ASHRAE Journal - May 2022 - 47

TECHNICAL FEATURE
improved technology-from catalytic converters to lowNOx
burners. Technologies in this type of appliance
have been and can be expected to improve. It is reasonable
to assume that nitrogen dioxide emissions might
be substantially lower in the future-in which case, NO2
may not be the critical contaminant of concern.
Consider the case where the NO2 production can be
shown to be substantially less than 20 ppm. In such a
case, nitrogen dioxide is no longer the critical contaminant
of concern; carbon dioxide is. The data in Figure 7
can be reconstructed to determine when the emission
of NO2 and CO2 cross. When the NO2 emission rate is
low enough, carbon dioxide becomes the critical contaminant,
and the situation becomes CO2 dependent.
Figure 8 delineates the combinations of capacity and
NO2 emission that are required for CO2 dependence.
Anywhere below the curve in Figure 8, carbon dioxide
is the critical contaminant, not nitrogen dioxide, and
can be controlled by a reasonable amount of extra ventilation.
The maximum capacity for a CO2-dependent
case is approximately 0.47 Btu/h · ft3 (4.9 W/m3). At the
ventilation rates in Standard 62.2-2019 (i.e., 0.35 ACH),
CO2 dependence begins at about 17 ppm air-free
emission for NO2 (for methane as the fuel). At a low
enough emission rate (e.g., below 5 ppm air-free),
nitrogen dioxide ceases to be a contaminant of concern,
of physics, one can often fi nd that systems don't work as
intended.
There are not, unfortunately, fi eld measurements
of homes and equipment that minimally meet both
standards, but some measurements of earlier versions
exist of each. Perhaps the best published fi eld study
is by Francisco,25 which measured use patterns and
contaminant concentration in 30 occupied homes. The
equipment was not known to meet the current ANSI Z21
standard, nor were the homes known to meet ASHRAE
Standard 62.2 . Inferences are accordingly limited for the
purposes of this article, but several important inferences
can be gleaned from the data.
The measurements did not show humidity and carbon
dioxide levels as elevated as this analysis predicts. This is
likely due to large ventilation rates through infi ltration.
The ventilation rate was not measured, but the enclosure
air tightness was. The mean air tightness (12 ACH50) is
much larger than modern, code-compliant homes allow.
FIGURE 8 NO2 air-free emission below which CO2 becomes the critical contaminant
as a function of space heater capacity. The higher the capacity, the higher
will be the necessary ventilation rate to control the contaminant of concern.
Contaminant Dependence
20
15
10
5
NO2 Dependent
CO2 Dependent
2
4
6
Normalized Capacity (Btu/h · ft3)
regardless of capacity, as long as carbon dioxide and
water vapor are controlled through ventilation.
An opportunity in the CO2-dependent case is allowing
the use of a well-developed ventilation strategy
to extend the allowed capacity of unvented heaters to
larger capacity values. Demand-controlled ventilation
(DCV) is a strategy used to adjust the ventilation rate
based on CO2 levels. If the DCV system is set to control
the carbon dioxide concentration to no more than
2,500 ppm, the capacity of the space heater would only
be limited by the capacity of the DCV system-as shown
by the CO2 curve in Figure 7.
In winter, this is going to produce much larger ventilation
rates than Standard 62.2 minimum requirements
and thus substantially increase contaminant dilution
compared to modern homes.
Despite the likely high ventilation rates, the data show
excessive levels of NO2 and CO. This is presumably due
to excessive emissions from the equipment. There were
no measurements to determine whether the equipment
met the air-free emission limits used in this analysis or
required by the current ANSI Z21 standard. Thus, we
cannot determine if other issues exist (e.g., local mixing)
or if this is just an equipment evolution issue. It
would be interesting to repeat such a study to determine
if there are any surprises for current ANSI Z21.11.2compliant
equipment and 62.2 -compliant homes,
but for now none of these issues impact this article's
fi ndings.
Poor mixing could cause incomplete combustion,
and it could also subject occupants to more elevated
M AY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
47
8
10
Air Free ppm
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ASHRAE Journal - May 2022

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