for low-boiling point compounds or when the heat exchanger is not well installed.

The effects of improved units and improved maintenance on energy use were assessed by numerous simulations, using a comprehensive model developed on purpose. One of the interesting results is that hybrid ventilation is very effective to ensure a comfortable indoor climate in office buildings at low energy cost. Another results from experiments on real units is that heat recovery can be partly or completely hindered by lack of building envelope air tightness or by parasitic recirculation.

The results of the AIRLESS project were not just meant to be of a scientific character but were thought, and most of them also planned, to become tools for the intervention in the real world of the design, commissioning and maintenance of HVAC-systems for better IAQ.

From the point of view of source control, the various HVAC-components were characterised as potential pollution sources. It was found that filters, ducts and humidifiers are the most prominent potential pollution sources. Protocols to assess the degree of cleanliness of the different components and to define a strategy for clean HVAC-systems, as well as guidance to better design, better commissioning and better maintain air-handling units, under the IAQ perspective are published.

From the energy aspect, it was found that envelope air tightness is paramount for achieving good ventilation at low cost. Humidification and mechanical cooling are significant energy users. Mechanical ventilation, when used in conjunction with night natural ventilation for night cooling, can ensure a low flow hygienic air change, without the necessary high intensity cooling. In many cases, when applicable, naturally ventilated buildings do present similar heating demand than low flow mechanically ventilated systems and smaller electricity consumption.

photo air handling unit

One of the measured air handling units

Project leader: Claude-Alain Roulet

Collaborators : Marie-Cécile Pibiri, Laurent Deschamps

External participants in Europe: TNO Netherlands Organization for Applied Scientific Research Helsinki University of Technology Technical University of Denmark Instituto de Engenharia Mechanica, Porto Technische Universitat Berlin AICIA, Associacion de Investigacion y cooperacion Industrial de Andalucia Heinrich Nickel GmbH Halton Oy

Swiss sub-contractors: E4Tech, Flavio Foradini EMPA: Viktor Dorer, Andreas Weber, R. Gehrig, Matthias Hill, Peter Hofer SORANE: Pierre Jaboyedoff Sulzer Infra Lab: Joachim Borth, Hanspeter Krüttli

Related publications

  • Roulet, C.-A., F. Foradini, and L. Deschamps. Measurement of Air Flow Rates and Ventilation Efficiency in Air Handling Units. in EPIC’98. 1998. Lyon (France).
  • Roulet, C.-A., M.-C. Pibiri, and F. Foradini, AIRLESS Project – Results From Diagnosis of Air Handling Units. 1999, LESO-PB, EPFL: Lausanne.
  • Roulet, C.-A., et al. Measurement of Air Flow Rates and Ventilation Efficiency in Air Handling Units. in CISBAT’99. 1999. Lausanne: LESO-PB.
  • Roulet, C.-A., F. Foradini, and L. Deschamps. Measurement of Air Flow Rates and Ventilation Efficiency in Air Handling Units. in Indoor Air’99. 1999. Edinburgh.
  • Roulet, C.-A., et al., DAHU: Diagnosis of Air Handling Units, in Air Distribution in Rooms – Ventilation for Health and Sustainable Development, H.B. Awbi, Editor. 2000, Elsevier: Reading, UK. p. 861-866.
  • Roulet, C.-A. and H. Kruttli, Recommendations for improving performances of air handling units equipped with rotating heat exchangers. 2000, LESO-PB, EPFL: Lausanne. p. 9.
  • Roulet, C.-A., M.-C. Pibiri, and R. Knutti. Measurement of VOC Transfer In Rotating Heat Exchangers. in Healthy Buildings 2000. 2000. Helsinki: SIY Indoor Air Information Oy, Vantaa, Finland.
  • Roulet, C.-A., et al. Is Heat Recovery in Air Handling Units Efficient? in 11.Schweizerisches Status Seminar. 2000. Zurich: ZEN.
  • Roulet, C.-A., L. Deschamps, and M.-C. Pibiri, DAHU: Diagnosis of Air Handling Units. 2000, LESO-PB, EPFL: Lausanne.
  • Roulet, C.-A., et al., Real heat recovery with air handling units. Energy and Buildings, 2001. 33(5): p. 495-502.
  • Roulet, C.-A., et al. Rendement réel de la récupération de chaleur dans les installations de ventilation des bâtiments. in CIFQ 2001 – Vème Colloque Interuniversitaire Franco-Québecois – Thermique des Systèmes. 2001. Lyon: INSA de Lyon.
  • Roulet, C.-A., M.-C. Pibiri, and F. Foradini. Diagnostic des installations de ventilation – Méthodes et quelques résultats. in CIFQ 2001 – Vème Colloque Interuniversitaire Franco-Québecois – Thermique des Systèmes. 2001. Lyon: INSA de Lyon.