Human activity generates a broad range of compounds that become dangerous in high-enough concentrations, and pollutants from outdoor sources can also degrade indoor air quality. Ventilation systems keep these substances at low levels by constantly renewing indoor air, and they also contribute to keeping moisture within the range of 30 to 60 percent as recommended by ASHRAE. Without ventilation, it would not be possible to keep indoor environments comfortable and healthy.
Natural ventilation relies on wind currents, outdoor temperature and other weather conditions to supply fresh air. The advantage of natural ventilation is that it comes for free, and in new buildings it is possible to optimize architectural design to maximize its effects. However, natural ventilation is uncontrollable, and generally insufficient to fully meet the requirements of modern buildings.
Normally, to meet ASHRAE standards and NYC Building Codes, mechanical ventilation must be deployed. Depending on their configuration, whole-house ventilation systems can be classified into three main types:
- Exhaust ventilation systems,which only use extractor fans.
- Supply ventilation systems,which only use injector fans.
- Balanced ventilation systems,which use both injector and extractor fans.
Balanced systems can be enhanced with energy-recovery ventilation, a technology that exchanges energy between the supply and exhaust airflows to maximize performance and minimize the overall running cost of the system.
Exhaust Ventilation Systems
As implied by their name, exhaust ventilation systems only deploy extractor fans. When the system starts to run, it creates a negative pressurization effect in occupied spaces, drawing in fresh outdoor air to renew that which is exhausted. It is important to note, however, that exhaust ventilation is not possible in air-tight buildings, since outdoor air must be allowed to leak in. If the building envelope has been tightened with caulking and weatherstripping, exhaust ventilation must be complemented with intake vents.
Exhaust ventilation systems have a single set of fans and ducts, which makes them affordable while reducing their installation time and cost. Energy expenses are relatively low because there is only one set of fans in operation, and maintenance is simplified as well. The system layout can be designed to target specific areas where pollutants are generated, ensuring they are removed before they spread indoors.
Exhaust ventilation generally achieves the best results in cold and dry climates, where outdoor air does not require dehumidification. It is not recommended for tropical and mixed climates, because warm and humid outdoor air is drawn in without control, driving up cooling and dehumidification expenses. Also, keep in mind that depressurization draws air from all surrounding spaces, with little control over pollutant content. In general, exhaust ventilation is recommended for cold weather, and when outdoor air pollution is low.
Another risk of exhaust ventilation is backdraft, which occurs when a combustion-based appliance suddenly draws in a lot of air, potentially causing a flashover. Since exhaust ventilation causes negative pressurization and does not control air supply, there is an increased chance of backdraft.
Supply Ventilation Systems
Supply ventilation only uses injector fans, pressurizing rooms and causing indoor air to leak out constantly. The main advantage of supply ventilation is control, since outdoor air can be filtered, humidified or dried as needed. In addition, the pressurization effect prevents the inflow of pollutants from surrounding spaces or from outdoors.
Another benefit of supply ventilation is that it eliminates the risk of backdraft from combustion appliances due to positive pressurization. Installation, operation and maintenance expenses are also reduced thanks to the simple system configuration.
Supply ventilation is better suited for tropical or mixed climate conditions, where dehumidification and filtering are often required. This configuration tends to cause trouble in cold weather, since the pressurization effect can cause condensation of indoor air humidity, leading to moisture accumulation and its common side-effects: furniture damage and the proliferation of mold, bacteria and dust mites.
Balanced Ventilation Systems
A balanced ventilation system is the result of combining exhaust and supply ventilation: both airflows can be controlled, providing the benefits of both system configurations. Of course, this comes at a higher installation and operation cost, since there are now two sets of fans and ducts.
Balanced ventilation is suitable for all weather conditions, and airflows can be adjusted to provide any pressurization effect as required – positive, negative or neutral. The recommended locations for each set of ducts are the following:
- Supply ducts should focus on areas where occupants spend most of their time, including living rooms and bedrooms. This ensures that these areas always have a supply of fresh and clean air.
- Exhaust ducts should focus on areas where moisture and humidity are released frequently, such as kitchens, bathrooms, laundries and boiler rooms.
Of course, it is possible to install supply and exhaust rooms for every room, but system costs are increased significantly. With the approach presented above, system costs are optimized without compromising performance.
Energy-Recovery Ventilation: Improving the Efficiency of Balanced Ventilation
Energy-recovery ventilation consists on exchanging energy between the supply and exhaust air, so that overall HVAC costs are minimized. These systems can be classified into two main types:
- Heat-Recovery Ventilation (HRV)systems only exchange heat between the supply and exhaust airflows.
- Enthalpy-Recovery Ventilation (ERV)systems exchange both heat and moisture.
In the summer, outdoor air typically requires cooling and dehumidification. However, when air is exhausted, it is still cooler and drier than the supply air; therefore, a part of the energy used for cooling and dehumidification is lost.
- The use of a heat exchanger (HRV) can improve energy efficiency: the exhaust air is used to precool the supply air without mixing both airstreams.
- If an ERV system is used, moisture is also transferred from the supply air to the exhaust air, further improving air-conditioning efficiency because there is less moisture to remove.
During the winter, HVAC needs are reversed because outdoor air typically requires heating and humidification. The operating principle of HRV and ERV is the same, but the direction in which heat and humidity are transferred is inverted.
- The exhaust air is warmer, and the heat exchanger captures a part of that thermal energy to preheat the supply air.
- If ERV is used, moisture is also retrieved from the exhaust air and provided to the supply air.
General Recommendations for HRV and ERV
It is important to note that HRV and ERV systems are significantly more complex than the ventilation systems presented before. They can only be installed and serviced by qualified personnel, which increases their cost of ownership. Compared with a basic balanced ventilation system, HRV and ERV systems have a higher running cost, but overall HVAC expenses are reduced.
HRV and ERV systems increase in effectiveness where temperature and moisture extremes are reached during the summer or winter, or when heating fuel costs are high. Their benefits are diminished under moderate weather conditions, where the added running cost may be higher than the savings achieved – balanced ventilation is a better alternative in these cases.
Spot Ventilation: A Complement for Whole-House Ventilation
Spot ventilation consists on using exhaust fans to extract pollutants and humidity at the room where they are released, preventing them from being spread throughout other indoor spaces. In residential settings, spot ventilation is most commonly used in bathrooms and kitchens to meet the minimum exhaust air levels established in both the NYC Mechanical Code and ASHRAE standards:
- Bathrooms require 50 cfm of intermittent ventilation or 20 cfm of continuous ventilation.
- Kitchens require 100 cfm or intermittent ventilation or 25 cfm of continuous ventilation.
Spot ventilation can be a great complement for supply ventilation systems, removing pollutants from key areas. This combination provides many of the benefits of a balanced ventilation system without having to install a full set of exhaust fans and ducts. The only disadvantage of this combination is that HRV and ERV systems are unfeasible, since there is no point where heat or moisture can be exchanged between airflows.