Mechanical ventilation is addressed in depth in Chapter 4 of the NYC Mechanical Code. Depending on building conditions, mechanical ventilation may be mandatory or optional. The code also provides detailed information on the required airflow for each type of occupancy.

Before discussing mechanical ventilation requirements it is necessary to distinguish between habitable space and occupiable space:

• Habitable space refers to most indoor spaces within residential dwellings (Group R) and special needs housing facilities (Group I-1). The main exceptions are kitchenettes, bathrooms, toilet rooms, laundry rooms, corridors, passageways and private halls. Dining spaces up to 55 ft2 and located off a kitchen, foyer or living room are not considered habitable space. Foyers used as entrance halls are not considered habitable space unless they exceed the maximum area conditions specified in Section BC 1202 of the NYC Building Code.

• Occupiable space is a more general term, referring to enclosed spaces intended for human activities. This excludes areas that are only occupied for short periods, such as equipment rooms.

In occupiable spaces, the NYC Mechanical Code allows the designer to choose between natural and mechanical ventilation. Natural ventilation is always mandatory in habitable spaces; mechanical ventilation is required in some cases, but as an addition to natural ventilation, not an alternative like in occupiable spaces.

The advantage of natural ventilation is that its energy consumption is zero. However, retrofitting a building that was not originally designed for natural ventilation can be very expensive. Natural ventilation has a higher potential in new projects, where it can be integrated into the building from the design phase. In existing buildings originally designed for mechanical ventilation, the best approach is to improve the efficiency of existing systems through equipment upgrades, automatic controls and energy-recovery ventilation.

It is also important to note that ventilation upgrades tend to have a higher return on investment in office buildings than in multifamily residential buildings: mechanical ventilation accounts for 13% of energy consumption in the office sector, but only 1% in the multifamily residential sector.

Mechanical Ventilation: General Requirements

The two key elements of mechanical ventilation are supplying fresh air and exhausting indoor air. The NYC Mechanical Code requires these two airflows to be approximately balanced, but one may be specified higher than the other if required in a specific application: a higher air supply results in positive pressurization, while a higher air exhaust produces negative pressurization. There is a minimum outdoor airflow to meet, which changes by occupancy.

Air recirculation is allowed, as long as the minimum outdoor airflow is met, and subject to the following conditions:

• Air cannot be recirculated to a different dwelling or between different occupancies.
• Air supplied to a swimming pool area can only be recirculated if its is dehumidified until relative humidity is 60% or less. In addition, it cannot be recirculated to other areas.
• Some areas require dedicated mechanical exhaust. Once air is supplied to these locations, all of it must be exhausted without any recirculation.

The main areas for which recirculation is prohibited are summarized in the following table:

The air supplied to areas requiring dedicated exhaust can be recirculated from other areas in the same dwelling or occupancy, as long as they are not areas requiring a dedicated exhaust themselves.

Calculation Procedure for Outdoor Air Supply

Supply airflows are calculated based on square footage of the space served and the number of occupants. The formula changes depending on occupancy: sometimes only square footage is considered, and sometimes only the number of occupants. Table 403.3 of the NYC Mechanical Code provides detailed information to carry out this calculation procedure:

• CFM per ft2.
• CFM per occupant.
• Default occupant density.

If both values are provided, two separate airflow values are calculated and then added together. When only one value is provided, a single calculation is enough. As an example, assume a cafeteria is designed for 100 persons with a total area of 5,000 ft2. Table 403.3 of the NYC Mechanical Code requires 7.5 cfm/person and 0.18 cfm/ft2, so the total outdoor airflow required is the following:

• Outdoor airflow = (100 persons x 7.5 cfm/person) + (5,000 ft2 x 0.18 cfm/ft2)
• Outdoor airflow = 1,650 cfm

The NYC Mechanical Code allows the use of variable-air-volume systems as an energy efficiency measure, as long as they are equipped with a control system that ensures the VAV systems never reduces the outdoor airflow below the minimum specified value.

Calculation Procedure for Exhaust Airflow

In areas that require dedicated air exhaust, airflow values are provided in CFM per ft2. However, note that in some cases the value provided is per room. An example of this are kitchens in residential dwellings, where the required exhaust airflow is 25 cfm for a continuously operating system, and 100 cfm for an intermittent system.

Checking the specific requirements of each case is highly recommended, since there are many exceptions and some occupancies have extra requirements. However, Table 403.3 indicates these cases clearly.

Difference Between Mechanical Ventilation and Exhaust Systems

The NYC Mechanical Code distinguishes between ventilation systems and exhaust systems. While ventilation serves indoor spaces, exhaust systems serve specific appliances and are covered in Chapter 5 of the NYC Mechanical Code. The following are some examples:

• Clothes dryer exhaust
• Cooking appliance exhaust
• Hazardous exhaust systems
• Dust, stock and refuse conveyors
• Subslab soil exhaust
• Smoke control systems
• Energy recovery ventilation

Conclusion

Mechanical ventilation requirements vary significantly depending on the type of occupancy, but the end goal is always to ensure acceptable levels of indoor air quality. If you are considering a new construction or a major renovation for existing building systems, get in touch with a qualified design professional to determine the ventilation system configuration that can meet your needs at the minimum energy cost.