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, passive vents 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 represent an
exhaust ventilation system in its simplest form.
One concern with exhaust
ventilation systems is; they may draw pollutants, along with
fresh air, into the structure, such as:
- Radon and molds from a
- Dust from an attic
- Fumes from an attached
- Flue gases from a fireplace
or fossil-fuel-fired water heater and furnace.
This concern is pronounced 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 to
energy recovery ventilation systems. This is because
exhaust ventilation system do not temper or remove moisture from
the make-up air before it enters the structure.
Supply ventilation systems work
by pressurizing the building. They use a fan to force
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).
As with 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) room/s of the structure
that are occupied most often (e.g., bedrooms, living room.)
This system may include an adjustable window or wall vents in
Supply ventilation systems allow
better control of the air that enters the house than do exhaust
ventilation systems. By pressurizing the house, supply
ventilation systems discourage the entry of pollutants from
outside the living space and prevent back-drafting 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
ventilation systems work best in hot or mixed climates.
Because they pressurize the house, supply ventilation 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, some
moisture may condense in the attic or cold outer parts of the
exterior wall where it can promote 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 structure. It may
contribute to higher heating and cooling costs compared to
energy recovery ventilation systems. Because air is
introduced in the structure 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 will increase operating costs.
systems, if properly designed and installed, neither
pressurize nor depressurize a house. Rather, they
introduce and exhaust approximately equal quantities of
fresh outside air and polluted inside air, respectively.
A balanced ventilation
system usually has two fans and two duct systems. It
faciliatates good distribution of fresh air by placing
supply and exhaust vents in appropriate areas. Fresh
air supply and exhaust vents can be installed in every room.
Typically a balanced ventilation system is designed to
supply fresh air to bedrooms and living rooms where people
spend the most time. It also exhausts air from rooms
where moisture and pollutants are most often generated
(kitchens, bathrooms, and laundry rooms.) Some designs
may use a single point exhaust. Because they directly
supply outside air, balanced ventilation systems allow the
use of filters to remove dust and pollen from outside air
before introducing it into the structure.
Balanced ventilation systems
are appropriate for all climates. However, because
they require two duct and fan systems, 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.
Recovery Ventilation Systems
Energy recovery ventilation
systems provide a controlled way of ventilating a structure
while minimizing energy loss. They reduce the costs of
heating ventilated air in the winter by transferring heat
from the warm inside air being exhausted to the fresh (but
cold) supply air. In the summer, the inside air cools
the warmer supply air to reduce ventilation 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 some 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
Most energy recovery
ventilation systems can recover about 70%–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.
Installation and Maintenance
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 structures, 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
use ordinary duct tape on ducts.)
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.
U.S. Department of Energy