All habitable buildings require ventilation, and when this system is implemented incorrectly or works poorly, the resulting poor air quality can have negative consequences such as unpleasant odors and health problems.
Ventilation can be classified into two subcategories, natural ventilation and mechanical ventilation. As the name suggests, natural ventilation is that which occurs spontaneously thanks to air temperature gradients and convection; while mechanical ventilation is ventilation that depends on energy-consuming equipment, such as fans or air handling units.
Achieve a ventilation design that meets codes while improving indoor air quality.
For residential areas, the New York City Mechanical Code requires that the mechanical ventilation system provide airflow equivalent to the greater of the following:
- 0.35 air changes per hour, calculated based on the total floor area
- 15 cfm (cubic feet per minute) per person
Any space not heated or maintained below 50°F and not subject to continuous occupancy is exempt from these requirements. There are also special requirements for kitchens and bathrooms:
- Kitchens: 100 cfm (intermittent) or 25 cfm (continuous)
- Bathrooms: 50 cfm (intermittent) or 20 cfm (continuous)
Natural ventilation through operable windows
The main advantage of using operable windows for natural ventilation is that the energy cost is zero, since there are no injectors and extractors consuming energy. The disadvantage of natural ventilation is that it is insufficient on its own in many applications, where the New York Mechanical Code points this out and makes mechanical ventilation mandatory.
Regardless of whether mechanical ventilation is mandatory or optional, Chapter 12 of the New York Building Code requires natural ventilation for all habitable spaces, and the permitted means of ventilation are windows and glazed doors. In the specific case of occupancies classified in Residential Group R-3, skylights and translucent panels are also valid as means of natural ventilation. The following requirements are established:
- The total area of windows or similar ventilation means must be at least 5% of the total area of the living space
- Each individual window or opening shall have a glazed area of at least 12 square feet, of which the openable area shall be at least 6 square feet. In bathrooms and powder rooms, the minimum glazing area per window is 3 square feet, with an opening area of 1.5 square feet
- Adjacent rooms connected via a door are considered separate spaces for ventilation purposes: requirements must be determined individually for each
- If there is a mechanical ventilation system that provides at least 40 cfm of fresh air, the required window area will be reduced to 2.5% of the total space. The required opening area per window is also reduced to 5.5 square feet
It is important to highlight that, whenever there is a mezzanine or a room divided into two levels, the total floor area must be taken into consideration when designing ventilation, and not just the area of the lower level.
Compartmentalized mechanical ventilation via PTAC or ERV units
When mechanical ventilation systems are required by codes, there are two main approaches: using centralized systems or compartmentalizing them by housing unit. The second approach is viable with both integrated terminal air conditioners (PTAC) and energy recovery ventilation (ERV) units.
The main advantage of this approach is isolation between housing units, which increases overall air quality and allows individual and independent control of ventilation systems.
PTAC Units
Packaged terminal air conditioners operate on the same principle as window-type air conditioners: they are installed directly between the ventilated space and the outside, extracting the intake air and heating or cooling it according to current needs of HVAC. The advantage of PTACs is that they consolidate heating, cooling, and ventilation into a single unit and are duct-free.
ERV Units
Energy recovery ventilation is a very energy-efficient alternative, which is based on using exhaust air to pre-condition the intake air and reduce the total cooling or heating load. Its basic principle is as follows:
- In summer, the exhaust air is used to pre-cool and dehumidify the intake air.
- On the other hand, in winter the exhaust air preheats and humidifies the intake air.
It is important to note that ERV units can only serve full HVAC load under specific temperature and humidity conditions. Otherwise, they must work in conjunction with an air conditioner or heater
Injection of external air into the corridor and extraction through the toilet exhaust
The New York Mechanical Code does not allow exhaust air from bathrooms to be used to ventilate other areas, but suctioning air from other areas of the building is valid. In the case of apartment buildings, a common injection system can supply air to corridors, which then enters individual dwellings before being finally exhausted by toilet fans. With this ventilation system, there are some important considerations:
- Bathroom extractors support the entire ventilation load of each apartment and must be sized accordingly. The New York Mechanical Code allows for a combined kitchen and bathroom exhaust system
- The injectors providing the outside air supply to each hallway must provide an airflow that balances the combined exhaust from all individual apartments
When installing air exhaust, there are several free spaces that must be taken into account:
- Two meters from windows belonging to the same apartment
- More than one meter from windows of adjacent buildings or belonging to another type of occupancy in the same building
- Three meters away from any air intake opening and three meters above the public sidewalk
A final consideration is that corrosion-resistant grilles, screens, or shutters are required for all intake and exhaust openings. The opening size should be between 1/4 and 1/2 inch
Simplicity is the main advantage of this ventilation system. When all the doors in a hallway are closed, positive pressure tends to increase in the hallway, while negative pressure increases inside the residential units. However, the Mechanical Code does not consider this a problem and the pressure tends to even out throughout the day as apartment doors are opened and closed.
The main limitation of this approach is that the compartmentalization is poor, which has some effects that can be detrimental to the overall performance of the ventilation system. The impact of the stack effect, in particular, can be significant:
- When the outside temperature is lower than that of the building, air tends to leak into the building at lower levels and out of it at higher levels.
- The opposite effect occurs when the outside temperature is warmer: air tends to leak into the upper floors and out of the lower floors.
The overall effect on ventilation equipment is increased energy consumption and reduced performance:
- On floors where outside air tends to enter, the extractors have to work harder
- On the other hand, levels where air leakage is directed outside tend to have air quality problems – air can be drawn in from other levels, carrying odors or pollutants.
Individual mechanical ventilation risers for each apartment
The construction of an individual riser for each apartment offers excellent ventilation control, as the exhaust from each unit is completely isolated from the others. Problems with stack effect are also avoided – since each housing unit has a dedicated ventilation system, air tightness between different areas can be maximized without compromising overall air quality. Therefore, this ventilation approach improves the overall energy efficiency of a building because isolated systems do not have to work against mass air movements.
The efficiency of this ventilation layout is further improved by the fact that each system can be controlled individually, something that is not possible when a single injection system provides air intake for an entire floor. Humidity control is also improved when ventilation is compartmentalized by housing unit.
Of course, installing a dedicated ventilation riser for each apartment involves a higher initial cost, as each unit has individual injection and extraction systems, and may only be a viable option in new construction due to space requirements.
General Energy Efficiency Recommendations
The performance of the ventilation system can be greatly improved with the use of variable frequency drives. Basically, a VFD adjusts the fan speed according to the ventilation load, rather than turning it on and off. Speed control is superior to duty cycle reduction because its savings are quadratic, while duty cycle reduction provides only linear savings.
- A fan operating at a duty cycle of 75% consumes about 75% of the energy compared to another with the same characteristics operating all the time.
- On the other hand, a fan running at 75% speed consumes only about 42% of the energy consumed by a fan running at full speed.
- In this scenario, reducing the duty cycle to 75% saves 25%, but operating the fan at 75% speed saves 58% of the energy. In other words, speed control generates 33% extra savings.
Along with space heating and cooling, ventilation plays a key role in maintaining ideal indoor conditions for human health and comfort. There are several potential system layouts, each with advantages and limitations, and ultimately the ideal choice is determined by project conditions and available budget. Compartmentalized ventilation systems tend to have higher energy efficiency at a higher cost; while centralized systems tend to suffer from the stack effect in taller buildings but are cost effective in lower height buildings where usage habits are similar across separate residential units.
Editor's Note: This post was originally published in November 2016 and has been reworked and updated for accuracy and comprehensiveness.
