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The house is a “system”

Modern homes have several interactive systems. Mechanical components of the system are envelope design, infiltration/exfiltration, HAC system, sources/ remediation and general occupant contamination. The envelope design, infiltration/exfiltration and HAC system must be appropriate for the climate to support effective ventilation. Assuming that they are, the house will have these things in common: people and their activities are the major source of indoor humidity; showers are the greatest contributor; breathing and perspiration, second; and cooking, third.

Showers and cooking always produce high concentrations of moisture for a short time in predictable spots, and are best controlled by intermittent “spot” ventilation.

Breathing and perspiration, in contrast, take place throughout the house and are best controlled by general continuous ventilation.

Keeping these two quite different strategies in mind makes control of excess moisture more logical.

If high moisture concentrations produced by the shower or cooking are not controlled at the source, they are dumped into the whole house. That causes excess moisture and makes it difficult if not impossible to provide good indoor air quality.

Continuous ventilation is for general health and breathing, and the installation must be done with that in mind. A quiet unit properly located and installed will be running continuously, 24/7.

Design of central ventilation requires decisions related to distribution. Each room requires fresh air, and mixing can be good if doors are open between rooms. The central heating and cooling systems ensure the air is well mixed between the rooms and a single inlet for fresh air is usually acceptable. The furnace return is a good way to introduce the fresh air. A single outlet for stale exhaust, centrally located, is being used more and more as the technology matures.

The key is to ensure that homeowners will use what has been installed. The quieter the fans are, for instance, the more often they will be used. It is important to remember that the simpler the design, the more likely it is to be effective. For spot ventilation, some type of automatic control would be beneficial.