W415-2845 / B / 11.10.20
40
IOM
IOM
FIGURE 27 - MANOMETER MEASURING GAS MANIFOLD PRESSURE
H18.6.4A
IMPORTANT:
NEVER ADJUST THE INPUT
OF THE FURNACE TO
EXCEED THE INPUT SHOWN
ON THE RATING PLATE.
Input =
Heating Value of Gas x 3600
Time in Sec. for 1 cu. ft.
where:
• input is expressed in Btu/hr
• heating value of the gas is expressed in Btu/ft
3
• and time is the number of seconds required for
the test dial to indicate 1 cubic foot.
If using a gas meter with SI (metric) units:
• 1 cubic foot = 0.0283 cubic meters
• 1 cubic meter = 35.315 cubic feet
• 0.01 cubic meter = 0.3531 cubic feet
• 0.5 cubic meter = 1.766 cubic feet
The natural gas supply pressure should be a maximum of 10.5” w.c. and minimum of 5” w.c (recommended 7”
w.c.). The LP gas inlet supply pressure should be 11” to 13” w.c.. The input rating of the furnace is based on
1000 Btu/ft.³ gas with a specifi c gravity of 0.6.
Since heating values for the gas vary geographically, the actual furnace input and output will vary accordingly.
For example, natural gas with a 900 Btu/ft.³ heating value will reduce the input to 90% of the rated input.
Natural gas with a 1100 Btu/ft.³ heating value will increase the input to approximately 110% of the rated input.
This is not usually a problem; however, adjustments to compensate for this can be made by minor adjustments
to the burner manifold pressure or by changing the burner orifi ce size. Contact the fuel supplier for specifi c
heating content values.
ny adjustments to the burner manifold pressure should be carried out with the use of a manometer or
calibrated Magnehelic® gauge. Do not adjust the gas valve pressure regulator more than ±0.3” w.c. from that
specifi ed on the rating label. Refer to Figure 27.
13.6 CHECKING FURNACE INPUT
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