The effects of faults on an air handling unit (AHU) are often poorly evaluated (assuming indoor air quality (IAQ) problems), and under-estimated in consumption balances. Based on an experimental site, this paper presents a quantitative study of some different faults that can appear on AHUs. Through a systemic analysis and a suitable modelling, the effects of several faults are simulated, using graphical software. Three families of faults are studied: control of a three-way valve, mixing box dampers (flow problems), and sensor inversion. Most of them find expression in an increase of the system consumption. The IAQ, seen here as the CO2 rate, is studied thanks to the developed piece of software; the IAQ impact of faults is not accessible without such tool. The software is then applied to a combination of several faults, in order to bring out major tendencies.