035-20499-001 Rev. B (1004)
18
Unitary Products Group
In the USA use the following formula to calculate the furnace input.
For natural gas multiply the heat content of the gas BTU/SCF (or Default 1030 BTU/SCF (38.4 MJ/m
3
), times 2 cubic ft. (0.056 m) of gas measured
at the gas meter, times a barometric pressure and temperature correction factor of 0.960; times 3600, then divided by the time (In seconds) it took to
measure 2 cubic ft. (0.056 m) of gas from the gas meter.
For propane (LP) gas multiply the heat content of the gas BTU/SCF (or Default 2500 BTU/SCF (93.15 MJm
3
), times 1 cubic ft. (0.028 m) of gas mea-
sured at the gas meter, times a barometric pressure and temperature correction factor of 0.960; times 3600, then divided by the time (In seconds) it
took to measure 1 cubic ft. (0.028 m) of gas from the gas meter.
The formula for US input calculation using a cubic foot gas meter:
In Canada you will use the following formula to calculate the furnace input if you are using a cubic foot gas meter.
For Natural Gas multiply the Heat content of the gas MJ/m
3
(or Default 39.2), times 2 cubic ft. of gas x 0.02831 to convert from cubic feet to cubic
meters measured at the gas meter, times a barometric pressure and temperature correction factor of 0.960; times 3600, then divided by the time it
took to measure 2 cubic ft. (0.056 m) of gas from the gas meter.
For Propane (LP) Gas multiply the Heat content of the gas MJ/m
3
(or Default 93.14), times 1 cu. ft. of gas x 0.02831 to convert from cubic feet to
cubic meters measured at the gas meter, times a barometric pressure and temperature correction factor of 0.960; times 3600, then divided by the
time it took to measure 1 cubic ft. (0.028 m) of gas from the gas meter.
The formula for metric input calculation using a cubic foot gas meter:
In Canada use the following formula to calculate the furnace input if you are using a gas meter that measures cubic meters.
For Natural Gas multiply the Heat content of the gas MJ/m
3
(or Default 39.2), times 0.0566 m
3
of gas measured at the gas meter, times a barometric
pressure and temperature correction factor of 0.960; times 3600, then divided by the time it took to measure 2 cubic ft. (0.0566 m
3
) of gas from the
gas meter.
For Propane (LP) Gas multiply the Heat content of the gas MJ/m
3
(or Default 93.14), times 0.00283 m
3
of gas measured at the gas meter, times a
barometric pressure and temperature correction factor of 0.960; times 3600, then divided by the time it took to measure 1 cubic ft. (0.0283 cm) of gas
from the gas meter.
The formula for metric input calculation using a cubic foot gas meter:
DO NOT ADJUST the manifold pressure regulator if the actual input is equal to or within 8% less than the furnace input specified on the rating plate
or if the furnace rise is above the specified rise range on the rating plate.
If the actual input is significantly higher than the furnace input specified on the rating plate
then replace the gas orifice spuds with the gas ori-
fice spuds of the proper size for the type of gas you are using.
For altitudes above 2,000 ft. (610 m) the furnace input MUST BE DERATED. Refer to the GAS CONVERSION FOR PROPANE (LP) AND HIGH
ALTITUDES IN SECTION IV for information on high altitude conversions.
BTU/f
3
x 2 cu.ft. x 0.960 x 3600
=
BTU/H
BTU/f
3
x 2 cu.ft. x 0.960 x 3600
=
BTU/H
Seconds it took to measure the 2 cu.ft. of gas
Seconds it took to measure the 2 cu.ft. of gas
NATURAL GAS INPUT CALCULATION
PROPANE (LP) GAS INPUT CALCULATION
EXAMPLE:
EXAMPLE:
1030 x 2 x 0.960 x 3600
=
79,997.38
2500 x 1 x 0.960 x 3600
=
80,000.00
90.5
108
Natural Gas
Propane Gas
BTU/SCF 1030
BTU/SCF 2500
MJ/m
3
x (2 cu.ft. x 0.028) x 0.960 x 3600
=
MJ/H
x
0.2777
=
kW
x
3412.14
=
BTU/H
Seconds it took to measure the 2 cu.ft. of gas
NATURAL GAS INPUT CALCULATION
EXAMPLE:
39.2 x (2 x 0.028) x 0.960 x 3600
=
84.83
x
0.2777
=
23.28
x
3412.14
=
79,433
90.5
Natural Gas
BTU/SCF 1030 = 39.2 MJ/m
3
PROPANE (LP) GAS INPUT CALCULATION
EXAMPLE:
93.15 x (1 x 0.028) x 0.960 x 3600
=
84.41
x
0.2777
=
23.45
x
3412.14
=
80,000.00
108
Propane Gas
BTU/SCF 2500 = 93.15 MJ/m
3
MJ/m
3
x m
3
x 0.960 x 3600
=
MJ/H
x
0.2777
=
kW
x
3412.14
=
BTU/H
Seconds it took to measure the 2 cu.ft. of gas
NATURAL GAS INPUT CALCULATION
EXAMPLE:
39.2 x 0.1 x 0.960 x 3600
=
84.67
x
0.2777
=
23.51
x
3412.14
=
80,231
160
Natural Gas
BTU/SCF 1030 = 39.2 MJ/m
3
PROPANE (LP) GAS INPUT CALCULATION
EXAMPLE:
93.15 x 0.028 x 0.960 x 3600
=
84.48
x
0.2777
=
23.18
x
3412.14
=
79,107
107.97
Propane Gas
BTU/SCF 2500 = 93.15 MJ/m
3
