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Estimating Rate
Manifold to Furnace Pressure information can be used to estimate the burner’s fi ring rate when it is not possible to
clock a meter for the rate.
To estimate the burner’s fi ring rate:
Measure the furnace pressure over fi re
Measure the manifold pressure (at the manifold pressure test connection in Figure 10).
Subtract the furnace pressure from the manifold pressure
Compare the result to the data in Figure 12 or 13 as appropriate for your burner.
Clocking a gas meter
Before you clock the meter you must make sure that the burner is operating at a steady rate, and that it is the only
thing that is using gas that would fl ow through the meter.
Uncorrected fl ow rate
Locate the dial that moves the fastest and determine how many cubic feet are represented by one revolution of its
dial.
Use your watch for timing and observe how many cubic feet are used in one minute.
Multiply by 60
Example: The dial is a one cubic foot dial, and goes around 7 ½ times in one minute. That gives 7.5 cubic feet per
minute. Multiply by 60 to get 450 cubic feet per hour (CFH) as your uncorrected fl ow rate.
Pressure correction
Gas is compressible, the higher the pressure on it, the smaller the number of cubic feet it occupies. To correct for that
you will need to know the gas pressure at the meter.
If there’s a gauge at the meter, read it while the gas is fl owing. (If not, measure the gas pressure in the line
downstream of the meter before any valves or regulators, with no gas fl owing.)
Convert the gas pressure to PSI if you measure it in any other units. (1 PSI = 27.7 inches water column)
Add 14.7 to the meter pressure, divide their sum by 14.7, and multiply that result times the uncorrected fl ow rate to
obtain the pressure corrected fl ow rate.
Example: The meter in the example above is operating at 14” WC.
14 / 27.7 = .5 PSI at the gauge; 14.7 + .5 =15.2; 15.2 / 14.7 = 1.034 correction factor;
1.034 x 450 = 465 CFH corrected for pressure
Temperature correction
Gas volume also changes with the temperature of the gas. Most gas meters are temperature compensated, and say so
on the face of the index and don’t require temperature correction. The temperature we correct to is 60° F. That’s close to
the temperature of the ground, so usually it’s not necessary to correct for temperature from underground lines.
Measure the gas temperature at the meter.
Add 460 to the gas temperature and divide 520 by the result to obtain the correction factor.
Multiply the pressure corrected fl ow rate times the temperature correction factor.
Example: The meter in the example is connected to an above ground line on a hot day and shows 100° F gas
temperature. 460 + 100 = 560; 520 / 560 = .929; 465 x .929 = 432 CFH corrected for pressure and temperature.
This fi gure is often called SCFH (for standard cubic feet per hour).
BTU’s and MBH
The BTU content of natural gas varies from one location to another, and even from day to day at a fi xed location. For
fi ring estimates it is usually assumed to be 1000 BTU’s per standard cubic foot.
To calculate BTU/Hr fi ring rates multiply SCFH times 1000.
Example: 432 SCFH x 1000 = 432,000 BTU/Hr.
MBH stands for thousands of BTU’s per hour. Since 1 cubic foot of natural gas contains 1000 BTU, 1 MBH equals 1
SCFH of gas fl ow.
Example: 432 SCFH = 432 MBH
Section: Start the Burner