What is the central question of this study? The goal of this study was to investigate the effect of alterations in tidal volume and alveolar volume on the elevated physiological dead space and the contribution of ventilatory constraints thereof in heart failure patients during submaximal exercise. What is the main finding and its importance? We found that physiological dead space was elevated in heart failure via reduced tidal volume and alveolar volume. Furthermore, the degree of ventilatory constraints was associated with physiological dead space and alveolar volume. ⋯ Patients who have heart failure with reduced ejection fraction (HFrEF) exhibit impaired ventilatory efficiency [i.e. greater ventilatory equivalent for carbon dioxide ( V̇E/V̇CO2 ) slope] and elevated physiological dead space (VD /VT ). However, the impact of breathing strategy on VD /VT during submaximal exercise in HFrEF is unclear. The HFrEF (n = 9) and control (CTL, n = 9) participants performed constant-load cycling exercise at similar ventilation ( V̇E ). Inspiratory capacity, operating lung volumes and arterial blood gases were measured during submaximal exercise. Arterial blood gases were used to derive VD /VT , alveolar volume, dead space volume, alveolar ventilation and dead space ventilation. During submaximal exercise, HFrEF patients had greater V̇E/V̇CO2 slope and VD /VT than CTL subjects (P = 0.01). At similar V̇E , HFrEF patients had smaller tidal volumes and alveolar volumes (HFrEF 1.11 ± 0.33 litres versus CTL 1.66 ± 0.37 litres; both P ≤ 0.01), whereas dead space volume was not different (P = 0.47). The augmented breathing frequency in HFrEF patients resulted in greater dead space ventilation compared with CTL subjects (HFrEF 15 ± 4 l min-1 versus CTL 10 ± 5 l min-1 ; P = 0.048). The HFrEF patients exhibited greater increases in expiratory reserve volume and lower inspiratory capacity (as a percentage of predicted) than CTL subjects (both P < 0.05), which were significantly related to VD /VT and alveolar volume in HFrEF patients (all P < 0.03). In HFrEF, the reduced tidal volume and alveolar volume elevate physiological dead space during submaximal exercise, which is worsened in those with the greatest ventilatory constraints. These findings highlight the negative consequences of ventilatory constraints on physiological dead space during submaximal exercise in HFrEF.