Residential Demand Controlled Ventilation (DCV) Strategy Based on ASHRAE
Demand controlled ventilation has been deployed mostly in commercial and industrial buildings. Residential applications of demand control ventilation have been more limited, since many homes use natural ventilation. The multifamily residential sector has provided more opportunities to deploy demand control ventilation, especially in the common areas of buildings.
Ventilation requirements for homes have been historically simpler than in commercial buildings. A common practice has been using only extractor fans in locations like bathrooms, kitchens and closed garages. These are the most humid areas where most air pollutants are produced in homes, and the purpose is removing them at the source.
Many modern home designs use an airtight construction to improve energy efficiency and temperature control. However, this also makes natural ventilation less viable. Standards such as ASHRAE 62.2 have introduced mechanical ventilation requirements for homes. However the installations are simpler than in commercial applications, and their controls are simplified accordingly.
ASHRAE Requirements for Residential Ventilation
The ASHRAE standard above mentioned has two basic requirements for home ventilation: Whole-House Ventilation and Local Mechanical Exhaust. This standard applies for single-family homes and multi-family buildings up to three stories high. Larger residential buildings must follow the commercial ventilation requirements in ASHRAE 62.1.
Just like in commercial ventilation systems, the Whole-House Ventilation requirement is based on both area and occupants. The required flow of air is calculated as follows:
- 7.5 cfm multiplied by the number of bedrooms, plus 1. For example, a home with three bedrooms will need 30 cfm (7.5 cfm x 4).
- 0.1 cfm per square foot of floor space. For example, if the 3-bedroom home has an area of 2,500 sq.ft, it needs 250 cfm.
- Both values are added together, which leads to 280 cfm in this example.
The ASHRAE standard allows exhaust fans, supply fans, or a combination of both. In addition, the system operation can be either intermittent or continuous. Energy recovery ventilation can be deployed when the system has both an air intake and an air exhaust, boosting efficiency.
Under the Local Mechanical Exhaust requirements, this standard specifies one extractor for every kitchen and bathroom in the home. Both continuous and intermittent operation are allowed, but the airflow requirement is different for each case:
- If the extractor fans are intermittent, the requirement is 100 cfm for kitchens and 50 cfm for bathrooms.
- If the extractor fans operate continuously, the requirement is 5 air changes per hour for kitchens and 20 cfm for bathrooms.
Using DCV in Residential Applications
Home ventilation systems are much simpler than those in commercial buildings. However, the basic principles of demand control ventilation can still be applied to conserve energy and improve IAQ. The ASHRAE 62.2 standard allows both continuous and intermittent operation, and this can be used as a starting point for the ventilation strategy.
Intermittent operation according to occupants is the most efficient option, since the total energy consumed by ventilation equipment is reduced. However, intermittent ventilation also provides more time for indoor air pollutants to accumulate, especially if the home is empty for many hours. A dedicated air pollution sensor, or a collection depending on floor area, can be deployed in this case, to activate the ventilation system in response to contamination, even when the home is empty.
CO2 concentration monitoring is generally too expensive and complex for the needs of a home. For example, a bathroom extractor can be activated along with the light switch, and a kitchen range hood can be activated while cooking. CO2 sensors are unnecessary in these cases, since ventilation can respond to simpler controls.
Just like in commercial applications, demand control ventilation can improve the efficiency of air conditioning and space heating systems. When the outdoor air supply is optimized according to number of occupants and pollutant emissions, the total energy needed to condition that air is reduced. On the other hand, a continuous ventilation system increases cooling needs during summer and heating needs during winter. In the case of air conditioning, more efficiency can be gained with an airside economizer, just like in commercial buildings.
Apartment buildings higher than three stories are getting more common everyday as a natural answer to the gathering of people in big cities and the lack of available land. These residential buildings must follow the same standards as commercial buildings. This means demand control ventilation will most likely be mandatory, and air pollution should also be taken into account.
Thanks to the use of Machine Learning algorithms Foobot Smart Air System’s technology is able to automatically control HVAC systems to counteract spikes of some pollutants, humidity, temperature, or occupancy, while keeping comfort. This way building occupants will breathe healthier air, and see energy bills tank.
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