A method of monitoring an air flow in a zone (1) of an HVAC system (10) is described, the zone (1) comprising a supply port (11) and a return port (12), a first flow sensor (111) configured to measure a supply flow (ϕ1) through the supply port (11), and a second flow sensor (121) configured to measure a return flow (ϕ2) through the return port (12), the method comprising: recording by a control system (20, 20′) the supply flow (ϕ1) measured by the first flow sensor (111) and the return flow (ϕ2) measured by the second flow sensor (121); determining by the control system (20, 20′) an infiltration and exfiltration component (ϕinf/exf), using the supply flow (ϕ1) and the return flow (ϕ2) recorded by the control system (20, 20′); and determining by the control system (20, 20′) an operational sensor state of at least one of the first flow sensor (111) and the second flow sensor (121), using the supply flow (ϕ1), the return flow (ϕ2), and the infiltration and exfiltration component (ϕinf/exf).