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Multiport supply ventilation (C) J Wiley, Steven Bliss Supply-Only Fresh Air Ventilation System Design

Supply air ventilation system design:

this article explains how and why to use supply-only ventilation to improve indoor air quality in homes.

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Supply-Only Fresh Air Ventilation System Design & Features

Open windows, the Alhambra, Spain (C) Daniel Friedman At the Alhambra in Spain (our photo at left) windows are always open and fresh air flows through the buildings continuously. But in modern enclosed buildings, a different strategy is needed to provide adequate fresh air and good indoor air quality.

As reported in Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) :

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.

[Click to enlarge any image]

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.

  1. Forced-air supply piggybacking off of an existing HVAC system
  2. Multiport supply ventilation systems

Forced-Air Supply Ventilation Piggybacks on Existing HVAC System

Stuck fresh air damper control arm (C) InspectApedia MLA 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

Multiport supply ventilation (C) J Wiley, Steven Bliss

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 (Steve Bliss, J Wiley & Sons) .

Fresh Air Ventilation Supply-Only Systems: Ducts & Controllers

Honeywell Y8150A Fresh Ar Ventilation Sytem & Controller cited & discussed at InspectApedia.com

Illustration: Honeywell's Fresh Air Ventilation System, cited below. [Click to enlarge any image]

Soler & Palau ventilatino system controller cited & dicussed at InspectApedia.com

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Lennox Y5862 - V109322 Fresh Air Ventilation Fan controller cited & discussed at InspectApedia.com

Question: How do I turn off our fresh air ventilation system

(Feb 5, 2013) Michelle said:

How do we turn it off? It blows ice cold air in winter and hot air in summer, which is separate from AC and heat.

Reply:

Michelle, you will find an electrical switch for the supply vent fan, typically located on the equipment or next to it as well as in the main electrical panel. But it sounds as if your supply ventilation system is either not properly designed or not installed correctly.

As we illustrate with this Lennox active ventilation controller, on just about all fresh air ventilation systems you will find an independent controller with buttons or a dial that allow you to specify the amount of "on-time" or to turn the ventiation system off completely.

This ventilation controller and others like it are discussed and their installation and operation manuals are provided

at VENTILATION, BALANCED.

You will read that where ventilation systems are installed in very cold climates the control should provide for keeping the venting system off under certain conditions such as very low outdoor temperatures or when indoor relative humidity is above 55% RH.

Watch out: do not turn off fresh air supply provided to building heating equipment such as boilers or water heaters or furnaces.

Doing so may result in inadequate combustion air, back drafting, and fatal carbon monoxide hazards - ie. you could kill building occupants.


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