The New York City Mechanical Code requires dedicated exhaust systems for areas such as kitchens and bathrooms, which are characterized by a high release of pollutants and moisture. However, since mechanical ventilation is not mandatory for outdoor air supply, passive vents are a very common and cost-effective option. Trickle vents are one the best-know types of passive ventilation, and they are commonly installed on windows or other building envelope components. The most common scenario is brought on by OER (Office of Environmental Remediation). Often when trickle vents are used, natural ventilation by windows are not able to be used due to noise, therefore, alternate ventilation (trickle vents) must be used.
In 2016, the Consortium for Advanced Residential Buildings (CARB) completed a case study for three multifamily residential buildings in New York City, where the goal was to assess the effectiveness of passive ventilation strategies. All three buildings participated in the NYSERDA Multifamily Performance Program, and two of them use trickle vents for outdoor air supply, while the third relies on airlets.
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CARB concluded that the combination of exhaust fans and trickle vents struggles to provide adequate ventilation unless some key requirements are met:
- Correct installation of trickle vents is very important for them to provide the required airflow. The openings drilled in the window must add up the area suggested by the manufacturer.
- Trickle vents perform better in airtight buildings. With a leaky envelope, most air is drawn from other building spaces, not from outdoors.
- The intake through trickle vents is strongly determined by the pressure difference across them. Exhaust systems should be designed to produce a high enough pressure differential, even if that means increasing their capacity beyond the minimum requirements set forth by ASHRAE and NYC codes.
When these factors are not accounted for, the airflow through trickle vents can be reduced significantly. CARB determined that poor installation and unwanted air leaks can reduce trickle vent airflow to only 13 to 36% of the design value.
Another common ventilation approach is using only exhaust fans in dwelling units, and air injectors in corridors. The issue with this approach is that it often conflicts with fire codes: the sudden flow of air from the corridor can cause backdraft if there is a fire inside a dwelling unit.
It is also possible to supply outdoor air through dedicated injection fans and air ducts, but this increases project costs significantly. Exhaust-only systems with passive vents for fresh air supply are preferred by property developers to minimize costs and increase profits.
1) Importance of Installing Trickle Vents Correctly
The CARB team determined that, when the airflow provided by trickle vents is insufficient, poor installation is often the cause. The following are some recommendations to achieve the best results during trickle vent installation:
- Window sashes often have an internal reinforcement, in many cases made from steel. Do not install your trickle vents on a part of the window where the reinforcement may interfere with airflow. Drilling through the reinforcement can be very difficult and is not worth the effort.
- When drilling the trickle vent holes, make sure you use the correct drill bit diameter. If the holes are too big, the trickle vent will not be able to block them in the closed position; on the other hand, if they are two small, airflow is restricted.
- Make sure clean holes are drilled: If shavings stick to their borders, airflow may be restricted.
Keep in mind that trickle vent installation is a labor-intensive process, due to the sheer number of holes that must be drilled on windows. If you must install trickle vents for many windows, consider hiring a contractor who has a table drilling machine. This will speed up the process significantly, while reducing the likelihood of installation defects.
2) Airtightness of the Building Envelope
When exhaust ventilation is combined with trickle vents, air leaks into the ventilated space through all openings available, not just the vents. Therefore, it is necessary to eliminate or minimize all unwanted air leaks. An added benefit of airtightness is improved indoor air quality, since pollutants and humidity are not allowed to spread through indoor spaces, and are removed by the exhaust fans serving the area where they are released.
Large amounts of air are drawn in whenever the main door of a dwelling unit is opened, but this is not an issue because hallways have their own ventilation systems. However, air leaks from adjacent dwellings or from crawlspaces may bring significant amounts of moisture and pollutants, so they must be controlled. Poor air-tightness may also cause issues due to bulk airflow across different floors:
- Warm air tends to rise, increasing the indoor pressure in upper floors. If the effect is strong enough to overcome the negative pressure induced by exhaust fans, trickle vents are rendered useless because outdoor air cannot flow in.
- This is not an issue in lower levels, where the rise of warm air actually favors trickle vents: more outdoor air is drawn in to replace it.
In the case of high-rise buildings, the effect of bulk air movements can be so drastic that not even airtightness can compensate for it. Therefore, trickle vents are not generally recommended in high-rise construction.
3) Negative Pressurization
The main factor that determines airflow across a trickle vent is the pressure differential. If the pressure required for a specified airflow is not established, the outdoor air supply will be insufficient even if the dwelling unit is completely airtight. When airtightness cannot be improved further, there are two options to further increase airflow through trickle vents:
- If there are more windows available, the number of trickle vents can be increased.
- If this is not enough to reach the required outdoor air flow, the capacity of exhaust fans can be increased to induce a higher-pressure differential, driving more air through the existing vents.
Keep in mind that increasing fan capacity also drives up energy expenses, so it should be done with moderation. As soon as the airflow through trickle vents is sufficient, any extra fan capacity only represents wasted power that brings no benefits.
Exhaust ventilation systems for multifamily apartments are more effective when each dwelling unit is equipped with its own extractor fans; central exhaust systems tend to perform poorly and may struggle to established the required pressure difference across passive vents. Dedicated exhaust fans generally provide more CFM per watt consumed, compared with central units.
Final Recommendations for Effective Use of Trickle Vents
The recommendations provided in this article can be summarized as follows:
- Make sure that trickle vents are installed properly. The total opening area must match the value recommended by the manufacturer.
- Compartmentalize individual dwelling units with airtightness measures to improve indoor air quality and eliminate unwanted air leaks.
- If the required airflow is not achieved even with proper installation and airtightness measures, increase the number of vents if possible, or increase the exhaust CFM to draw in more outdoor air through the vents.
Of course, like with any engineering upgrade, a professional opinion is highly recommended. An engineering consultant or design firm can help you select the ventilation system configuration that suits the needs of your building better.
Editors Note: This post was originally published in April 2017 and has been revamped and updated for accuracy and comprehensiveness.
