Backdrafting may cause comfort and IAQ problems
People spend the majority of their time in residences (Klepeis et al. 2001), making indoor air quality an increasing concern. It has been widely recognized that the health burden of indoor air is significant (Edwards et al. 2001; de Oliveira et al.2004; Weisel et al. 2005). Current ventilation standards are set to protect the health and provide comfort for residents, but the majority rely heavily on engineering judgement due to the limited existence of scientific justification. This section will describe current and potential methods for estimating required flow rates for ventilation and provide an overview of important existing standards.
HUMAN EFFLUENTS AND CARBON DIOXIDE
Pettenkofer Zahl bases for ventilation standards
Sweating seems to be the main body odour source determining perceived indoor air quality (Gids and Wouters, 2008). Odours create discomfort, as good air quality is often perceived as the absence of odour. In many cases occupants become used to odours that can be well perceived by someone entering the room. The judgement of a visiting test panel (Fanger et al. 1988) can be used to assess the odor intensity.
Carbon dioxide (CO2) is not a major health driver for indoor air exposure in residences. CO2 is a marker for the bioeffluents of people and can be related to the nuisance of odour. CO2 has been the basis for almost all ventilation requirements in buildings since the work of Pettenkofer (1858). He recognized that while CO2 was harmless at normal indoor levels and not detectable by persons, it was a measurable pollutant that ventilation standards could be designed around. From this study, he proposed the so-called “PettekoferZahl” of 1000 ppm as a maximum CO2 level to prevent odours from human effluents. He assumed an outside concentration of about 500 ppm. He advised to limit the difference in CO2 between inside and outside to 500 ppm. This is equivalent to a flow rate for an adult of about 10 dm3/s per person. This amount is still the basis of ventilation requirements in many countries. Later Yaglou (1937), Bouwman (1983), Cain (1983) and Fanger (1988) conducted further research on a “odour nuisance driven” ventilation approach based on CO2 as a marker.
Generally used CO2 limits in spaces (Gids 2011)
Table: Generally used CO2 limits in spaces (Gids 2011)
A recent study indicates that CO2 itself might influence the cognitive performances of people (Satish et al. 2012). In case the performance of people is the most important parameter in rooms such as classrooms, lecture-rooms and even in some cases offces, CO2 levels should determine the ventilation level rather than nuisance and/or comfort. In order to develop standards based on CO2 for cognitive performance, an acceptable level of exposure would have to be established. Based on this study, maintaining a level of around 1000 ppm appears to have no impairment on performance (Satish et al. 2012)
BASIS FOR FUTURE VENTILATION STANDARDS
VENTILATION FOR HEALTH
Pollutants are emitted in or enter into the space where the occupants then inhale them. Ventilation provides one option for removing pollutants to reduce exposure either by removing the pollutants at source, such as with cooker hoods, or by diluting air in the home via whole house ventilation. Ventilation is not the only control option for reducing exposures and may not be the right tool in many situations.
In order to design a ventilation or pollutant control strategy based on health, there must be a clear understanding of the pollutants to control, indoor sources and source strengths of those pollutants, and acceptable levels of exposure in the home. A European Collaborative Action developed a method for determining the ventilation requirement to achieve good indoor air quality as a function of these pollutants (Bienfait et al. 1992).
Most important pollutants indoors
Pollutants that appear to drive the chronic health risks associated with exposure to indoor air are:
• Fine particles (PM2.5)
• Second-hand tobacco smoke (SHS)
• Radon
• Ozone
• Formaldehyde
• Acrolein
• Mould/moisture related pollutants
Currently there is insufficient data about source strengths and specific source contributions to exposure in homes to design a ventilation standard based on health. There is significant variability in source characteristics from home to home and the appropriate ventilation rate for a home may need to take indoor sources and occupant behaviour into account. This is an ongoing area of research. Future ventilation standards may rely on health outcomes to establish sufficient ventilation rates.
VENTILATION FOR COMFORT
As described above, odours can play an important role in comfort and well-being. Another aspect of comfort is thermal comfort. Ventilation can influence thermal comfort by transporting cooled,
heated, humidified or dried air. The turbulence and air speed caused by ventilation can influence the perceived thermal comfort. High infiltration or air change rates can create discomfort (Liddament 1996).
Calculating required ventilation rates for comfort and health requires different approaches. Ventilation for comfort is mostly based on odour reduction and temperature/humidity control, while for health the strategy is based on reduction of exposures. A proposal of the concerted action guidelines (CEC 1992) is to separately calculate the ventilation rate needed for comfort and health. The highest ventilation rate should be use for the design.
EXISTING VENTILATION STANDARDS
UNITED STATES VENTILATION STANDARDS: ASHRAE 62.2
The American Society of Heating, Refrigerating and Air Conditioning Engineer’s (ASHRAE’s) Standard 62.2 is the most widely accepted residential ventilation standard in the United States. ASHRAE developed Standard 62.2 “Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings” to address indoor air quality (IAQ) issues (ASHRAE 2010). ASHRAE 62.2 is now required in some building codes, such as California’s Title 24, and is treated as a standard of practice in many energy efficiency programs and by organisations that train and certify home performance contractors. The standard specifies an overall, residence-level outdoor air ventilation rate as a function of floor area (a surrogate for material emissions) and the number of bedrooms (a surrogate for occupantrelated emissions) and requires bathroom and cooking exhaust fans. The focus of the standard generally is considered to be the overall ventilation rate. This emphasis has been based on the idea that risks indoors are driven by continuously emitted, distributed sources such as formaldehyde from furnishings and bioeffluents (including odours) from humans. The required level of whole residence mechanical ventilation was based on the best judgement of experts in the field, but was not based on any analysis of chemical pollutant concentrations or other health-specific concerns.
EUROPEAN VENTILATION STANDARDS
There are a variety of ventilation standards in various European countries. Dimitroulopoulou (2012) provides an overview of existing standards in table format for 14 countries (Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Italy, Netherlands, Norway, Portugal, Sweden, Switzerland, United Kingdom) along with a description of modelling and measurement studies done in each country. All countries specified flow rates for whole house or specific rooms of the home. Airflow was specified in at least one standard for the following rooms: living room, bedroom, kitchen, bathroom, toilet Most standards only specified airflow for a subset of rooms.
The basis for ventilation requirements varies from country to country with requirements based on number of people, floor area, number of rooms, room type, unit type or some combination of these inputs. Brelih and Olli (2011) aggregated ventilation standards for 16 countries in Europe (Bulgaria, Czech Republic, Germany, Finland, France, Greece, Hungary, Italy, Lithuania, Netherlands, Norway, Poland, Portugal, Romania, Slovenia, United Kingdom). They used a set of standard homes to compare resulting air exchange rates (AERs) calculated from these standards. They compared required airflow rates for the whole house and task ventilation. Required whole house ventilation rates ranged from 0.23-1.21 ACH with highest values in the Netherlands and lowest in Bulgaria.
Minimum range hood exhaust rates ranged from 5.6-41.7 dm3/s.
Minimum exhaust rates from toilets ranged from 4.2-15 dm3/s.
Minimum exhaust rates from bathrooms ranged from 4.2-21.7 dm3/s.
There seems to be a standard consensus between most standards that a whole house ventilation rate is required with additional higher levels of ventilation for rooms where pollutant emitting activities may occur, such as kitchens and bathrooms, or where people spend the majority of their time, such as living rooms and bedrooms.
STANDARDS IN PRACTICE
New home construction is ostensibly built to meet requirements specified in the country in which the home is built. Ventilation devices are selected that meet required flow rates. Flow rates can be affected by more than just the device selected. Backpressure from the vent attached to a given fan, improper installation and clogged filters can result in drops in fan performance. Currently there is no commissioning requirement in either the US or European standards. Commissioning is mandatory in Sweden since 1991. Commissioning is the process of measuring actual building performance to determine if they meet requirements (Stratton and Wray 2013). Commissioning requires additional resources and may be considered cost prohibitive. Due to the lack of commissioning, actual flows may not meet prescribed or designed values. Stratton et al (2012) measured flow rates in 15 California, US homes and found that only 1 met the ASHRAE 62.2 Standard completely. Measurements across Europe have also indicated that many homes fail to meet prescribed standards (Dimitroulopoulou 2012). Commissioning should potentially be added to existing standards to assure compliance in homes.
Original Article
