improve the coherence in the following text.
In contrast, dedicated low emission gas heaters do not require a flue system. With the introduction of oxygen depletion sensors and thermostatic controls, they do not place critical reliance on ventilation as had been the case.
In contrast, dedicated low emission gas heaters do not require a flue system. Furthermore, with the introduction of oxygen depletion sensors and thermostatic controls, they do not place critical reliance on ventilation as had been the case.