There are many ways to ventilate a house – in the past people didn’t install mechanical systems, instead relying solely on passive ventilation – doors, windows, air leakage, drafts etc. as good enough. Now houses are tighter, making them more energy efficient, but require a mechanical system to maintain good indoor air quality. Some methods include using fans in bathrooms and kitchens, or you can have a whole house ventilation system like a Heat Recovery Ventilator (HRV). An HRV has 4 different ducts coming into and out of it: incoming and outgoing indoor air (fresh air in, exhaust air out) and incoming and outgoing outdoor air (fresh cold outdoor air, stale exhaust air). All ducts come together into the core of the system where the airstreams cross diagonally and come into a thermal contact while not being mixed. This allows the heat from the outgoing airstream to be transferred into the incoming cold air, recovering heat that would otherwise be wasted. The core has filters that can help with PM 2.5 in the winter and summer during wildfire season.