Open Windows or Mechanical Ventilation?

Several construction concepts exist that intend to dramatically reduce building energy consumption. Among them is Passive House, which describes design and construction methods proven to reduce heating and cooling energy use 75% or more, compared to most new buildings. A certified Passive House will have met both energy-savings and occupant comfort goals.

Specifiers refer to the Passive House concept to design and build every imaginable type of building, including commercial offices, hospitals, hotels, multi-family, schools and single-family residences. Key features of Passive House buildings are measurable airtightness, climate-specific insulation, minimal thermal bridging, high-performance doors and windows and energy-recovery air handling units (AHU). Of these, only the AHU consumes energy, which may seem odd, considering the goal of Passive House is to save energy.

Why not just throw open the windows for comfort, like our predecessors? Wouldn’t that save the energy consumed by the AHU?

The airtightness of the Passive House technique is responsible for tremendous energy savings, but airtightness also prevents airborne pollutants and moisture from escaping on their own. Robust insulation along with minimal thermal bridging also reduces building energy use yet increases the risk for unacceptably high indoor air temperatures. These Passive House features limit the usefulness of operable windows, especially in larger buildings.

Operable windows exist that meet Passive House thermal and solar criteria. These high-performance operable windows can be strategically selected and located to meet ventilation and cooling requirements in some climates and in some buildings, like single family residences. At the other end of the spectrum, imagine a meeting room in the core of a code-compliant office space, with no exterior windows. How does the code-required amount of outside air get into that meeting room from an open window on the perimeter, especially when the meeting room is at capacity on a hot day with the door closed for privacy?

A proven method to continuously replace contaminated indoor air with fresh conditioned air throughout a building is to use AHU, which conditions outside air to adjust the temperature and remove excess moisture. AHU also pressurize the air, causing it to flow through ducts to every occupied space in the building. Passive House certified air handling units (often referred to as Heat or Energy Recovery Ventilators) perform these critical functions using energy recovery to reduce heating and cooling costs.

Passive House requires occupant comfort to be designed into buildings. In most climates, large buildings require supplemental mechanical cooling and heating to maintain occupant comfort. Passive House Institute certifies air handling units partly on the basis of their ability to capture heat leaving the building when it is cold outside. The same process of energy recovery also rejects excess heat and moisture from the outside air entering the building on a hot day. Energy recovery air handling units temper outside air so that on most days, little or no additional energy is required to keep occupants comfortable. On the extreme hot and cold days, small amounts of energy are consumed to heat or cool the Supply Air leaving the air handler to keep occupants comfortable. Alternately, neutral temperature air is delivered to occupied zones, and terminal devices provide local heating and cooling.

Not all airborne contaminants are created indoors. Allergens and fine particles like dust originate outdoors. Well-designed AHU filtration removes most of those fine particles, contributing to occupant well-being and overall building cleanliness. Filters increase life-cycle costs because they require routine replacement and because they add a pressure drop that causes the AHU to work harder and consume more electricity. Filters reduce life-cycle costs, however, by keeping air handler energy recovery devices clean, ensuring peak energy recovery effectiveness at all times, and virtually eliminating the need to clean the energy recovery element.

Opening windows may be an effective method of providing ventilation and occupant comfort in single family homes, or even along the perimeter of large non-residential spaces. But this natural ventilation alone cannot be expected to provide code-required ventilation in all types of buildings all the time. Natural ventilation may play a valuable role in the year-round energy use profile, but chances are, your large Passive House building will benefit from energy-efficient AHU providing mechanical ventilation over a large part of the year.