WHOLE-HOUSE VENTILATION SYSTEMS

CONTROLS

Ventilation experts usually recommend that whole-house

ventilation systems be designed to operate automatically so

that fresh air is supplied to the house without occupant

intervention. An on-off control may also be required by codes.

C ONTINUOUS C ONTROL

Some experts recommend continuous ventilation to simplify

controls and to avoid unhealthy indoor air for the several hours

it may take a system to flush out pollutants after having been

off for an extended time. If pollutants continuously released

from furnishings or building materials are the principal

concern, continuous ventilation is most effective.

P ROGRAMMABLE TIMER

Rather than operate a whole-house ventilation system

continuously, a programmable timer can be employed to operate

the system intermittently for a selected period of time each day.

To provide equivalent ventilation, such a system must have a

higher capacity than one that operates continuously.

OCCUPANCY C ONTROL

Some experts recommend that whole-house ventilation

systems be controlled to provide fresh air only when

occupants are home to minimize costs. If the occupants and

their activities are the principal cause of indoor air pollution,

occupancy-based ventilation controls can be most effective.

Detecting occupancy is difficult. Motion detectors are effective

unless the occupants are sitting or sleeping quietly or are out

of the view of the sensor. Good coverage may require one or

more motion detectors in each room. Carbon dioxide sensors

can be effective but are expensive. Setting the detection level

may be difficult because a sleeping person gives off little

carbon dioxide. Thus, it is difficult to distinguish between a

house with sleeping people and one that is unoccupied.

L O W -HIGH S WITCH

A two-speed fan installed in a whole-house ventilation system

might be controlled by a low-high switch. This allows the

ventilation fan to provide a low level of ventilation

continuously, but provides the occupant with the capability to

boost the ventilation rate if needed.

L ABELING

If a ventilation control switch is used, it should be labeled to

provide clear guidance for its proper operation. For example, it

might be labeled to say “This switch controls the ventilation

system. It should be ON whenever the home is occupied.”

DESIGN AND INSTALLATION CRITERIA

F ANS

Noise is a major reason people avoid using ventilation

systems. Fans used in whole-house ventilation systems and

installed inside the house should be quiet (less than 2 sones,

but preferably less than 1 sone) or installed remotely outside

the living space so that the noise caused by their operation is

not perceived.

A significant part of the operating cost associated with a

ventilation system is the electricity used to operate the fan.

Energy-efficient fans should be used to reduce these costs.

Fans should be sized and selected to provide necessary air

flows based on the type, length, and design of the duct

system. Fans selected for whole-house ventilation systems

should be manufactured for continuous operation and long life

(greater than 10 years), and installed in a location that is easily

accessible for regular maintenance.

DUCTS

The most efficient ventilation ducts are smooth, short, straight,

and properly sized. Smooth sheet metal ducts offer low airflow

resistance. Because corrugated ducts have greater flow

resistance, it is important to keep them as short as possible—

stretch the corrugated material to its full length and cut off the

excess. Minimize the number of elbows. Provide adequate

support. Use mechanical fasteners and sealants (preferably

duct mastic) at all joints. Ducts located outside the conditioned

space should be insulated.

DUCT TERMINATIONS

Ducts expelling water vapor or other pollutants must exhaust

directly to the exterior—never into attics or crawlspaces prone

to moisture problems. Use wall caps or roof jacks with flap

dampers, screens, or both to deter access and to reduce air

infiltration. Unless they are already integrated into the system

(e.g., wall cap with flapper), equip ventilation ducts with

backdraft dampers at or near the insulated building boundary.

I NDOOR AI R CIRCULATION

Indoor air must be free to flow between supply and return

ports of whole-house ventilation systems. If a supply and

return port is not installed in every room, then through-thewall transfer grills should be installed above doors in rooms

with doors that are often closed, or the doors should be

undercut to facilitate air flow.
WHOLE-HOUSE VENTILATION SYSTEM DESIGNS

The decision to use whole-house ventilation is typically

motivated by concern that natural ventilation is not providing

adequate air quality, even with source control by spot

ventilation. Whole-house ventilation systems are usually

classified as exhaust ventilation if the mechanical system

forces inside air out of the home, supply ventilation if the

mechanical system forces outside air into the home, or

balanced ventilation if the mechanical system forces equal

quantities of air into and out of the home.

E XHAUST VENTILATION SYSTEMS

Exhaust ventilation systems work by depressurizing the

building. By reducing the inside air pressure below the outdoor

air pressure, they extract indoor air from a house while makeup air infiltrates through leaks in the building shell and through

intentional, passive vents.

Exhaust ventilation systems are relatively simple and

inexpensive to install. Typically, an exhaust ventilation system

is composed of a single fan connected to a centrally located,

single exhaust point in the house. A preferable design option is

to connect the fan to ducts from several rooms (preferably

rooms where pollutants tend to be generated, such as

bathrooms). Adjustable, passive vents through windows or

walls can be installed in other rooms to introduce fresh air

rather than rely on leaks in the building envelope. However,

their use may be ineffective because larger pressure

differences than those induced by the ventilation fan may be

needed for them to work properly. Spot ventilation exhaust

fans installed in the bathroom but operated continuously can

represent an exhaust ventilation system in its simplest form.

Exhaust ventilation systems are most applicable in cold

climates. In climates with warm humid summers,

depressurization can draw moist air into building wall cavities,

where it may condense and cause moisture damage.

One concern with exhaust ventilation systems is that they may

draw pollutants, along with fresh air, into the house. For

example, in addition to drawing in fresh outdoor air, they may

draw in radon and molds from a crawlspace, dust from an

attic, fumes from an attached garage, or flue gases from a

fireplace or fossil-fuel-fired water heater and furnace. This can

especially be of concern when bath fans, range fans, and

clothes dryers (which also depressurize the home while they

operate) are run when an exhaust ventilation system is also

operating. Also, exhaust ventilation systems can contribute to

higher heating and cooling costs compared with heat-recovery

systems because exhaust systems do not temper or remove

moisture from the make-up air before it enters the house.

CENTRAL VENTILATION STRATEGIES

○○○○○○○○ ○○○○○○○○○○○○○○○

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Central exhaust fan

Exhaust air

outlet

Central supply fan

Fresh air

Air flow inlet

Air infiltration

Positive air pressure

Negative air pressure

Exhaust ventilation Supply ventilation
WHOLE-HOUSE VENTILATION SYSTEM DESIGNS

The decision to use whole-house ventilation is typically

motivated by concern that natural ventilation is not providing

adequate air quality, even with source control by spot

ventilation. Whole-house ventilation systems are usually

classified as exhaust ventilation if the mechanical system

forces inside air out of the home, supply ventilation if the

mechanical system forces outside air into the home, or

balanced ventilation if the mechanical system forces equal

quantities of air into and out of the home.

E XHAUST VENTILATION SYSTEMS

Exhaust ventilation systems work by depressurizing the

building. By reducing the inside air pressure below the outdoor

air pressure, they extract indoor air from a house while makeup air infiltrates through leaks in the building shell and through

intentional, passive vents.

Exhaust ventilation systems are relatively simple and

inexpensive to install. Typically, an exhaust ventilation system

is composed of a single fan connected to a centrally located,

single exhaust point in the house. A preferable design option is

to connect the fan to ducts from several rooms (preferably

rooms where pollutants tend to be generated, such as

bathrooms). Adjustable, passive vents through windows or

walls can be installed in other rooms to introduce fresh air

rather than rely on leaks in the building envelope. However,

their use may be ineffective because larger pressure

differences than those induced by the ventilation fan may be

needed for them to work properly. Spot ventilation exhaust

fans installed in the bathroom but operated continuously can

represent an exhaust ventilation system in its simplest form.

Exhaust ventilation systems are most applicable in cold

climates. In climates with warm humid summers,

depressurization can draw moist air into building wall cavities,

where it may condense and cause moisture damage.

One concern with exhaust ventilation systems is that they may

draw pollutants, along with fresh air, into the house. For

example, in addition to drawing in fresh outdoor air, they may

draw in radon and molds from a crawlspace, dust from an

attic, fumes from an attached garage, or flue gases from a

fireplace or fossil-fuel-fired water heater and furnace. This can

especially be of concern when bath fans, range fans, and

clothes dryers (which also depressurize the home while they

operate) are run when an exhaust ventilation system is also

operating. Also, exhaust ventilation systems can contribute to

higher heating and cooling costs compared with heat-recovery

systems because exhaust systems do not temper or remove

moisture from the make-up air before it enters the house.

CENTRAL VENTILATION STRATEGIES

○○○○○○○○ ○○○○○○○○○○○○○○○

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Central exhaust fan

Exhaust air

outlet

Central supply fan

Fresh air

Air flow inlet

Air infiltration

Positive air pressure

Negative air pressure

Exhaust ventilation Supply ventilation
WHOLE-HOUSE VENTILATION SYSTEM DESIGNS

The decision to use whole-house ventilation is typically

motivated by concern that natural ventilation is not providing

adequate air quality, even with source control by spot

ventilation. Whole-house ventilation systems are usually

classified as exhaust ventilation if the mechanical system

forces inside air out of the home, supply ventilation if the

mechanical system forces outside air into the home, or

balanced ventilation if the mechanical system forces equal

quantities of air into and out of the home.

E XHAUST VENTILATION SYSTEMS

Exhaust ventilation systems work by depressurizing the

building. By reducing the inside air pressure below the outdoor

air pressure, they extract indoor air from a house while makeup air infiltrates through leaks in the building shell and through

intentional, passive vents.

Exhaust ventilation systems are relatively simple and

inexpensive to install. Typically, an exhaust ventilation system

is composed of a single fan connected to a centrally located,

single exhaust point in the house. A preferable design option is

to connect the fan to ducts from several rooms (preferably

rooms where pollutants tend to be generated, such as

bathrooms). Adjustable, passive vents through windows or

walls can be installed in other rooms to introduce fresh air

rather than rely on leaks in the building envelope. However,

their use may be ineffective because larger pressure

differences than those induced by the ventilation fan may be

needed for them to work properly. Spot ventilation exhaust

fans installed in the bathroom but operated continuously can

represent an exhaust ventilation system in its simplest form.

Exhaust ventilation systems are most applicable in cold

climates. In climates with warm humid summers,

depressurization can draw moist air into building wall cavities,

where it may condense and cause moisture damage.

One concern with exhaust ventilation systems is that they may

draw pollutants, along with fresh air, into the house. For

example, in addition to drawing in fresh outdoor air, they may

draw in radon and molds from a crawlspace, dust from an

attic, fumes from an attached garage, or flue gases from a

fireplace or fossil-fuel-fired water heater and furnace. This can

especially be of concern when bath fans, range fans, and

clothes dryers (which also depressurize the home while they

operate) are run when an exhaust ventilation system is also

operating. Also, exhaust ventilation systems can contribute to

higher heating and cooling costs compared with heat-recovery

systems because exhaust systems do not temper or remove

moisture from the make-up air before it enters the house.

CENTRAL VENTILATION STRATEGIES

○○○○○○○○ ○○○○○○○○○○○○○○○

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Central exhaust fan

Exhaust air

outlet

Central supply fan

Fresh air

Air flow inlet

Air infiltration

Positive air pressure

Negative air pressure

Exhaust ventilation Supply ventilation
WHOLE-HOUSE VENTILATION SYSTEMS

CONTROLS

Ventilation experts usually recommend that whole-house

ventilation systems be designed to operate automatically so

that fresh air is supplied to the house without occupant

intervention. An on-off control may also be required by codes.

C ONTINUOUS C ONTROL

Some experts recommend continuous ventilation to simplify

controls and to avoid unhealthy indoor air for the several hours

it may take a system to flush out pollutants after having been

off for an extended time. If pollutants continuously released

from furnishings or building materials are the principal

concern, continuous ventilation is most effective.

P ROGRAMMABLE TIMER

Rather than operate a whole-house ventilation system

continuously, a programmable timer can be employed to operate

the system intermittently for a selected period of time each day.

To provide equivalent ventilation, such a system must have a

higher capacity than one that operates continuously.

OCCUPANCY C ONTROL

Some experts recommend that whole-house ventilation

systems be controlled to provide fresh air only when

occupants are home to minimize costs. If the occupants and

their activities are the principal cause of indoor air pollution,

occupancy-based ventilation controls can be most effective.

Detecting occupancy is difficult. Motion detectors are effective

unless the occupants are sitting or sleeping quietly or are out

of the view of the sensor. Good coverage may require one or

more motion detectors in each room. Carbon dioxide sensors

can be effective but are expensive. Setting the detection level

may be difficult because a sleeping person gives off little

carbon dioxide. Thus, it is difficult to distinguish between a

house with sleeping people and one that is unoccupied.

L O W -HIGH S WITCH

A two-speed fan installed in a whole-house ventilation system

might be controlled by a low-high switch. This allows the

ventilation fan to provide a low level of ventilation

continuously, but provides the occupant with the capability to

boost the ventilation rate if needed.

L ABELING

If a ventilation control switch is used, it should be labeled to

provide clear guidance for its proper operation. For example, it

might be labeled to say “This switch controls the ventilation

system. It should be ON whenever the home is occupied.”

DESIGN AND INSTALLATION CRITERIA

F ANS

Noise is a major reason people avoid using ventilation

systems. Fans used in whole-house ventilation systems and

installed inside the house should be quiet (less than 2 sones,

but preferably less than 1 sone) or installed remotely outside

the living space so that the noise caused by their operation is

not perceived.

A significant part of the operating cost associated with a

ventilation system is the electricity used to operate the fan.

Energy-efficient fans should be used to reduce these costs.

Fans should be sized and selected to provide necessary air

flows based on the type, length, and design of the duct

system. Fans selected for whole-house ventilation systems

should be manufactured for continuous operation and long life

(greater than 10 years), and installed in a location that is easily

accessible for regular maintenance.

DUCTS

The most efficient ventilation ducts are smooth, short, straight,

and properly sized. Smooth sheet metal ducts offer low airflow

resistance. Because corrugated ducts have greater flow

resistance, it is important to keep them as short as possible—

stretch the corrugated material to its full length and cut off the

excess. Minimize the number of elbows. Provide adequate

support. Use mechanical fasteners and sealants (preferably

duct mastic) at all joints. Ducts located outside the conditioned

space should be insulated.

DUCT TERMINATIONS

Ducts expelling water vapor or other pollutants must exhaust

directly to the exterior—never into attics or crawlspaces prone

to moisture problems. Use wall caps or roof jacks with flap

dampers, screens, or both to deter access and to reduce air

infiltration. Unless they are already integrated into the system

(e.g., wall cap with flapper), equip ventilation ducts with

backdraft dampers at or near the insulated building boundary.

I NDOOR AI R CIRCULATION

Indoor air must be free to flow between supply and return

ports of whole-house ventilation systems. If a supply and

return port is not installed in every room, then through-thewall transfer grills should be installed above doors in rooms

with doors that are often closed, or the doors should be

undercut to facilitate air flow.
WHOLE-HOUSE VENTILATION SYSTEMS

CONTROLS

Ventilation experts usually recommend that whole-house

ventilation systems be designed to operate automatically so

that fresh air is supplied to the house without occupant

intervention. An on-off control may also be required by codes.

C ONTINUOUS C ONTROL

Some experts recommend continuous ventilation to simplify

controls and to avoid unhealthy indoor air for the several hours

it may take a system to flush out pollutants after having been

off for an extended time. If pollutants continuously released

from furnishings or building materials are the principal

concern, continuous ventilation is most effective.

P ROGRAMMABLE TIMER

Rather than operate a whole-house ventilation system

continuously, a programmable timer can be employed to operate

the system intermittently for a selected period of time each day.

To provide equivalent ventilation, such a system must have a

higher capacity than one that operates continuously.

OCCUPANCY C ONTROL

Some experts recommend that whole-house ventilation

systems be controlled to provide fresh air only when

occupants are home to minimize costs. If the occupants and

their activities are the principal cause of indoor air pollution,

occupancy-based ventilation controls can be most effective.

Detecting occupancy is difficult. Motion detectors are effective

unless the occupants are sitting or sleeping quietly or are out

of the view of the sensor. Good coverage may require one or

more motion detectors in each room. Carbon dioxide sensors

can be effective but are expensive. Setting the detection level

may be difficult because a sleeping person gives off little

carbon dioxide. Thus, it is difficult to distinguish between a

house with sleeping people and one that is unoccupied.

L O W -HIGH S WITCH

A two-speed fan installed in a whole-house ventilation system

might be controlled by a low-high switch. This allows the

ventilation fan to provide a low level of ventilation

continuously, but provides the occupant with the capability to

boost the ventilation rate if needed.

L ABELING

If a ventilation control switch is used, it should be labeled to

provide clear guidance for its proper operation. For example, it

might be labeled to say “This switch controls the ventilation

system. It should be ON whenever the home is occupied.”

DESIGN AND INSTALLATION CRITERIA

F ANS

Noise is a major reason people avoid using ventilation

systems. Fans used in whole-house ventilation systems and

installed inside the house should be quiet (less than 2 sones,

but preferably less than 1 sone) or installed remotely outside

the living space so that the noise caused by their operation is

not perceived.

A significant part of the operating cost associated with a

ventilation system is the electricity used to operate the fan.

Energy-efficient fans should be used to reduce these costs.

Fans should be sized and selected to provide necessary air

flows based on the type, length, and design of the duct

system. Fans selected for whole-house ventilation systems

should be manufactured for continuous operation and long life

(greater than 10 years), and installed in a location that is easily

accessible for regular maintenance.

DUCTS

The most efficient ventilation ducts are smooth, short, straight,

and properly sized. Smooth sheet metal ducts offer low airflow

resistance. Because corrugated ducts have greater flow

resistance, it is important to keep them as short as possible—

stretch the corrugated material to its full length and cut off the

excess. Minimize the number of elbows. Provide adequate

support. Use mechanical fasteners and sealants (preferably

duct mastic) at all joints. Ducts located outside the conditioned

space should be insulated.

DUCT TERMINATIONS

Ducts expelling water vapor or other pollutants must exhaust

directly to the exterior—never into attics or crawlspaces prone

to moisture problems. Use wall caps or roof jacks with flap

dampers, screens, or both to deter access and to reduce air

infiltration. Unless they are already integrated into the system

(e.g., wall cap with flapper), equip ventilation ducts with

backdraft dampers at or near the insulated building boundary.

I NDOOR AI R CIRCULATION

Indoor air must be free to flow between supply and return

ports of whole-house ventilation systems. If a supply and

return port is not installed in every room, then through-thewall transfer grills should be installed above doors in rooms

with doors that are often closed, or the doors should be

undercut to facilitate air flow.