ASHRAE Journal - May 2022 - 33

TECHNICAL FEATURE
outdoor air exchange rates (ventilation), especially in
winter. These large ventilation rates diluted indoor-generated
contaminants that might otherwise be problematic
but this quantity of ventilation air could cause discomfort
and require large heating and cooling systems.
Increased concerns about energy costs, comfort and
climate change motivated greater levels of energy effi -
ciency. Homes became tighter and eventually there was
a concern that being too tight could lead to unacceptable
IAQ unless systems were designed and operated to provide
minimum ventilation quantities. In 2003, ASHRAE
published the fi rst ANSI standard on residential ventilation,
Standard 62.2, Ventilation and Acceptable Indoor Air
Quality in Residential Buildings.1 The most recent version of
this standard was released in 2019.
Standard 62.2 contains a variety of requirements,
including minimum dwelling ventilation rates, source
control measures and equipment specifi cations. The
ventilation rate depends on the size and occupant density
of the home.
The purpose of Standard 62.2 is to provide acceptable
indoor air quality (AIAQ), which is defi ned as when a substantial
majority of occupants express no dissatisfaction with
respect to odor and sensory irritation and in which there are not
likely to be contaminants at concentrations that are known to
pose a health risk. The standard notes that this may not
be achieved when high-polluting events occur. These
are isolated and occupant controllable events that release
pollutants in excess quantities. An activity that produces
occupant dissatisfaction or where there are likely to be
contaminants that pose a health risk qualifi es as a high
polluting event.
In this article we will attempt to quantify what " excess
quantities " means for unvented room heaters in the
context of Standard 62.2. Note that any recommendations
or requirements ASHRAE might make about
unvented combustion in homes must be made in the
context of not only Standard 62.2, but also other related
documents of the Society.
Position Document on Unvented Combustion
Many ASHRAE documents and much published
research exist relating to unvented combustion, but
a key piece is the ASHRAE Position Document (PD) on
Unvented Combustion Devices and Indoor Air Quality.2
This PD was fi rst approved by ASHRAE in 2012 and
was reaffi rmed in 2020. This section highlights and
summarizes particular parts of the PD, and the reader
is encouraged to consult it directly for references and
other details.
The primary products of combustion are carbon
dioxide (CO2), water vapor (H2O), carbon monoxide
(CO) and nitrogen dioxide (NO2). The fi rst two are the
unavoidable products from burning hydrocarbons, and
the latter two are the practically unavoidable products
from not being able to burn hydrocarbons perfectly. The
latter two can have very severe health impacts. We shall
examine all of these contaminants in more detail in later
sections.
Depending on the fuel and technology used, unvented
combustion is also known to produce fi ne and ultrafi ne
particles, highly irritating compounds (e.g., aldehydes
such as acrolein) and carcinogenic compounds (e.g.,
benzene or polycyclic aromatic hydrocarbons), among
others.3 All of these compounds are known to be harmful
to occupants exposed to them, but suffi cient research
on appliance production of these harmful compounds
has not been accomplished to enable quantitative estimates
of the risks posed. Currently, engineering judgement
would need to be applied to a particular situation
to determine if any of these risks are signifi cant, and
they will not be analyzed further here.
Nitrogen dioxide (NO2) is what the Environmental
Protection Agency calls a criteria pollutant; outdoor limits
have been set by the EPA at 100 ppb for a one-hour
exposure.4 EPA does not explicitly state that its level
is appropriate for use indoors, but the World Health
Organization (WHO) sets a level of 110 ppb for both
indoor and outdoor one-hour levels,5 suggesting that
EPA's outdoor level could be equally well used indoors.
Carbon monoxide is also a criteria pollutant. The
EPA eight-hour (outdoor) exposure limit is 9 ppm.4 For
comparison, physical symptoms of carbon monoxide
poisoning do not tend to appear until approximately 100
ppm. In properly functioning equipment, carbon monoxide
does not tend to be produced in large amounts
except in a low-oxygen environment, which can be
caused by insuffi cient ventilation or re-entrainment of
exhaust gases. Some appliances have oxygen depletion
sensors, which can help limit life-threatening levels of
CO production.
ASHRAE Position
The PD on unvented combustion lists 18 Society
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ASHRAE Journal - May 2022

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