In the post-pandemic era, Signaled Manual Airing (SMA, i.e. windows opening driven by alarms and real-time IAQ measurements) is still expected to play a role in schools, either independently or in support of mechanical ventilation (MV) systems, as a cost-effective solution to meet more stringent regulatory requirements for indoor CO2 levels. The present study in-depth evaluates the impact of SMA on improved levels of CO2 but also its thermal downside in winter via a dedicated IoT-IAQ monitoring platform equipped with fully tuneable multi-threshold alarm schemes. An aged middle-school building, representative of a large amount of old existing school buildings in Italy (and Europe) and a Northern Italy location, were selected to investigate cold and high humidity winter conditions as the most critical for SMA. The mean indoor CO2 difference with reference classrooms without alarms ranged from 20% to an impressive nearly 70% reduction. We demonstrate a clear dependence on signalling mode and level of system usage learning. The best results were in fact attained with acoustically enhanced signalling (acu-SMA). As a downside effect, operating SMA at even moderately cold latitudes caused also thermal discomfort during the coldest days, with worsened response to the alarms. The observed mean CO2 levels maintained by classroom groups appear to be correlated with the average indoor temperature and its inverse product [CO2*Tindoor]-1 to a cumulative fatigue function with memory effect, referred as “thermal behavioural fatigue“. Bounds of generalization, further developments towards hybrid ventilation solutions and most suitable Italian regions for operating SMA in schools are finally discussed.