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Buildings designed to meet Passive House criteria are known primarily for very low energy consumption. Occupant comfort, however, has always been one of the stated goals of the Passive House. According to Passipedia, a Passive House Institute (PHI) resource, Passive House is “a building standard that is truly energy efficient, comfortable, affordable and ecological at the same time.” PHIUS, states “Passive building comprises a set of design principles used to attain a quantifiable and rigorous level of energy efficiency within a specific quantifiable comfort level.”
Modern construction techniques allow buildings to be built with a high degree of airtightness. With low levels of airtightness, air can leak into (“infiltrate”) or out of (“exfiltrate”) in an uncontrolled manner that consumes unnecessary energy and causes occupant discomfort.
Passive House buildings feature airtightness that is designed into the building and tested during construction to save energy otherwise wasted through infiltration and exfiltration. In turn, this airtightness requires continuous ventilation provided by heat recovery or energy recovery air handlers, collectively described as “ERV” in this post.
An interesting benefit of making a building airtight is that the ventilation system is able to provide greater control over occupant comfort – most of the outside air entering the airtight Passive House building enters through the ERV, which tempers the ventilation air before blowing it downstream through ducts for occupants to breathe.
Compare this to buildings with inferior airtightness. Cold winter air, for example, blows into the building through cracks or the leaky facade, and the occupant experiences cold, drafty conditions. Warm air provided by the HVAC system in a leaky building is often unable to counteract this drafty condition. Extra warm air may be blown in as a countermeasure, but this wastes energy. Even with more warm air blowing out of the ducts, occupants may still feel a cold draft. And under certain conditions, the extra warm air may exfiltrate outdoors. High-performance ERV are also a key characteristic of Passive House buildings, yet their efficiency alone does not completely address comfort. In colder climates, even the most efficient ERV cannot recycle enough heat to provide comfort on the coldest days. Most large-scale Passive House specifiers in North America have not taken full advantage of available ventilation system control options to control comfort, settling for lowest first cost control instead. With a small first-cost investment in ventilation controls, Passive House buildings can enjoy a much-improved life-cycle cost benefit.
In particular, time-tested demand control ventilation (DCV) methods temporarily increase the rate of ventilation, providing enhanced comfort on-demand by delivering more warmed ventilation air without adding heat mechanically. Passive House Institute describes DCV in General Minimum criteria for all Standards in Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standard. Specifically, the criteria for the ventilation system controllability is stated as “The ventilation volume flow rate must be adjustable for the actual demand. In residential buildings the volume flow rate must be user-adjustable for each accommodation unit (three settings are recommended: standard volume flow / standard volume flow +30 % / standard volume flow -30 %)”.
Ventilation systems for Passive House can meet this criteria by offering 3 modes of operation.
Occupied Mode
Whenever the space is occupied, the system delivers the required ventilation flowrate. This is equivalent to Standard Volume flow recommended in Criteria, and provides the amount of air required by local code.
Boost Mode
Increased number of occupants, and occupant activities like cooking may reduce perceived comfort due to diminished indoor environmental quality. Temperature, CO2 and VOC are all measures of indoor air quality, and an increase in either can make occupants feel uncomfortable. Boost mode allows occupants to temporarily increase the amount of ventilation air provided to improve the indoor environmental quality. Criteria recommends designing the ventilation system so that in Boost mode, occupants receive 30% more ventilation air than the Standard Volume (Occupied Mode). The ventilation supply air remains the same temperature, but the amount of air increases, resulting in greater heating or cooling capacity. The extra air volume may also be designed to increase the room air velocity, which also impacts occupant comfort.
Setback or Unoccupied Mode
The volume of ventilation air delivered to unoccupied zones may often be reduced to save energy. By reducing the zone airflow, the ERV can be ramped down to reduce energy used to move air. An unoccupied zone does not need to provide occupant comfort, but maintaining a low volume of airflow prevents the build up of odors.
Demand control ventilation strategies provide the combined benefit of extra comfort and extra energy savings. Extra benefits of DCV and the incorporation of DCV into the Passive House design will be the topic of future posts.