SCHEDULED MAINTENANCE
16
3. Once heating cycle is at a steady state (typically 15
minutes of operation), use a stopwatch to time how
long it takes the smallest unit of measure dial on the
gas meter to make a full revolution. In Table 1, one
cubic foot is selected. The smallest unit of measure
will vary depending on the gas meter.
Table 1
4.
Using Table 2 below, find the number of seconds it
took for the dial to make a full revolution. To the right
of that number of seconds and below the Size of Test
Dial (selected in step 3 and shown in Table 1) will be
the Cubic Feet per Hour (CFH).
5. Use this formula to verify the Cubic Feet per Hour
(CFH) input determined in step 4 is correct:
(3600 x Gas Meter Dial Size) / Time (seconds) =
Cubic Feet per Hour (CFH)
6. Check with your local utility for actual BTU content
(caloric value) of natural gas in the area (the average
is 1025 BTU’s).
7. Use this formula to calculate the BTU/HR input (See
BTU/HR Calculation Example):
Cubic Feet per Hour (CFH) x BTU content of your
natural gas = BTU/HR input
8.
Should the figure you calculated not fall within five
(5) percent of the nameplate rating of the unit, adjust
the gas valve pressure regulator or resize orifices.
To adjust the pressure regulator on the gas valve,
turn downward (clockwise) to increase pressure and
input, and upward (counterclockwise) to decrease
pressure and input. A properly operating unit must
have the BTU per hour input and CFM of air, within
the limits shown to prevent short cycling of the
equipment. As the external static pressure goes up,
the temperature rise will also increase. Consult the
proper tables for temperature rise limitation.
BTU/HR Calculation Example:
The unit being tested takes 40 seconds for the 1 cubic foot
dial to make one complete revolution. Using the chart,
this translates to 90 cubic feet per hour. Based upon the
assumption that one cubic foot of natural gas has 1,025
BTU’s (Check with your local utility for actual BTU content),
the calculated input is 92,250 BTU’s per hour.
Furnace Nameplate Input in this example: 90,000 BTU/
HR
Calculated Gas Input in this example: 92,250 BTU/HR
This example is within the 5% tolerance input and does
not need adjustment.
Table 2
