This investigation studies a scalable control method for multi-zone heating, ventilation and air-conditioning (HVAC) systems with the objective to reduce energy cost while satisfying thermal comfort and indoor air quality (IAQ) (represented by CO2) simultaneously. This problem is challenging as we need to cope with the complex system behaviours, various couplings and the conflicting nature of objectives and constraints. To address the computational challenges, we propose a two-level distributed method (TLDM) by exploring the problem structures. Specifically, the upper level control (ULC) computes the optimal zone mass flow rates for maintaining zone thermal comfort, and the lower level control (LLC) regulates zone mass flow rates calculated from the upper level and the ventilation rate to achieve IAQ. As both the upper and lower level subproblems can be solved in a distributed manner w.r.t the zones, the proposed method is scalable and computationally efficient. The sub-optimality of the method is demonstrated through comparison with centralized method in a benchmark. Through comparison with the distributed-based scheduling strategy (DTBSS) for HVAC control [1] which hasn't been able to involve IAQ, we find that the proposed method can achieve both thermal comfort and IAQ with a slight increase of energy cost. Finally, we compare the proposed method with the commonly-used demand controlled ventilation strategies (DCVs) for IAQ management [2,3]. The numeric results imply an 8-10% energy cost saving for the proposed method.Note to Practitioners-Designing energy-efficient controllers for HVAC systems has stimulated extensive discussions. However, the status quo has mostly focused on thermal comfort requirements only. The indoor air quality (IAQ) (usually represented by CO2 level), which closely relates to human's health and working productivity, has been less aware of.This work is mainly motivated by our previous work [4] where we observed that the CO2 level may arise insufferably during the periods with high occupancy if only the temperature is cared while designing energy-efficient controllers for HVAC systems. Therefore, this paper aims to involve the requirements on IAQ into HVAC controller design. This task is usually computationally challenging as i) the necessities to cope with the complex thermal and CO2 dynamics simultaneously, which usually makes it intractable even to find a viable control both to