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28-07-2009, 10:18 PM
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Guide to Sizing House Ventilation Systems

Guide to Sizing House Ventilation Systems - How Many CFM of Vent Fan Capacity are Needed?

ASHRAE Standard 62-1989 recommends a minimum ventilation rate in houses of 15 cfm per person, or .35 air changes per hour (ACH), whichever is greater. Based on the ACH method, a three-bedroom house of 1,500 sq ft with 8 ft ceilings would require:
(1,500 x 8) x .85 x .35 / 60 = 60 cfm
Multiplying the volume by .85 accounts for partitions and exterior wall thickness.
Using the per person method and assuming two people in the master bedroom and one in each other bedroom, the rate is also 60 cfm.
The revised ASHRAE standard 62.2, released in 2003, uses the formula of 7.5 cfm per person (based on the number of bedrooms plus one) plus an factor of .01 cfm for each square foot of house area. For example, based on the new ASHRAE standard, the same three-bedroom, 1,500-square-foot house would require:
(7.5 x 4) / (1,500 x .01) = 45 cfm.
As these calculations show, a low ventilation rate is adequate if run on a continuous basis. A higher continuous rate would be advisable for a home with higher-than-average moisture levels or pollutant sources such as smoking. Intermittent ventilation can also work as long as the total daily ventilation rate is equivalent, but is most effective when the system is timed to operate when people are home breathing air and generating pollutants.
A two-speed or variable-speed fan provides flexibility, allowing the ventilation rate to be raised when needed, for example when painting a room or during a party. More important than the precise number of cubic feet per minute, however, is a well-designed system that is quiet, reliable, and low- maintenance, ensuring it will actually be used.
TABLE 7-1 Whole House Ventilation Strategies

Installation Tips for Whole House Ventilation Systems

Whole-house ventilation systems should be installed by people familiar with the equipment. Since they normally operate at 100 to 200 cfm rather than the much larger fans found in air handlers, they are less forgiving of errors. Numerous field studies have found heat-recovery ventilators performing poorly due to installation errors and poor maintenance.
For good performance with whole-house ventilation systems, follow these general guidelines:
  • Size the whole house ventilation system correctly. Oversizing will increase heating and cooling costs.

    Choose quiet, efficient fans in the house ventilation equipment

    Keep HVAC or ventilation duct runs as short and straight as possible.

    Locate fresh air intakes away from pollution sources such as cars, pesticides, and outlets from HVAC equipment or exhaust fans.

    Seal all ducts and insulate where required. Examples: Insulate intake ducts that run though a hot attic or exhaust ducts that pass through a cold, unheated space.

    Integrate spot ventilation in bathrooms or provide separately.

    Use separate spot ventilation in kitchens due to grease.

    Place supply registers high on walls and away from beds, sofas, chairs, and other places likely to cause occupant discomfort.

    Keep controls as simple and automatic as possible.
  • Educate homeowners about the system and maintenance requirements.

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  post #2  
28-07-2009, 10:23 PM
Senior Member
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Exhaust House Venting Systems

Single-Port Exhaust House Venting Systems

The simplest and least expensive central ventilation system consists of an automatic timer wired to one centrally located bathroom or laundry fan so it cycles on and off for a portion of every hour or for the 8 to 12 hours per day when most people are home, typically mornings and evenings.
See the figure at left (click the image for details) about a single port exhaust house ventilation system.
The simplest ventilation system uses a single, centrally located exhaust fan that runs on a timer or continuously at a low speed. The fan may also serve as a bathroom or laundry fan, but a dedicated fan is optimal.
Passive air inlets are sometimes installed but will only work properly in very tight homes.
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. Exhaust fan control switch: Since the house ventilation exhaust fan is doing double duty as a bath or laundry fan, it must have a manual override switch for intermittent use. In larger homes, two fans at separate locations can be used. Another upgrade is to use a dedicated fan in a central location, such as a hallway ceiling, which will provide better distribution of both exhaust and supply air.
Exhaust fan noise: For the house ventilation system to work well, it is important to use a quiet exhaust fan of one sone or less and choose a central location. Also, the door to the bathroom with the exhaust fan must be undercut by 3/4 to 1inch, along with doors to all 4 bedrooms and other rooms that require ventilation. An alternative is to connect the rooms with through-the-wall transfer grilles.

The biggest drawback to exhaust-only ventilation is that there is little control over distribution of the incoming air. Makeup air will come via the path of least resistance. In a leaky house, this might be a window or drop ceiling in the bathroom with the exhaust fan, leaving the rest of the house un served by the ventilation system. For this reason, single-port exhaust-only ventilation works well only in relatively small, tight houses.
  • Passive air inlets. Some contractors install passive air inlets in an effort to direct makeup air into bedrooms and main living areas. For these to work properly, however, the house must be extremely tight and doors must be left open or be cut at least an inch above the carpet. If a house is too leaky or rooms are cut off from household airflows, the inlets will function like other random holes in the building shell, leaking air inward or outward, depending on the wind, stack effect, and imbalances in the HVAC system.

    The inlets typically require at least 10 Pascals of negative pressure to operate. They do not eliminate depressurization as sometimes thought. In fact, they require it to work properly.
Multiport Exhaust House Venting Systems

This type of system uses a more powerful exhaust fan that is remotely mounted, typically in the attic or basement. See the figure at left for details of a multi-port whole house exhaust fan vent system).
A multiport exhaust system improves air distribution by picking up air from bathrooms and main living areas. These are often used in conjunction with passive air inlets. Exhaust-only systems are best used in homes with electric heating or sealed-combustion appliances where backdrafting is not a concern.
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.
The multiport house exhaust fan system is ducted to exhaust grilles in bathrooms, laundries, and other wet areas, and sometimes to a centrally located pickup point in the main living space. A room with no outside walls would also benefit from a pickup point.
Systems typically run on a low background speed with timer switches in bathrooms for higher-powered spot ventilation. If installed correctly, these systems are very quiet and provide good distribution of ventilation.
Multiport exhaust systems may incorporate passive air inlets (see description above) that install either in windows or through the wall, providing some control over supply air. The inlets, typically three or four for a small house, go in bedrooms, main living areas, and other occupied rooms, such as dens or home offices. Inlets should be placed high on the wall away from beds, chairs, or other places where drafts might cause discomfort. Placement near a window is preferred.
Because these systems use more powerful fans that depressurize the house, they should not be used in houses with fireplaces or atmospherically vented combustion appliances. They are also not recommended in hot climates, since hot, moist exterior air may be drawn into walls and condense behind interior surfaces chilled from air conditioning.

Ventilating Heat-Pump Water Heater
This variation on exhaust-only ventilation passes the exhaust air through a heat-pump water heater, reclaiming heat from the outgoing air stream. Some systems can be reversed in summer, functioning as a supply ventilation system while cooling and dehumidifying the incoming air. A packaged heat-pump ventilating system is available from Therma-Stor.


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