Procedures For Residential Mechanical Ventilation

BACKGROUND

Better construction practices result in tighter homes with significantly reduced air infiltration. Unlike commercial buildings, residential fresh air ventilation is rarely automated and is primarily the responsibility of the occupants. With new construction being relatively air tight, natural ventilation through windows, along with intermittently operated kitchen and bathroom exhaust fans, are important sources of outside air. It is important that the occupants be educated about the use of these systems to achieve adequate ventilation.

Concern about occupants not adequately opening windows and operating bathroom and kitchen exhaust fans has led some builders to consider mechanical ventilation to alleviate health and safety concerns. The benefits of ventilation, such as avoidance of problems that can occur due to inadequate ventilation of the house, should be weighed against the costs, including the first costs of the equipment and the energy costs to the homeowner to run the fan.

To reduce litigation exposure from indoor air pollution and humidity/condensation problems, some builders have opted to install mechanical ventilation. This mechanical ventilation can be provided simply through installation of a small, quiet fan ducted to the outside that will continuously ventilate the whole house.

These procedures are intended to be a guide for those builders who have decided to provide mechanical ventilation. The underlying principle behind this system is that the introduction of outside air into the conditioned space be intentional and controllable rather than incidental and variable.

CALIFORNIA ENERGY CODE CREDIT FOR VENTILATION

The California Energy Efficiency Standards for Residential Buildings (Title 24, Part 6) will provide a credit for tight construction beginning in July 1999. If credit is taken for construction tighter than 3 specific leakage area (SLA), mechanical ventilation is required. If the air tightness of the home is measured and is found to be below 1.5 SLA, mechanical supply ventilation is required.

These procedures will result in a ventilation system that meets the mechanical ventilation requirements of Title 24. If the builder intends to comply with Title 24 through the performance standards approach, then the total power consumption of any mechanical ventilation must be included as part of the total energy analysis for the home. Builders who wish to understand how to obtain Title 24 credit for tight construction and the requirements for mechanical ventilation credit should refer to the Residential Manual published by the California Energy Commission.

SIZING

Central mechanical ventilation shall be sized to provide 0.047 cfm/square foot of house conditioned area (size the fan upwards), or according to Table 1, for typical size fans:

Square footage of house	Total ventilation (CFM¹)	Min Duct diameter²
up to 1300	60	4"
1300 to 1900	90	5"
1900 to 2600	120	6"
2600 to 3200	150	6"
3200 to 3800	180	7"
3800 to 4500	210	7"
4500 to 5100	240	8"

CFM = cubic feet per minute -- to determine CFM:
Alternative 1: Use the fan rating from manufacturer's specifications with duct sized according to manufacturer's specifications. Fan airflow (in CFM) shall be calculated to account for pressure loss due to ducting and fittings. System capacity shall be based on actual installation rather than a simple rating based on air flow at atmospheric or other simplified conditions. Refer to manufacturer's data for maximum duct lengths.
Alternative 2: Use fan rating value at 0.25" water gauge.
Minimum main trunk duct diameter for maximum air velocity of 700 feet per minute. Branch ducts should be sized based on the CFM serving each branch.

INSTALLATION

Supply-Fan Approach

The preferred method for continuous mechanical ventilation is for the fan to be installed to push air into the house -- called the "supply-fan approach." This approach is preferred because it directly controls the source of the ventilation air, and because it will not depressurize the house, which could cause back-drafting of fireplaces and/or vented combustion appliances. In areas of high moisture generation (such as kitchens, showers) exhaust fans should be used in addition to the continuous supply-fan ventilation to remove the moisture generated in these areas from the house. In cold climates (such as Climate Zone 16 in California) care should be taken with the supply-fan approach to ensure that moisture does not accumulate in the envelope. If large, unvented sources of moisture are present (such as large fish tanks, waterfalls, saunas, spas) the exhaust-fan approach is preferred.

If the supply-fan approach is used, the fan should not be placed in a bathroom (due to possible distribution of polluted air). The outlet should be placed in a more central location, preferably away from bedrooms (due to possible noise concerns). The fan should be located so as to be accessible for maintenance. To avoid contamination of fresh air, the outside air intake should be ducted directly from the outside, and located away from flues, chimneys, vents, or other pollutant sources. The intake should also have a rodent/insect screen.

If the occupants are likely to be sensitive to allergens or other pollutants in outside air, it may be advantageous to provide filtration for the supply ventilation air. Filters should be accessible for maintenance and be located upstream of the fan. The installation should minimize the potential for entrance of unfiltered air between the filter and the fan.

Exhaust-Fan Approach

In the coldest areas of California it is possible for the supply-fan approach to drive moist, warm air into walls, where it can condense and potentially cause mold and mildew or structural damage. For these areas, it may be preferred to use a continuous bathroom exhaust fan (properly sized and sound rated for 1 sone or less) for continuous ventilation. This exhaust-fan approach is also acceptable in all California microclimates, as long as it does not result in house depressurization of more than -5 Pascals (house pressure compared to outside pressure). An exhaust fan can depressurize the house; therefore, care should be taken to insure that the continuous-running fan does not effect combustion air requirements and cause unsatisfactory or potentially dangerous operation of fireplaces or vented combustion appliances either installed by the builder or by the homeowner after occupancy.

MATERIALS AND EQUIPMENT

For all installations a quiet fan should be selected to minimize noise. Fans should be rated for continuous duty and be noise rated at no more than 1 sone.

Flow resistance in all ducting shall be minimized. To get air flow to bedrooms and other spaces, all doors to closeable rooms shall be undercut by 1/2" to 3/4" above the carpet or threshold to allow for unobstructed airflow. Alternatively, transfer grilles may be installed.

Ducts shall be sized for maximum air velocities of 700 feet per minute (see Table 1 above or use formula: [minimum x-sectional area of duct in square feet] = [flow in cubic feet per minute]/ [700 feet per minute]).

All ventilation devices (exhaust fans, heat exchangers, operable windows, ducting, grilles, vents, etc.) shall meet all appropriate listing, certification, installation, and/or features required by applicable codes, including but not limited to UL, UBC, UMC, and Title 24.

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