In some well-designed, well-characterized, well-maintained HVAC environments, the use of fixed CO2;monitors can be informative. When used, these monitors are often incorporated into demand-controlled ventilation (DCV) systems that are designed with a primary intent of maximizing energy efficiency through reductions in outdoor air delivery. However, during times of high community transmission, guidance is often to deactivate DCV systems and exceed minimum ventilation whenever possible, in addition to enhanced filtration, and other intervention-focused considerations. Opposite of exhaust-only systems, supply-only systems ventilate outdoor air into the home. The indoor air then leaves the home through exhaust fan ducts. Although this method works in tempered climates like California, it is not suitable for colder climate regions because the incoming air is not tempered. Additionally, these ventilation systems are particularly difficult to create balance within airtight homes.