Ventilation Questions Posed by Home Energy Magazine
(It should be noted that whole-house ventilation is discussed here, not source
ventilation for kitchens and baths.)
1. What is the purpose of installing a mechanical ventilation
system in a home?
In short, the primary purpose of a whole-house mechanical ventilation
system in a home is to provide, throughout the habitable and conditioned
space, a controlled amount of unpolluted outside air for indoor pollutant
dilution and removal, and for the sensory satisfaction of occupants. Since
my definition considers excessive moisture to be a pollutant, a corollary
to the stated purpose would be, to control interior moisture.
An important secondary purpose is to control interior pressure, with
respect to outside, to maximize building durability, combustion safety,
and indoor air quality.
2. What are the qualities of a good ventilation system? That
is; what should a contractor strive for (and a homeowner
expect from) a good ventilation system?
First of all, a good ventilation system should achieve the purpose
stated in response to question 1. In more specific terms, the system
should:
- have at least a 15 year useful life at the design operational time;
- be acceptable to operate so that the occupants will not seek
alternative means to achieve the purpose; this would include
acceptable noise and operating cost;
- not detract from, but possibly enhance, the safety and durability of
the house as a system.
3. Should mechanical ventilation be standard in homes built
today? What is the one most important consideration in
making this decision?
Most people like their homes better when they are not expensive,
drafty, stuffy, odoriferous, moldy and wet. On the other hand, they like
their houses to be affordable, comfortable, healthy and durable. For
homebuilders who want to accomplish that, random natural infiltration
should be minimized and controlled mechanical ventilation should be
employed. Natural infiltration occurs when ever-changing forces of wind
and stack (due to inside to outside temperature difference) move outside
air through randomly distributed, unintentional holes in the building
enclosure. Properly designed and installed, mechanical ventilation provides
a consistent and uniform building air change for pollutant dilution and
removal, and for the sensory satisfaction of occupants. The most important
consideration in making the decision for or against mechanical ventilation
is, therefore, "How lucky do you feel?"
4. What minimum criteria would determine if a new home needs
mechanical ventilation? In what cases would you retrofit
a home for mechanical ventilation?
For new construction, a minimum criteria could be demonstration of the
greater of 40 ft3/min and 10 ft3/min per person, or the equivalent volume
over a two hour period, of unpolluted outside air distributed uniformly
throughout the habitable and conditioned space 24 hours a day while the
home is occupied.
A retrofit criteria for existing construction is much harder to
determine, or imagine in the real world. A first criteria, based on common
business sense, would be if the homeowner wanted to pay for it. A second
criteria, based purely on my personal observation and satisfaction in many
homes that I have tested with fan pressurization and tracer gas, might be
homes that test less than somewhere between 4 and 5 air changes at 50 Pa
pressure difference.
5. Out of the three main types of mechanical ventilation (a
sidebar will briefly describe exhaust, supply, and
balanced systems), certain types work better in certain
climates and markets. What type of system would you recommend
for "typical" homes in the following locations: Phoenix,
Arizona; Seattle, Washington; Fargo, North Dakota; Tampa,
Florida?
I will give my recommendations with respect to two different markets in
new construction- production homes (very first-cost sensitive) and custom
homes (not first-cost sensitive within the range of any ventilation
system).
Phoenix, Arizona; Production homes:
My recommendation for production homes in the Phoenix climate is,
without a doubt, central-fan-integrated supply ventilation. This system
would typically use a 5 to 8 inch insulated outside air duct, located to
maximize the quality of ventilation air, connected to the return side of
the central air distribution system, at a location that will provide
sufficient potential to draw in the design quantity of outside air. A
balancing damper should be installed in the outside air duct to field
adjust the air flow upon system startup, and provisions should be made to
filter the outside air prior to entering the central fan. See the example
schematic in Figure 1. As in any good home, the air distribution ducts
should be substantially airtight (sealed at every joint with approved
mastic or tape), or, better yet, inside conditioned space. The minimum fan
run time should be controlled by a fan recycling system, which cycles the
fan after a preset period of inactivity. An upgrade would be to include a
motorized damper in the outside air duct, also controlled by the fan
recycling system to limit the introduction of outside air to the design
level.
This type of central-fan-integrated supply ventilation system is an
energy efficiency strategy that utilizes the normal cycling of the fan, as
the fan operates to distribute conditioned air in response to calls from
the thermostat, to also distribute ventilation air at the same time. So,
for the amount of time that the air handler operates for heating or
cooling, ventilation air is distributed for free. Hourly simulations have
shown that the cost to periodically operate the air handler (33% duty
cycle), when there is no thermostat demand, is less than $50 per year
across the U.S.
An added benefit of this system, which can be very important to
builders and customers alike, is enhanced temperature and humidity comfort
control in conditioned spaces. Thermostats are typically located in a
central area and are expected to serve an entire zone that usually
includes closed rooms, and often, more than one floor level. Temperature
conditions can vary widely between the thermostat location and extremities
of the space the thermostat serves. This has been demonstrated in a
recently published survey of 1000 homeowners by a major controls
manufacturer. The top complaint was inconsistent temperature between
rooms, levels, or at different times of day. The central-fan-integrated
supply ventilation system will serve to smooth out temperature, humidity,
and air quality conditions in the house by circulating and mixing indoor
air.
Phoenix, Arizona; Custom homes:
My recommendation for custom homes in the Phoenix climate would be a
heat recovery ventilation (HRV) unit combined with a fan recycling system
on the central air handler. The purpose of the HRV unit would be to lower
the cost of conditioning ventilation air. The HRV would be ducted such
that it pulled inside air from the master bedroom and delivered the
ventilation air to the central area. When heating and cooling is active,
the ventilation air would be distributed throughout the house via the
central fan, the central fan recycling system would make sure that the
ventilation air was distributed when there was no demand for heating or
cooling. This strategy would be more effective overall, considering the
mixing benefits described above, and cost less, than installing a complete
separate air distribution system for the HRV. Keep in mind that most HRV/ERV
manufacturers recommend continuous operation of the central air handler
with operation of the HRV/ERV; the fan recycling system produces the same
effect with lower operating cost.
Figure 1 Schematic of central-fan-integrated supply ventilation
used in Phoenix
Seattle, Washington:
My recommendations for homes in the Seattle climate are the same as for
Phoenix. Referring to Figure 2, the outdoor dewpoint remains moderately
below the indoor comfort conditions throughout the year and outside air is
useful for interior moisture dilution. For extended mild periods in
Seattle, when no space conditioning is needed and windows are used for
ventilation, the central fan recycling system may be turned off.
Fargo, North Dakota; Production homes:
My recommendation for production homes in the Fargo climate, is a
continuous exhaust fan pulling air from the central area combined with a
lower level of central-fan-integrated supply ventilation with fan
recycling. This system provides a base rate of exhaust ventilation with
intermittent supply ventilation and mixing. While the central fan is
operating, the ventilation system will be balanced, and the source of
outside air is known and can be filtered. The continuous exhaust allows
the outside air fraction of the central-fan-integrated system to be less
than would be used for warmer climates.
An upgrade to this system would be to pull the exhaust air from two or
three locations in the house (central area, master bedroom, family bath)
instead of the central area alone. This will improve ventilation air
distribution when the central fan is not operating, and, if all duct
connections are made in the same mechanical room, it can be relatively
easy to convert to an ERV/HRV system (see recommendation for Custom home
below). The creative "ERV/HRV ready" concept was first suggested
by collaborators in Minnesota.
Fargo, North Dakota; Custom homes:
My recommendation for custom homes in the Fargo climate would be an
energy recovery ventilation (ERV) unit combined with a fan recycling
system on the central air handler. The purpose of the ERV unit would be to
lower the cost of conditioning ventilation air, and, in winter, to recover
back some interior generated moisture to keep the house from being too
dry. The ERV would be ducted such that it pulled inside air from the
master bedroom and delivered the ventilation air to the central area. When
heating and cooling is active, the ventilation air would be distributed
throughout the house via the central fan, the central fan recycling system
would make sure that the ventilation air was distributed when there was no
demand for heating or cooling. This strategy would be more effective
overall, considering the mixing benefits described above, and cost less,
than installing a complete separate air distribution system for the ERV.
Keep in mind that most ERV/HRV manufacturers recommend continuous
operation of the central air handler with operation of the ERV/HRV; the
fan recycling system produces the same effect with lower operating cost.
ERV's as opposed to HRV's are most useful when the average outdoor
dewpoint temperature is a lot below the indoor dewpoint during winter and
above the interior dewpoint temperature in summer. The closer the outdoor
dewpoint is to the indoor dewpoint, the lower the ERV benefit. Where the
outdoor dewpoint is a lot below the interior dewpoint, the ERV benefit is
in recovering back some of the interior generated moisture to keep the
house from being too dry. Where the outdoor dewpoint is above the interior
dewpoint, the ERV benefit is to reduce the incoming moisture load in the
outside air to reduce cooling/dehumidification energy consumption and to
potentially keep the house dryer. Referring to Figure 2, it can be seen
that an ERV can be most effective in Fargo, ND and Tampa, FL. However,
contrary to a common misconception, the ERV cannot be used as a
dehumidifier. In Tampa, FL, over extended periods with no cooling system
operation, an ERV will not keep interior moisture below the interior
dewpoint threshold shown. Table 1 shows the corresponding interior drybulb
temperature and relative humidity used to calculate the interior dewpoint
threshold for each season.
Figure 2 Mean monthly outdoor dewpoint temperature for
various locations, and interior dewpoint threshold temperature
Table 1 Typical interior conditions by
season: drybulb temperature
and relative humidity, giving interior dewpoint temperature threshold
Tampa, Florida; Production homes:
My recommendation for production homes in the Tampa climate is
central-fan-integrated supply ventilation, with central fan recycling, and
a stand-alone dehumidifier. The dehumidifier and the air handler can be
installed in the same mechanical closet and use the same condensate drain
line. The dehumidifier is not a penalty resulting from the ventilation
system, it is needed in that climate anyway, with or without mechanical
ventilation, to maintain good humidity control throughout the year.
Throughout much of the year, even during summertime at night, humidity
control cannot be maintained with the cooling system alone. The cooling
system may not operate for long periods because the sensible heat load is
low, and the thermostat, which only responds to temperature, not humidity,
is satisfied. Exterior moisture continues to enter the house through
infiltration or ventilation and interior generated moisture is added to
that. During those times, a separate dehumidifier is required to maintain
relative humidity below a healthy 50% to 60%.
Supply ventilation will tend to pressurize an interior space relative
to the outdoors, causing inside air to be forced out through leakage
openings located randomly throughout the building enclosure. This strategy
is advantageous in warm-humid climates to minimize moisture entry into the
building structure from outdoors. In mechanically cooled buildings in
hot-humid climates, if interior negative pressure causes moisture laden
outdoor air to enter the building enclosure, moisture can condense on cool
surfaces, and, if restricted from drying to the inside, material
durability and indoor air quality problems can result.
Tampa, Florida; Custom homes:
My recommendation for custom homes in the Tampa climate is a
high-efficiency dehumidifier that filters and pre-conditions a mixture of
inside and outside air and delivers the ventilation air to the central
area, with central fan recycling for whole-house circulation and mixing.
The dehumidifier can be energized by a ventilation timer and an indoor
dehumidistat. This is a premium system for the hot-humid climate, and will
provide the highest quality of ventilation and indoor humidity control.
6. Other than climate, what else must be considered in terms
of choosing a system? What are the pros and cons of each system?
The source of outside air must be considered. The source is unknown for
exhaust ventilation systems. Exhaust ventilation systems draw outside air
from leakage openings and pathways located randomly throughout the
building enclosure, thus, it is not possible to treat the outside air
before it enters the living space. The "ventilation" air could
be fresh and healthy, or it could be coming from locations with high
pollutant concentrations. For example, ventilation air drawn from a
garage, crawl space, basement sump, or from underneath a concrete slab may
induce entry of fuel vapors, combustion gases, insecticides, radon gas,
excessive water vapor, and fungal or mold spores. The source of outside
air is known for supply and balanced ventilation systems. The ventilation
air can be treated before distribution to the living space (i.e. one or
more of: heated, cooled, dehumidified, filtered, cleaned).
Marketability must be considered. While many energy purists would like
to see systems installed that have the absolute lowest operating cost,
even if it amounts to a relatively small dollar value, getting off ground
zero with first-cost sensitive production homebuilders is a very important
step in the process of change in the, understandably so, conservative and
evolutionary, not revolutionary, homebuilding industry.
Ease of use and maintenance should be considered. An advantage of
central-fan-integrated ventilation is that the same air handler that
distributes air for heating and cooling is used to distribute ventilation
air, and that fan and filter system is likely to be kept in working
condition and be used. Other ventilation systems may not be repaired or
replaced if broken, and regular maintenance may not be as obvious.
7. Exhaust systems are the least expensive and most common
type of mechanical ventilation system, and the most frequently installed
in homes. How severe are the hazards associated with the house
depressurization caused by exhaust ventilation? What is the best way to
mitigate such hazards?
Building depressurization in hot-humid climates, other than due to
normal intermittent operation of bathroom fans, kitchen range hood, and
dryer, is hazardous. Use of low permeability interior finishes increases
the problem greatly. Humid outdoor air will randomly enter the building
enclosure, even following circuitous paths within partitions and
interstitial spaces, where it comes in contact with surfaces that are
below the air dewpoint temperature. If those materials stay moist long
enough, mold will result.
Combustion appliances inside houses should be at least power vented,
and preferably sealed combustion. In such houses, it would be unlikely
for whole-house exhaust ventilation systems alone to cause a problem
with combustion safety. Whole-house exhaust ventilation along with other
exhaust devices- clothes dryer, kitchen range hood or down-draft,
bathroom fans, power vented gas hot water heater and furnace- can cause
more severe depressurization requiring an intended make-up air source. |
