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Supply air ventilation system design: this article explains how and why to use supply-only ventilation to improve indoor air quality in homes. Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman. Supply-Only Fresh Air Ventilation System Design & Features
While not widely used, supply-only systems have distinct advantages over exhaust systems. The incoming air is easily filtered and can be directed to bedrooms and main living areas. The slight positive pressure helps guard against radon, backdrafting, and other problems associated with negative pressures. In cold climates, however, delivering nontempered air can lead to uncomfortable drafts. Also, forcing moist, interior air out through gaps in the building shell could contribute to condensation problems in building cavities and between prime and storm windows. In airtight homes in very cold climates, supply-only systems have reportedly iced up keyholes to entry doors as exfiltrating warm, moist air came in contact with the cold metal. Adding a single exhaust fan wired to operate whenever the supply fan switches on can alleviate these problems. Below we discuss two types of supply-only building ventilation approaches, followed by a discussion of VENTILATION, BALANCED designs.
Forced-Air Supply Ventilation Piggybacks on Existing HVAC SystemA multiport forced air home ventilation system piggybacks on the ductwork of a central heating or cooling system by running an intake duct from outside into the return ductwork. The screened intake has a motorized damper that is timed to open when ventilation is needed, blending fresh air into the HVAC system and slightly pressurizing the house. A special controller is needed to control the damper and fan, activating the damper whenever ventilation is needed and activating the fan whenever the air handler has not run for a set period of time, typically 20 minutes. Several manufacturers, including Lipidex Corporation, Tjernlund, and Honeywell, make controllers for this application. One manufacturer, American Aldes, offers a packaged system for warm, humid climates: the DHV-100V, which dehumidifies and filters incoming air and integrates with the home’s central air-conditioning system. This approach is relatively inexpensive since it uses existing ductwork, and it provides good distribution of fresh air. The chief drawback is that ventilation is required at regular intervals, often when the HVAC system is not operating. At those times, the controller will switch on the air handler, which is typically noisy and inefficient, making this the most expensive system to operate. Also, delivering untempered outside air through the duct system can cause discomfort in very hot or cold weather. And if the return ducts are not well sealed, they can pull contaminants from attic or basement spaces into the ventilation system. The operating costs can be cut in half by replacing the standard air-handler blower with an efficient, variable- speed fan with an integrated control motor (ICM). The fan would work on high speed for heating and cooling and continuous low speed for ventilation-only, cutting operating costs in half. However, the damper adjustment that provides the right amount of ventilation air at 500 cfm Laboratory (LBNL) as the optimal system for tract homes will produce too little when run at 100 cfm. Either the based on first cost, operating costs, air distribution, and the setting has to be changed seasonally or a compromise level potential health and safety benefits of positive pressures. found. It is also easy to filter and, if necessary, to dehumidify the incoming air (Figure 7-4 below). Multiport Supply House Ventilation System to Improve Indoor Air Quality
Although the least common type of ventilation system, the multiport supply system was identified in a recent study by Lawrence Berkeley National Laboratory (LBNL) as the optimal system for tract homes based on first cost, operating costs, air distribution, and the potential health and safety benefits of positive pressures. It is also easy to filter and, if necessary, to dehumidify the incoming air. See our figure at left for details about a multi-port fresh air supply whole house ventilation system. Though not widely used, supply-only ventilation has several advantages: incoming air can be easily filtered and distributed to the rooms where it is needed, and positive pressures help guard against backdrafting and radon entry. Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above. While few packaged systems are currently available, one manufacturer, Tamarack Technologies, offers a unit with a replaceable filter and an efficient variable-speed fan that provides 90 cfm of ventilation through one 3-inch and three 2-inch ducts. Since these systems pressurize the house, the LBNL study recommends that, in cold climates, the supply fan be balanced by a single-port exhaust fan, which could also serve as a bathroom fan. In this type of system, a central fan, typically in the attic or basement, draws in outside air through a filter and delivers it through ducts to bedrooms and main living areas. The supply ductwork should be in conditioned space or insulated and sealed airtight. Supply grilles should be placed high on the wall away from beds, chairs, or other places where drafts could cause discomfort. Since these systems pressurize the house, the LBNL study recommends that, in cold climates, the supply fan be balanced by a single-port exhaust fan, which could also serve as a bathroom fan. In this type of system, a central fan, typically in the attic or basement, draws in outside air through a filter and delivers it through ducts to bedrooms and main living areas. The supply ductwork should be in conditioned space or insulated and sealed airtight. Supply grilles should be placed high on the wall away from beds, chairs, or other places where drafts could cause discomfort. -- Adapted with permission from Best Practices Guide to Residential Construction. Frequently Asked Questions (FAQs)... Ask a Question or Search InspectApediaQuestions & answers or comments about supply-only fresh air ventilation system designs, installation, troubleshooting. Ask a Question or Enter Search Terms in the InspectApedia search box just below. Technical Reviewers & ReferencesRelated Topics, found near the top of this page suggest articles closely related to this one.
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