Whole House Ventilators

How They Worked and Why They're Problematic Today

The original purpose of these fans was to provide a form of cooling by creating a large-scale air exchange. On a hot evening, a homeowner would open a few windows and turn on the attic fan. The fan would quickly pull all the hot, stale air out of the house, drawing in the cooler evening air through the open windows.

However, from a modern building science perspective, these systems are a very poor solution for ventilation and are considered a significant energy and safety liability.

1. Massive Energy Waste: These fans are extremely energy inefficient, and are only intended to be used during the cooling season in conjunction with opening windows.

2. Poor Indoor Air Quality: Because these fans vent air from the living space into the attic, they can also pull in unconditioned air from uncontrolled, unknown areas of the envelope such as crawlspaces or basements, which can be filled with dust, insulation fibers and other contaminants into the living space.

3. Safety Hazard: Like any powerful exhaust fan without a dedicated makeup air source, an attic fan can create a massive negative pressure inside the home. This can lead to a backdrafting of combustion appliances, pulling deadly carbon monoxide (CO) from furnaces or water heaters back into the home.

4. Ineffective in Humid Climates: The system relies on cool, dry air from outside. In hot, humid climates, these fans simply pull in more humid air, which can lead to condensation and potential mold growth inside the home.

5. Huge unconditioned air source: The dampers that close when the blower is off often do not shut all the way, get debris in them, and will allow unconditioned air to enter the structure.  This leads to a source of heat & humidity in the summer, as well as cold dry air for HVAC systems to offset leading to increased energy costs.

Modern Alternatives

The modern alternative to the whole-house attic ventilator is a balanced ventilation system. These systems are designed to provide controlled, energy-efficient, and healthy air exchange without creating pressure imbalances or wasting conditioned air. The most common modern solutions are:

• Heat Recovery Ventilators (HRVs): An HRV brings in a controlled amount of fresh outdoor air and exhausts an equal amount of stale indoor air. During this process, it transfers the heat from the outgoing air to the incoming air, making it highly energy efficient.

• Energy Recovery Ventilators (ERVs): An ERV does the same as an HRV but also transfers moisture, making it a good choice in both humid and very dry climates.

These systems run continuously at a low, controlled rate, providing the recommended air exchange without the risks and energy waste of a traditional whole-house fan.

In short, while the old attic ventilator was a clever cooling trick, it is a building science anachronism. It’s a tool that is fundamentally incompatible with the principles of energy efficiency, air quality, and safety that modern homes are designed around.