Ventilation Airflow Control
Regardless of the performance criteria or particular ventilation system, contractors should follow a ventilation strategy. The ventilation strategy should dilute or remove the background emissions and the occupant-related emissions, to prevent chronic and acute exposure to indoor air pollution. As a result, current standards and regulations, such as ASHRAE 62.2-2016 and others in Europe, often prescribe ventilation strategies requiring three constraints on airflow rates.
- A constant airflow based on a rough estimation of the emissions of the buildings: for instance, one that considers size of the home, the number and type of occupants, or combinations of both
- Minimum airflows (for instance during unoccupied periods and severe temperatures)
- Provisions for short-term forced airflows to dilute and remove a source pollutant generated by activities as cooking, showering, house cleaning, etc.
This portion of the strategy allows for benefits such as shifting ventilation from times when thermal loads associated with ventilation are high to those when it will be lower. This extends some strategies already discussed in the literature, such as night flushing or pre-ventilating before expected occupancy periods.
Airflow Control Variables
These sources of information may include outdoor conditions such as temperature, humidity, pollutant concentrations, wind speed, and wind direction. Ventilation systems should respond to these indoor conditions.
- Occupancy, humidity, pollutant concentrations, and control set points (for example, static pressure reset)
- Whole-house conditions such as predefined schedules and the operation of other mechanical equipment
- Global inputs such as community- or regional-scale demand for electricity or the price of electricity.
With this information, a smart ventilation controller can then make decisions based on both current and future conditions. With this organization, the controller can choose various control strategies based on financial, energy, and air quality considerations.
Ventilation Demand Control
- Global demand control, which we call “smart whole-house strategy.”— This involves detection and response to global variables such as regional energy demand or price of electricity, outdoor conditions, and home occupancy, and modulation of ventilation airflows.
- Zonal demand control, which we call “smart room strategy.”— As the name implies, this strategy responds to variables measured on a per-room or per-zone basis, such as occupancy and pollutant concentration. In this strategy, ventilation controls should still first control the ventilation airflow globally. Airflow rates into and out of each room are not independent, but depend on the mass balance of air in the home.
- Local demand control, or “smart source-removal strategy”— When possible, pollutants are removed at their source. Sources include cooking, showering, house cleaning, and toilet use. Several studies in the literature describe specific short-term emissions. These emissions include indoor particle and gas generation by the following sources.
- Cooking, toasting, smoking, candle-burning and incense-burning,
- Use of hairdryers and vacuums,
- Chemical pollutants from cleaning products,
- NO2 emissions during gas cooking.
The ventilation system should detect these pollutants and exhaust them from the home. Recent studies have, however, demonstrated that ventilation systems vary widely in their capture efficiencies and may capture only about half of the emitted pollutants.
Individual Components of Ventilation Strategy
Use the equivalent ventilation principle to ensure that a building’s exposure to pollutants is the same for the smart ventilation system as for a continuously operating system.
- Use timers or temperature sensors to provide ventilation when the impact is the smallest. Shift ventilation from times of high temperature differences to times of low temperature difference. This strategy saves energy, reduces peak-demand, and improves grid reliability.
- Account for operation of other air-moving equipment, such as kitchen and bathroom exhaust fans and clothes dryers.
- Reduce ventilation during unoccupied times.
- Ventilate more at times to compensate for other times when controls reduce ventilation.