Mike Woolsey
2021-01-22 09:27

Passive House is a building design concept that is all about staying within a strict energy use budget, while providing occupant comfort. These goals are routinely accomplished by designing a well-insulated and airtight building. These primary design requirements leave little alternative but for Passive House certifying bodies like PHI and PHIUS to also require a certain type of air handling system – the ERV (energy recovery ventilator). Air handlers are the drivers of ventilation systems, and they are notoriously among the biggest energy consumers in a building, therefore, they deserve special attention in all energy efficient building design.

Passive House certification includes requirements for energy savings, comfort and project integration. Rather than simply reporting the measured test results, many of the criteria are pass-fail. Either the ERV falls within the criteria specified, or it simply does not get certified.

These criteria give design teams both opportunities and challenges. The opportunity to succeed in certifying the project to meet Passive House standards increases dramatically when the ERV specified is certified to meet Passive House requirements.

Passive House professionals generally agree on 5 key characteristics which define a Passive House building, and result in exceptionally low building energy use:

  1. Airtight construction
  2. Climate-specific insulation
  3. Minimized thermal bridging
  4. High-performance openings
  5. Continuous ventilation
Figure 1 – Key characteristics of a Passive House

The unusually airtight construction required by the Passive House concept has tremendous impact on the building’s HVAC, lowering the amount of energy used by the building for heating, cooling and ventilation. Here, we explore the reduction on energy use attributed to airtight construction.

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Passive House Ventilation – Introduction

Swegon offers an AIA Nano Learning Course introducing the basic ventilation concepts that are important in Passive House design.

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The Passive House concept has been around in its current form since 1990, when the Passive House Institute (PHI) standardized the key characteristics that could be used repeatably. Increasing evidence exists that as-built Passive projects do indeed save energy by reducing the amount of energy expended heating, cooling and ventilating. (Reference: Johnston, D., Siddall, M., Ottinger, O. et al. Are the energy savings of the passive house standard reliable? A review of the as-built thermal and space heating performance of passive house dwellings from 1990 to 2018. Energy Efficiency (2020). https://doi.org/10.1007/s12053-020-09855-7)

A key characteristic of Passive House construction is very low leakage of air between the outdoors and the indoors. During the cold season, heated air is not lost to the outdoors through exfiltration, nor does cold air leak in through infiltration requiring extra heating. The opposite is true during the cooling season. Low leakage construction is thus a main contributor to reduced energy consumption in Passive House buildings.

Traditionally, a substantial amount of the ventilation required in a multi-family building is provided by infiltration-ASHRAE 62.2 provides for an infiltration credit. When infiltration is dramatically reduced due to airtight construction, the amount of ventilation provided through cracks and leaks is also reduced. Continuous ventilation is therefore required in airtight buildings to provide fresh air to occupants, and to dilute airborne contaminants and excess moisture.

Figure 2 Hanac Corona – passive house project in Queens NY. photocredit https://thenyhc.org/projects/hanac-corona-senior-residence-3/

Continuous ventilation requires ventilation air handling units to operate whenever the building is occupied, and sometimes even when the building is not occupied. These air handlers thus must be selected and specified based on their ability to move air using the least amount of electricity. The common measure of this energy-efficient forced movement of air is Specific Power, expressed in terms of watts consumed by the air handler to move a specific volume of air.

react damper project
Figure 3 – 1020 Legacy Way passive house project in Whistler BC

Because so little cold air leaks into a Passive House building, the heating load in an airtight, well-insulated structure is often dominated by the temperature of cold outdoor air, which must be warmed to provide occupant comfort. The fact that this the cold air intentionally enters the airtight building through ductwork makes the work of heating the air much more controllable. Because the intent of a Passive House building is to operate using very little energy, great importance is placed on selecting and specifying air handling units with energy recovery – using special heat exchangers to capture heat leaving the building through the exhaust air stream. By capturing heat in this way, the amount of heat added using new energy input is minimized.

The common measure this energy-efficient heat recovery is Heat Recovery Effectiveness, expressed as a percentage. If a heat exchanger could perfectly capture all of the heat leaving the building and return that heat as useful heat back into the building, the heat recovery effectiveness would be 100%. Commercially available heat exchangers typically have a heat recovery effectiveness on the order of 85%.

Air handling units with heat recovery play an important role in passive house projects.  Selecting the proper air handler will contribute to the overall building energy efficiency, while providing occupants fresh air for their health and temperature control for their comfort.

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airtightness, continuous ventilation, ERV, heat recovery effectiveness, insulation, Passive House, PHI, PHIUS, specific power, and thermal bridging