Flue Gas Temperature and Heat in the Flue Gas Flow (Btu/hour)
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This computer program calculates the specific heat (Btu/lbF) and the enthalpy (Btu/lb) for
typical gases found in the flue gas of combustion systems. For instance if you just input an
assumed flue gas temperature the program will calculate the specific heat and enthalpy of each
component gas in the program. If you desire more information you can also input the mass flows (lbs/hour) of each component gas found in the flue gas (from a combustion analysis or
from a field test of the flue gas) and the program will calculate the energy flow (Btu/hour)
for each gas and then calculate the Total Flue Gas (Btu/hour) that is lost with the associated
flue gas flow.
Example #1  Determination of the Flue Gas Temperature from a Natural Gas Furnace
In this specific example the combustion device is an older lower efficiency Natural Gas
Home Furnace. The fuel is considered to be Standard Natural Gas (1040 Btu/std. ft3 ), the fuel consumption is 100,000 Btu/hr (HHV Basis), and the heat delivered to the home is 80,000 Btu/hour. The percent excess air for this combustion system is 10 %. The combustion air is drawn
from the home at 70 F and 35 % Relative Humidity. From a combustion analysis hand calculation (or from the NGBULT77 Program) you can determine the amount of combustion
air entering the furnace (mass flow or ACFM) and the mass flow of component flue gases that
are going up the stack. Below you will see this data along with the Total Flue Gas (Btu/hour)
flowing up the stack:
A. Furnace Energy Inputs
1. Fuel Consumed: 100,000.00 Btu/hour
2. Combustion Air Into the Furnace: 1,194.31 Btu/hour (17.8468 ACFM)
3. Total Energy Flow into the Furnace: 101,194.31 Btu/hour
B. Heat Recovered
1. Heat recovered (or delivered to the home): 80,000 Btu/hour
C. Heat in the Flue Gas
1. Heat in the Flue Gas = 101,194.31 – 80,000
= 21,194.31 Btu/hour
D. Component Gases In The Flue Gas (from combustion analysis or from a stack test)

Mass Flow 
Specific Heat 
Enthalpy 
Energy Flow 
1. Carbon Dioxide 
11.7888 lb/hr 
Unknown 
Unknown 
Unknown 
2. Nitrogen 
61.0971 lb/hr 
Unknown 
Unknown 
Unknown 
3. Oxygen 
1.6725 lb/hr 
Unknown 
Unknown 
Unknown 
4. SO2 
0.0001 lb/hr 
Unknown 
Unknown 
Unknown 
5. Argon 
0.0934 lb/hr 
Unknown 
Unknown 
Unknown 
6. Water Vapor 
9.6800 lb/hr 
Unknown 
Unknown 
Unknown 
7. Total Flue Gas 
84.3318 lb/hr 
Unknown 
Unknown 
21,194.31 Btu/hr 
E. Flue Gas Temperature Determination Using CPGASH56
The flue gas temperature can be determine in this case by an iterative process of
inputting the flue gas mass flows and an assumed flue gas temperature to see if
the Total Flue Gas Btu/hr is equal to 21,194.31 Btu/hour, if not you try another
assumed temperature.
Sample of a Complete Energy Analysis for a Lower Efficiency Induced Draft Natural Gas Furnace
Natural Gas Induced Draft Furnace (PDF)
Alternate Approach Used to Determine the Flue Gas Temperature for Natural Gas Combustion
There is another approach that can be used to determine the flue gas temperature from a
natural gas furnace . The attached diagram uses the Percent Excess Air and the Percent
Available Heat (this is the percentage of input energy that is delivered).
1. Percent Excess Air: 0 % to 1000 % (assumes 60 F combustion air)
2. Percent Available Heat =
Total Heat Delivered (Btu/hr) x 100
Total Heat Into The Furnace (Btu/hr)
For this specific case you enter the diagram with the following data:
Percent Excess Air = 10.00 %
Percent Available Heat =
80,000 Btu/hour heat delivered x 100
(100,000 + 1194.31) Btu/hr
= 79.06%
Chart Value for Flue Gas Temperature = 520 F
Flue Gas Temperature Illustration (PDF)
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