Exhaust Fans
Exhaust fans are small, electrically operated fans used to remove odors, heat, and moisture from kitchens and bathrooms. They are commonly mounted in the wall or ceiling. Kitchens also will have an exhaust fan in the range hood over the stove.
Exhaust fans require very little maintenance. Wash the fan housing and fan blades every 6 months. Check the fan in the range hood for a grease filter. This should be cleaned every 3 months or more often depending on how much cooking is being done.
Kitchen Exhaust Fans
In estimating the requirements for ventilating kitchens, it is customary to allow a complete change of air every 2 minutes. In many cases, it is also desirable to have all the extracted air leave via hoods or canopies located over ranges, steam tables, dishwashers, and similar sources of localized heat and contaminants.
Allowing for a complete change of air every 2 minutes only applies to average conditions, and modifications from this average should be made on the basis of the kitchen size and the type of heat- and vapor-producing equipment.
An entrance velocity at the hood opening of 100 fpm is considered satisfactory as an allowance for average conditions. For very light cooking, an entrance velocity of only 50 fpm is usually sufficient. Heavy cooking may require an entrance velocity of 150 fpm or higher.
Exhaust hoods are usually located overhead in the majority of kitchen exhaust systems. They should be placed directly over the heat- and vapor-producing equipment and approximately 80 inches from the floor line to allow sufficient head clearance.
An overhead exhaust hood should be larger in horizontal area than the source of the heat or fumes. When located not over 2 feet above the range, the hood should be 6 inches larger in all directions than the overall dimensions of the range when the distance exceeds 2 feet. Thus, a range 2 feet by 7 feet with a clearance of 2 feet would require a hood 3 feet by 8 feet. Such a hood would have an area of 24 square feet. Using an average entrance velocity at the hood of 100 fpm, the volume of air to be handled would be 2400 cfm.
The area of the branch duct leading from the hood should be made 1⁄16 of the hood area (that is, 24 square feet –: 16) or 1.5 square feet. With the hood located 4 feet above the range, the dimensions would be 4 feet by 9 feet with a branch duct area of 2.25 square feet.
If a supply system is required, the amount of exhaust air should be greater than the volume of supply air to prevent undesirable cooking odors spreading to adjoining rooms. The supply air is usually figured on the basis of 75 percent of the exhaust air.
Bathroom Exhaust Fans
An air change every 3 minutes, or 20 complete changes per hour, is desirable for bathroom ventilation. Systems of this type should be entirely different and separated from other ventilating systems. Bathrooms located on the inside of a structure require ducts to exhaust air to the outside.
Compact fans are especially recommended for use in bathroom exhaust systems. Their compact design requires a minimum of space, and they are capable of operating against the resistance of the system.
Note
In tightly constructed and insulated houses, vent fans, clothes dryers, and kitchen exhaust fans can create a negative pressure that draws air into the house through holes in the framing, chimneys, and even exhaust flues.This can cause backdrafting in combustion appliances, which can be a serious health hazard. While the bathroom can be maintained at a negative pressure to control odor problems, the remainder of the house should be maintained at a slightly positive pressure. In hot, humid climates, it is best to operate the exhaust fan only when the bathroom is in use, so that the negative pressure does not draw humid outside air into the building cavities.
Whole-House Ventilation
Whole-house ventilation is a ventilating system in which a large centrally located fan provides natural air-conditioning. In very dry climates where hot days and very cool nights are the norm, whole- house ventilating fans often provide a suitable replacement for air- conditioning. Cooler outside air is drawn in through the windows on the lower floors during the night and forced out through the attic vents (see Figure 7-24). This system produces a steady supply of filtered, fresh air to all the living spaces of the house. It works most efficiently when the outdoor temperature is below 82˚F. A whole-house ventilation system can also reduce air-conditioning costs by using it instead of the air conditioner to cool the house between the heating and cooling seasons, or by using it to ventilate the house before turning on the air conditioner.
Note
Whole-house ventilation is not very suitable for humid climates because it draws excess moisture into the house.
In cold climates, the type of whole-house ventilator used in the system is commonly designed to capture some of the heat from the
air before it is exhausted from the attic. These are sometimes called air-to-air heat exchangers or heat recovery ventilators (HRVs). These air-to-air heat exchangers are designed to recover as much as 80 percent of the heat energy from the indoor air before it leaves the attic.
Some whole-house ventilators are designed to add or remove moisture from the air. These units are sometimes called energy recovery ventilators (ERVs). An energy recovery ventilator operates by balancing the humidity levels between the air drawn into the structure and the air exhausted from the attic. During the winter when the outdoor air is drier, moisture is added to the air by the unit as it is drawn into the house. In the summer when the air is more humid, the energy recovery ventilator removes moisture from the air before it is blown out of the attic.
The fans used in whole-house ventilation are much larger than the standard attic ventilating fans because they are required to move much more air. These are high-velocity cooling fans commonly installed in a hallway ceiling directly beneath the attic. Because some of these fans are too large to be installed between ceiling joists, a box frame is constructed on top of the joists to house them. The box frame must be built by the home- owner or building contractor because it is not provided by the fan manufacturer.
These fans are either belt-driven or direct drive. Belt-driven fans are the quieter of the two, but they require belt replacement every 2 or 3 years. Direct-drive fans are almost maintenance-free.
Note
Low-velocity whole-house ventilating fans should not be confused with the high-velocity types used in whole-house ventilation systems. The former are used only to provide a continuous stream of fresh air and remove indoor pollutants.
A whole-house ventilation fan is switched on when the outside temperature falls below the indoor temperature and continues to operate throughout the night. The fan can be operated both manually and automatically through centrally located controls. Many installations have only a wall-mounted manual on-off switch.
Warning
Windows must be open when operating a whole-house ventilator in order to prevent backdrafting. Depressurization can be prevented if the total open area of the windows is approximately
equal to the total net free area of the attic.
Select a whole-house ventilator large enough to deliver 20 air changes per hour (ACH). A typical sizing method for these fans is to divide the volume of the house (width X height X length) by 3 to obtain the air changes per hour for the fan.
Whole-house ventilators require very little maintenance. If the unit is equipped with a filter, it will have to be periodically cleaned or replaced. Inspect the fan blades from time to time and clean them when there is a noticeable buildup of dirt or grease.