ASHRAE Journal - May 2022 - 44

TECHNICAL FEATURE
Propane
Fuel gasses are generally composed of hydrocarbons
called alkanes made up of carbon and hydrogen. All
alkanes have a ratio of hydrogen atoms to carbon atoms
of more than two. Methane-the principal component
of natural gas-has the highest ratio at four, specifi -
cally one carbon atom and four hydrogen atoms. When
alkanes are completely oxidized (i.e., cleanly burned)
they create water and carbon dioxide molecules at half
the ratio of the alkane. The weight and heat content of
each alkane also differs.
For simplicity the calculations in this article assume
that it is pure methane that is being burned. Natural
gas is mostly compromised of methane but may have
higher alkanes (e.g., ethane, which has two carbon
atoms and six hydrogen atoms) mixed in. The results
would be slightly, but not substantially, different for a
mix of alkanes.
Some equipment conforming to Z21.11.2 uses propane,
which has three carbon atoms and eight hydrogen
atoms per molecule, as fuel. To see if the fuel mix
is likely to have a signifi cant impact on the results from
the main article, some of the calculations are reexamined
using propane instead of methane.
The NO2 limit specifi ed in the Z21 standard is set
in terms of parts per million. While it is true that the
mixture of exhaust gasses is a bit different between
methane and propane, most of the exhaust is made up
of the nitrogen in the air that took no part in the combustion.
Thus, the emission rate in terms of mass of
The input for the mass balance model is summarized
in Table 1. The exact equations are available as supplemental
material that can be found at https://tinyurl.
com/48krk3e4 .
Results
We can now use the mass balance model to determine
the impact that Z21.11.2 minimally compliant appliances
will have on a minimally compliant Standard
62.2-2019 dwelling for the four contaminants of concern.
The incremental concentration increases in the
contaminants of concern will be shown as a function of
the capacity of the appliance. Like the ventilation, the
capacity is normalized by the volume of the space under
44
ASHRAE JOURNAL ashrae.o rg M AY 2 0 2 2
FIGURE 6 Incremental CO2 increase as a function of heater capacity for unvented
combustion fueled by methane and propane. Compare with Figure 3a.
CO2 Concentration: Propane versus Methane
6,000
5,000
4,000
3,000
2,000
1,000
CO2 Limit
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)
nitrogen dioxide is essentially the same. Accordingly,
the changes in the NO2-based limit of 0.42 Btu/h · ft3
(4.3 W/m3) are negligible and can be ignored.
The same cannot be said for the other two contaminants
of concern. Propane produces more CO2 and
less water than methane. Since carbon dioxide was
the more important contaminant in the methane case,
the humidity impacts of propane are not considered.
However, Figure 3a is revamped here to show the carbon
dioxide impacts of propane vs. methane.
Figure 6 shows the propane curve is almost 20% higher
and thus more relevant when CO2 is the critical contaminant.
In particular we see that the point at which the
propane production reaches the 1,500 ppm acceptance
limit is lower at 0.40 Btu/h · ft3 (4.1 W/m3). That then
becomes the relevant capacity limitation based on CO2.
consideration. The plots are in I-P units. The reader
should note that 1 Btu/h · ft3 is approximately 10 W/m3.
Carbon Dioxide
Based on CO2 limits and minimally meeting the relevant
standards, the indoor air quality is unacceptable
when the appliance is operated at a capacity greater
than 0.47 Btu/h · ft3 (4.9 W/m3) as shown in Figure 3a.
Water Vapor
Based on H2O limits and minimally meeting the relevant
standards, the indoor air quality will be unacceptable
when the appliance is operated at a capacity greater
than 0.77 Btu/h · ft3 (8.0 W/m3) as shown in Figure 3a.
Incremental CO2 (ppm)
Propane (C3H8)
Methane (CH4)
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ASHRAE Journal - May 2022

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