ASHRAE Journal - May 2022 - 46

TECHNICAL FEATURE
Increased Ventilation
Manufacturers of unvented heaters are certainly
aware that this class of appliance produces products of
combustion. Manufacturers' recommendations may
include advice to increase ventilation, but do not normally
inform the user how much the ventilation should
be increased. The position document on unvented
combustion1 states that additional ventilation should
be explored. This sidebar investigates the benefi ts of
increasing ventilation beyond minimum values and thus
how much ventilation is necessary to maintain acceptable
IAQ for a given capacity of unvented space heater.
Figure 7 shows the ventilation rate (as an air change
rate) necessary to meet acceptable contaminant levels
for each of the four contaminants of concern. The
capacity of the heater is normalized by the space volume
on the x-axis.
Standard 62.2-2019 has a formula for determining
ventilation rate as a function of home size and occupancy.
For large, low-occupancy homes, that can translate
into an air change rate as low as 0.3. For small highoccupancy
homes, it can be larger. For convenience,
0.35 is often used as a representative value.
In Figure 7, one sees how much ventilation is needed to
control the concentration of each of the four contaminants
of concern to the air quality acceptance levels
Nitrogen Dioxide
Based on NO2 limits and minimally meeting the relevant
standards, the indoor air quality will be unacceptable
when the appliance is operated at a capacity greater
than 0.42 Btu/h · ft3 (4.3 W/m3) with methane as the fuel
as shown in Figure 3b.
Carbon Monoxide
Based on CO and minimally meeting the relevant standards,
the indoor air quality will be unacceptable when
the appliance is operated at a fi ring rate greater than
1.64 Btu/h · ft3 (17.0 W/m3) as shown in Figure 3b.
The incremental production of the contaminants
of concern are compared as a function of normalized
capacity to determine the capacity at which production
exceeds acceptable levels. Figure 4 shows this data
relative to the acceptability criteria developed earlier
and shows that nitrogen dioxide and carbon dioxide
are the two most critical contaminants of concern
46
ASHRAE JOURNAL ashrae.o rg M AY 2 0 2 2
FIGURE 7 Total air exchange rate vs. normalized heater capacity to meet the
acceptable incremental levels of the four contaminants of concern. A reference
line is drawn at 0.35 ACH.
2.0
1.5
1.0
0.5
Ventilation at Incremental Contaminant Limits
0.35 ACH
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Normalized Heater Capacity (Btu/h · ft3)
developed in this article. The steepest curve is the most
stringent, and for any home minimally meeting the two
standards, NO2 is the critical contaminant (ignoring
any possible synergies). Because of the steepness of the
nitrogen dioxide curve, impractically large amounts of
extra ventilation would be necessary to go much above
the 0.42 Btu/h · ft3 (4.3 W/m3) capacity that is allowed at
0.35 ACH.
Improved NO2 emissions for these products, however,
is certainly a possibility. The EPA achieved reduction
in outdoor concentrations of the oxides of nitrogen
through a variety of means including encouraging
since they are the fi rst ones to reach their acceptability
limits.
For any specifi c space size, operating at the acceptability
limits can translate into a nominal appliance
capacity. Figure 5 shows this relationship for the four
contaminants of concern. As before, nitrogen dioxide
and carbon dioxide limits are the most restrictive for the
capacity of the equipment.
Discussion
The development earlier is based on calculations using
methane as the fuel source and minimal compliance
with standards (i.e., 0.35 ACH for the ventilation rate,
20 ppm air-free for NO2 emission and 200 ppm air-free
for CO emission). These assumptions can be expanded
to see if there are any countervailing issues. Ideally,
empirical fi eld measurements of equipment and homes
that minimally meet this standard are most helpful.
While measurements are not expected to violate the laws
Ventilation Rate (h-1 )
70 ppb NO2
1,500 ppm CO2
20% RH
9 ppm CO
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