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Maintaining Indoor Air Quality
Energy-efficient homes --
both new and existing -- require mechanical ventilation to maintain indoor air
quality.
There are four basic
mechanical whole-house ventilation systems -- exhaust, supply, balanced, and
energy recovery.
COMPARISON OF WHOLE-HOUSE VENTILATION
SYSTEMS
Ventilation
System
|
Pros
|
Cons
|
|
Exhaust
|
·
Relatively inexpensive and
simple to install
·
Work well in cold climates.
|
·
Can draw pollutants into
living space
·
Not appropriate for hot humid
climates
·
Rely in part on random air
leakage
·
Can increase heating and
cooling costs
·
May require mixing of outdoor
and indoor air to avoid drafts in cold weather
·
Can cause backdrafting in
combustion appliances.
|
|
Supply
|
·
Relatively inexpensive and
simple to install
·
Allow better control than
exhaust systems
·
Minimize pollutants from
outside living space
·
Prevent backdrafting of
combustion gases from fireplaces and appliances
·
Allow filtering of pollen and
dust in outdoor air
·
Allow dehumidification of
outdoor air
·
Work well in hot or mixed
climates.
|
·
Can cause moisture problems
in cold climates
·
Will not temper or
remove moisture from incoming air
·
Can increase heating and
cooling costs
·
May require mixing of outdoor
and indoor air to avoid drafts in cold weather.
|
|
Balanced
|
·
Appropriate for all climates
|
·
Can cost more to install and
operate than exhaust or supply systems
·
Will not temper or
remove moisture from incoming air
·
Can increase heating and
cooling costs.
|
|
Energy
Recovery & Heat Recovery Ventilators
|
·
Reduce heating and cooling
costs
·
Available as both small wall-
or window-mounted models or central ventilation systems
·
Cost-effective in climates
with extreme winters or summers and high fuel costs.
|
·
Can cost more to install than
other ventilation systems
·
May not be cost-effective in
mild climates
·
May be difficult to find
contractors with experience and expertise to install these systems
·
Require freeze and frost
protection in cold climates
·
Require more maintenance than
other ventilation systems.
|
|
Exhaust ventilation systems work by depressurizing your home.
The system exhausts air from
the house while make-up air infiltrates through leaks in the building shell and
through intentional, passive vents.
Exhaust ventilation systems are most appropriate for 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.
Exhaust ventilation systems are relatively simple and
inexpensive to install.
Typically, an exhaust
ventilation system consists of a single fan connected to a centrally located,
single exhaust point in the house.
A better design is to connect
the fan to ducts from several rooms, preferably rooms where pollutants are
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.
Passive vents may, however,
require larger pressure differences than those induced by the ventilation fan
to work properly.
One concern with exhaust ventilation systems is that -- along
with fresh air -- they may draw in pollutants, including:
·
Radon and molds from a crawlspace
·
Dust from an attic
·
Fumes from an attached garage
·
Flue gases from a fireplace or
fossil-fuel-fired water heater and furnace.
These pollutants are a particular 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.
Exhaust ventilation systems can also contribute to higher
heating and cooling costs compared with energy recovery ventilation
systems because exhaust systems do not temper or remove moisture from the
make-up air before it enters the house.
SUPPLY VENTILATION SYSTEMS
Supply ventilation systems use a fan to pressurize your home,
forcing outside air into the building while air leaks out of the building
through holes in the shell, bath, and range fan ducts, and intentional vents
(if any exist).
Like exhaust ventilation systems, supply ventilation systems are
relatively simple and inexpensive to install.
A typical supply ventilation
system has a fan and duct system that introduces fresh air into usually one --
but preferably several -- rooms that residents occupy most (e.g., bedrooms,
living room). This system may include adjustable window or wall vents in other
rooms.
Supply ventilation systems allow better control of the air that
enters the house than exhaust ventilation systems do.
By pressurizing the house,
supply ventilation systems minimize outdoor pollutants in the living space and
prevent backdrafting of combustion gases from fireplaces and appliances.
Supply ventilation also
allows outdoor air introduced into the house to be filtered to remove pollen and
dust or dehumidified to provide humidity control
Supply ventilation systems work best in hot or mixed climates.
Because they pressurize the
house, these systems have the potential to cause moisture problems in cold
climates.
In winter, the supply ventilation
system causes warm interior air to leak through random openings in the exterior
wall and ceiling.
If the interior air is humid
enough, moisture may condense in the attic or cold outer parts of the exterior
wall, resulting in mold, mildew, and decay.
Like exhaust ventilation systems, supply ventilation systems do
not temper or remove moisture from the make-up air before it enters the house.
Thus, they may contribute to
higher heating and cooling costs compared with energy recovery ventilation
systems.
Because air is introduced
into the house at discrete locations, outdoor air may need to be mixed with
indoor air before delivery to avoid cold air drafts in the winter.
An in-line duct heater is
another option, but increases operating costs.
BALANCED VENTILATION SYSTEMS
Balanced ventilation systems, if properly designed and
installed, neither pressurize nor depressurize your home.
Rather, they introduce and
exhaust approximately equal quantities of fresh outside air and polluted inside
air.
A balanced ventilation system usually has two fans and two duct
systems.
Fresh air supply and exhaust
vents can be installed in every room, but a typical balanced ventilation system
is designed to supply fresh air to bedrooms and living rooms where occupants
spend the most time.
It also exhausts air from
rooms where moisture and pollutants are most often generated (kitchen,
bathrooms, and perhaps the laundry room).
Some designs use a single-point exhaust. Because they directly
supply outside air, balanced systems allow the use of filters to remove dust
and pollen from outside air before introducing it into the house.
Balanced ventilation systems are appropriate for all climates.
Because they require two duct
and fan systems, however, balanced ventilation systems are usually more
expensive to install and operate than supply or exhaust systems.
Like both supply and exhaust systems, balanced ventilation
systems do not temper or remove moisture from the make-up air before it enters
the house.
Therefore, they may
contribute to higher heating and cooling costs, unlike energy recovery
ventilation systems.
Also, like supply ventilation
systems, outdoor air may need to be mixed with indoor air before delivery to
avoid cold air drafts in the winter.
ENERGY RECOVERY VENTILATION SYSTEMS
Energy recovery ventilation systems provide a controlled way of
ventilating a home while minimizing energy loss.
They reduce the costs of
heating ventilated air in the winter by transferring heat from the warm inside
exhaust air to the fresh (but cold) outside supply air.
In the summer, the inside air
cools the warmer supply air to reduce cooling costs.
There are two types of energy-recovery systems: heat-recovery
ventilators (HRV) and energy-recovery (or enthalpy-recovery) ventilators (ERV).
Both types include a heat
exchanger, one or more fans to push air through the machine, and controls.
There are some small wall- or
window-mounted models, but the majority are central, whole-house ventilation
systems with their own duct system or shared ductwork.
The main difference between a heat-recovery and an
energy-recovery ventilator is the way the heat exchanger works.
With an energy-recovery
ventilator, the heat exchanger transfers a certain amount of water vapor along
with heat energy, while a heat-recovery ventilator only transfers heat.
Because an energy-recovery ventilator transfers some of the
moisture from the exhaust air to the usually less humid incoming winter air,
the humidity of the house air stays more constant.
This also keeps the heat
exchanger core warmer, minimizing problems with freezing.
In the summer, an energy-recovery ventilator may help to control
humidity in the house by transferring some of the water vapor in the incoming
air to the theoretically drier air that's leaving the house.
If you use an air
conditioner, an energy-recovery ventilator generally offers better humidity
control than a heat-recovery system.
However, there's some
controversy about using ventilation systems at all during humid, but not overly
hot, summer weather.
Some experts suggest that it
is better to turn the system off in very humid weather to keep indoor humidity
levels low.
You can also set up the
system so that it only runs when the air conditioning system is running, or use
pre-cooling coils.
Most energy recovery ventilation systems can recover about 70%
to 80% of the energy in the exiting air and deliver that energy to the incoming
air.
However, they are most
cost-effective in climates with extreme winters or summers, and where fuel
costs are high.
In mild climates, the cost of
the additional electricity consumed by the system fans may exceed the energy
savings from not having to condition the supply air.
Energy recovery ventilation systems usually cost more to install
than other ventilation systems.
In general, simplicity is key
to a cost-effective installation. To save on installation costs, many systems
share existing ductwork.
Complex systems are not only
more expensive to install, but they are generally more maintenance intensive
and often consume more electric power.
For most houses, attempting
to recover all of the energy in the exhaust air will probably not be worth the
additional cost. Also, these types of ventilation systems are still not very
common. Only some HVAC contractors have enough technical expertise and
experience to install them.
In general, you want to have a supply and return duct for each
bedroom and for each common living area.
Duct runs should be as short
and straight as possible. The correct size duct is necessary to minimize
pressure drops in the system and thus improve performance.
Insulate ducts located
in unheated spaces, and seal all joints with duct mastic (never ordinary duct
tape).
Also, energy recovery ventilation systems operated in cold
climates must have devices to help prevent freezing and frost formation.
Very cold supply air can
cause frost formation in the heat exchanger, which can damage it. Frost buildup
also reduces ventilation effectiveness.
Energy recovery ventilation systems require more maintenance
than other ventilation systems.
They need to be cleaned
regularly to prevent deterioration of ventilation rates and heat recovery and
to prevent mold and bacteria on heat exchanger surfaces.
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