ASHRAE Journal - May 2022 - 42

TECHNICAL FEATURE
FIGURE 4 Impacts of unvented combustion compared to incremental contaminant
limits as a function of capacity. Concentration has been normalized to the
appropriate acceptability limit.
Limits of Acceptance: Contaminants of Concern
3.0
2.5
2.0
1.5
1.0
.5
0.0
0.2
0.4
0.6
0.8
1.0
FIGURE 5 Maximum unvented heater capacities for different size spaces and
contaminant acceptability limits. Note: 1 m2 = 10 ft2 and 1 Btu/h = 0.3 W.
Acceptance Limit
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
1.2
1.4
Normalized Heater Capacity (Btu/h·ft3)
cleaning. In normal use none of these mechanisms
apply to unvented space heaters and shall not be considered
here. The principal mechanism of contaminant
removal is dilution via outdoor air ventilation.
ASHRAE Standard 62.2 establishes the minimum
ventilation rate required for homes. Homes may have
more ventilation than the minimum, but this analysis
addresses those homes and appliances that are
minimally compliant. In the discussion (and sidebar
" Increased Ventilation " on page 46), the cases of
larger ventilation rates and smaller emission rates are
examined.
Standard 62.2 defines the minimum ventilation rate
requirements in terms of occupiable floor area and
number of bedrooms (the surrogate for occupants) in
the space. This means the minimum air change rate will
vary with ceiling height and occupant density. If normalized
by dwelling volume, the minimum ventilation
rate is expressed in air changes per hour (ACH) and is in
a relatively narrow band as shown in Figure 2, where we
have used 2016 U.S. Census data on floor area and number
of bedrooms to calculate the minimum ventilation
rate required by Standard 62.2.
We have chosen 0.35 ACH as the value to use for the air
change rate in this analysis. This value has an historical
basis in Standard 62, but the current version of Standard
62.2 allows a spread depending on house characteristics.
As seen in Figure 2, 0.35 ACH represents the 90% percentile
and thus keeps any conclusions from being too conservative.
Combining this air change rate with the emission
rates from earlier, the incremental concentration
increase of the contaminants of concern from operation
42
ASHRAE JOURNAL ashrae.o rg M AY 2022
of unvented appliances can be estimated. To decide on
acceptable contaminant levels, all four contaminants of
concern are evaluated individually from a health, safety
and perception viewpoint.
All of the contaminants of concern are removed by
dilution via the required ventilation rate. However, NO2
will also be removed at surfaces because it is a reactive
compound. Various rates for this reaction can be found
in the literature. For the reactive removal of nitrogen
dioxide, the rate of 0.8 h-1 from Traynor26 is used in this
analysis. This is a key value as our results would be more
conservative if we have overestimated it due to surface
reactivity. By the same token, if there were a catalytic
converter in operation, the issue would be far less
important.
Overall Mass Balances
The emission rates of carbon dioxide and water vapor
are directly proportional to the heater capacity based
on combustion stoichiometry. The Z21.11.2 limits for
nitrogen dioxide and carbon monoxide are also directly
proportional to the heater capacity as expressed by the
" air-free " emission measurement limits. The removal
mechanisms (dilution due to outdoor air ventilation)
and reactive decay for nitrogen dioxide are all directly
proportional to the volume of the space due to the basic
assumptions used in this analysis. One can show that
the resulting incremental concentrations of all four
contaminants are directly proportional to the normalized
unvented heater capacity (heater capacity per
unit space volume). Thus, the results shown below are
expressed as a function of the normalized unvented
heater capacity.
1.6
1.8
2
Acceptance Limits for Contaminants of Concern
1,000
2,000
3,000
Floor Area, ft2
4,000
5,000
Normalized Concentration
Maximum Heater Capacity, Btu/h
NO2
CO2
CO
CO2: 0.47 Btu/h·ft3
H2O
CO: 1.64 Btu/h·ft3
H2O: 0.77 Btu/h·ft3
NO2: 0.42 Btu/h·ft3
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