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What is Ventilation?

Ventilation is the intentional introduction of air from the outside into a building; it is further subdivided into natural ventilation and force ventilation. Natural ventilation is the intentional flow of air through open windows, doors, grilles, and other planned building envelope penetrations, and it is driven by natural and/or artificially produced pressure differentials. Forced ventilation is the intentional movement of air into and out of a building using fans and intake and exhaust vents; it is also called mechanical ventilation. Residential air-conditioning systems may introduce outdoor air, although it is not a code requirement in most localities. Positive ventilation should be considered, however, if the anticipated infiltration is less than about 0.5 ACH. When positive means of introducing outdoor air are used, controls, either manual or automatic, should be provided, and an energy recovery device should be considered.

More on Ventilation

Ventilation requirements for houses have traditionally been met on the assumption that the building envelope is leaky enough that infiltration will suffice. Possible difficulties with this approach include low ventilation when natural forces (temperature difference and wind) are weak, unnecessary energy consumption when such forces are strong, drafts in cold climates, lack of control of ventilation rates to meet changing needs, potential for interstitial condensation from exfiltration in cold climates, and lack of opportunity to recover the energy used to condition the ventilation air. The solution to these concerns is to have a reasonably tight building envelope and a properly designed and operated mechanical ventilation system. ASHRAE Standard 62 gives ventilation rate requirements for houses, essentially 0.35 ACH with at least 8 L/s per occupant. Canadian Standards Association (CSA) Standard F326 expands the requirements for residential mechanical ventilation systems to cover air distribution within the house, thermal comfort, minimum temperatures for equipment and ductwork, system controls, pressurization and depressurization of the dwelling, installation requirements, and verification of compliance. Verification can be by design or by test, but the total rate of outside air delivery must be measured. Mechanical ventilation is being used in houses, especially in energy-efficient housing demonstration programs. Possible systems can be characterized as local or central; exhaust, supply, or balanced; with or without heat recovery; and with continuous, occupant controlled, or demand controlled (i.e., by pollutant sensing) operation. Note that not all combinations are viable. The simplest systems use bathroom and kitchen fans to augment infiltration. Noise, installed capacity, lifetime under continuous operation, distribution to all rooms (especially bedrooms), and energy efficiency issues need to be addressed. Many present bath and kitchen fans are ineffective ventilators because of poor installation and design. However, properly specified and installed exhaust fans can form part of good whole-house ventilation systems and are so specified in some Canadian building codes. Central exhaust systems use leakage sites and, in some cases, intentional and controllable openings in the building envelope as the supply. Such systems are suitable for retrofit in existing houses. Energy can be recovered from the exhaust air stream with a heat pump to supplement domestic hot water and/or space heating. For new houses with tightly constructed envelopes, balanced ventilation systems with passive heat recovery (air-to-air heat exchangers or heat recovery ventilators) are appropriate. Fan-induced supply and exhaust air flows at nearly equal rates over a heat exchanger, where heat and sometimes moisture is transferred between the airstreams. This reduces the energy required to condition the ventilation air by typically 60 to 80 %. It also reduces the thermal comfort problem that occurs when untempered between streams, biological contamination of wet surfaces, and frosting are concerns associated with these systems.