ASHRAE Journal - May 2022 - 41

TECHNICAL FEATURE
TABLE 1 Input data used for mass balance model.
CONTAMINANT
OF CONCERN
EMISSION
REFERENCE
CO2
Stoichiometry
H2O Stoichiometry
CO
NO2
Z21.11.2
Z21.11.2
INCREMENT
LIMIT
1,500 ppm
20% RH
9 ppm
70 ppb
VENTILATION
RATE
0.35/h
0.35/h
0.35/h
0.35/h
REACTIVE
DECAY RATE
0.8/h
Vented gas equipment may exceed this capacity limit
but is presumed to not directly contribute to these contaminants.
Similarly, gas cooking appliances may exceed
this limit, but Standard 62.2 requires kitchen exhaust
go directly to outside to address cooking-related contaminants.
Furthermore, the Standard 62.2 committee
is currently developing kitchen hood capture effi ciency
criteria to improve cooking source control. Therefore,
the issue of such equipment will not be directly considered
here. Ineffi ciencies in controlling emissions from
cooking could, however, contribute to background contaminant
levels.
The length of time occupants will use unvented heaters
depends on their thermal preferences, the quality of the
thermal envelope and weather conditions-all of which
are independent of Standard 62.2 (and ANSI Z21.11.2).
From the point of view of determining impacts, the conservative
assumption is that appliances will be operated
at their capacity for long enough to reach steady-state
conditions. Francisco25 discusses this issue for the cases
they studied.
Source Emission Rates
To estimate the water and carbon dioxide emissions,
we use the chemistry of completely burning the fuel.
Specifi cally, the complete combustion of methane is
assumed, although propane appliances are also available,
and natural gas is a methane-heavy mixture of
gases. For every molecule of methane combusted, there
is one molecule of carbon dioxide and two molecules of
water created. (See " Propane " sidebar on page 44, which
discusses some of the impacts of using higher-order
alkanes.)
If combustion were perfectly stoichiometric, no
CO would be formed, but nothing is ever perfect.
The Z21.11.2 standard requires no more than 0.02%
(200 ppm) carbon monoxide (air-free) be emitted when
tested in the laboratory. Real-world performance could
be better or worse, but we shall use that value.
FIGURE 3A Incremental carbon dioxide and water vapor production as a function
of appliance capacity. The left-hand axis is for the CO2 curve and the right-hand
axis is for the H2O curve.
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
Incremental CO2 and H20 Concentration
20% RH
1,500 ppm
40
35
30
25
20
15
10
5
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)
FIGURE 3B Incremental nitrogen dioxide and carbon monoxide production as a
function of appliance capacity. The left-hand axis is for the NO2 curve and the
right-hand axis is for the CO curve.
200
180
160
140
120
100
80
60
40
20
Incremental NO2 and CO Concentration
70 ppb
9 ppm
20
18
16
14
12
10
8
6
4
2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Normalized Unvented Heater Capacity (Btu/h · ft3)
NO2 often forms during combustion in these appliances,
as oxygen and nitrogen react at higher temperatures.
The Z21.11.2 standard requires there be no
more than 0.002% (20 ppm) nitrogen dioxide (air-free).
Similarly, we shall use that value in our mass balance
calculations.
Note that " air-free " as used in the Z21.11.2 is an
adjusted concentration value that converts combustion
with excess supply air to an equivalent stoichiometric
combustion process with no excess air, but with the
same amount of carbon monoxide or nitrogen dioxide
generated per unit of fuel (methane). These air-free
adjustments are also based on measurements after all
water vapor is removed from the air sample.
Contaminant Removal Rate
Contaminants can be removed by a variety of mechanisms
including fi ltration, local exhaust and air
M AY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
41
Incremental NO2 (ppb)
Incremental CO2 (ppm)
Incremental CO (ppm)
Incremental H2O (% RH)
Nitrogen Dioxide (N02)
Carbon Monoxide (CO)
Carbon Dioxide (CO2)
Water Vapor (H2O)
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ASHRAE Journal - May 2022

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