This section discusses three options for design of whole-building ventilation systems.
ü Exhaust ventilation
ü Supply ventilation
ü Balanced ventilation
See “Fan and Duct Specifications” on page 363.
We begin by discussing ducts for all types of ventilation systems.
Exhaust ventilation systems employ an exhaust fan to remove indoor air, which is replaced by infiltrating outdoor air.
Installing a two-speed bathroom fan is a common ventilation strategy. The new fan runs continuously on low speed for whole-building ventilation. A built-in occupancy sensor switches the fan automatically to a high speed to remove moisture and odors from the bathroom quickly.
A remote fan that exhausts air from several rooms through ducts (4-to-6 inch diameter) may provide better ventilation for larger more complex homes, compared to a single-point exhaust fan.
ü Fans must conform to “Fan Specifications” on page 363.
ü Ducts must conform to “Duct Materials and Installation” on page 368.
ü Termination fittings must conform to“Termination Fittings” on page 365.
Exhaust ventilation systems create a negative house pressure, drawing outdoor air in through leaks in the shell. This keeps moist indoor air from traveling through building cavities, which would occur with a positive house pressure. The negative house pressure reduces the likelihood of moisture accumulation in building cavities during the winter months in cold climates. In hot and humid climates, however, this depressurization can draw moist outdoor air into the home through building cavities. Therefore we recommend supply ventilation for warm humid climates rather than exhaust ventilation.
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SWS Detail: 6.6102.3 Intake for Ventilation Air to Forced Air System Used for Heating or Cooling |
Supply ventilation, using the home’s air handler, is never operated continuously as with exhaust ventilation because the furnace or heat-pump blower is too large and would over-ventilate the home and waste electrical energy. Supply ventilation may not be appropriate for tight homes in very cold climates because supply ventilation can push moist indoor air through exterior walls, where moisture can condense on cold surfaces.
A motorized damper that opens when the air-handler blower operates must control outdoor-air supply. The furnace/air conditioner heats or cools the outdoor air as necessary before delivering it to the living spaces.
The damper control estimates how much ventilation air is needed. The damper closes after the required amount of ventilation air has entered during heating or cooling. The control also activates the damper and the blower for additional ventilation air as needed without heating or cooling the air, during mild weather.
Supply ventilation typically uses the furnace or heat pump as a ventilator. A 5-to-10 inch diameter duct connects the furnace’s main return duct to a termination outdoors.
ü The existing duct system must leak less than 10% of the air handler flow when measured at 25 Pa. WRT outside.
ü The outdoor air must flow through a MERV 6 or better air filter before flowing through heating and cooling equipment.
ü Ducts must conform to “Duct Materials and Installation” on page 368.
ü Termination fittings must comply with“Termination Fittings” on page 365.
Balanced ventilation systems exhaust stale air and provide fresh air through a ducted distribution system. Of the three ventilation systems discussed here, balanced systems do the best job of controlling pollutants in the home.
Balanced systems move equal amounts of air into and out of the home. Most balanced systems incorporate heat-recovery ventilators or energy-recovery ventilators that reclaim heat and moisture from the exhaust air stream.
Balanced ventilation systems can improve the air quality and comfort of a home, but they require a high standard of care. Testing and commissioning is vital during both the initial installation and periodic service calls.
The difference between heat-recovery ventilators (HRVs) and energy-recovery ventilators (ERVs) is that HRVs transfer heat only, while ERVs transfer both sensible heat and latent heat (moisture) between air streams.
HRVs are often installed as balanced whole-building ventilation systems. The HRV core is an air-to-air heat exchanger in which the supply and exhaust air streams pass one another and exchange heat without mixing.
The home’s residents can maintain good indoor air quality by using spot ventilation together with opening windows and doors. Depending on climate and season, residents can control natural ventilation to provide clean air, comfort, and energy efficiency.
ü Choose windows and screen doors in strategic locations to ventilate using prevailing winds.
ü Make sure that windows and screen doors, chosen for ventilation, open and close and have effective insect screens.
ü Open windows to provide make-up air when an exhaust fan or the clothes dryer is operating.
ü Understand that dust and pollen may enter through windows or screen doors and consider the consequences.