Carrier 48FC 08-16 Series - GAS HEATING SYSTEM; General; Fuel Types and Pressures

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GAS HEATING SYSTEM
General
The heat exchanger system consists of a gas valve feeding multi-
ple in-shot burners off a manifold. The burners fire into matching
primary tubes. The primary tubes discharge into combustion ple-
num where gas flow converges into secondary tubes. The second-
ary tubes exit into the induced draft fan wheel inlet. The induced
fan wheel discharges into a flue passage and flue gases exit out a
flue hood on the side of the unit. The induced draft fan motor in-
cludes a flue gas pressure switch circuit that confirms adequate
wheel speed through the Integrated Gas Control (IGC) board.
Safety switches include a Rollout Switch (at the top of the burner
compartment; see Fig. 52) and a limit switch (mounted through
the heat exchanger cover panel, over the tubes). (See Fig. 53.)
Fig. 52 — Burner Section Details
Fig. 53 — Gas Limit Switch Location
Fuel Types and Pressures
NATURAL GAS
The 48FC unit is factory-equipped for use with natural gas (NG)
fuel at elevation under 2000 ft (610 m). See section “Orifice Re-
placement” for information in modifying this unit for installation
at elevations above 2000 ft (610 m).
Gas line pressure entering the unit’s main gas valve must be within
specified ranges (see Table 6). Adjust unit gas regulator valve as
required or consult local gas utility.
Manifold pressure is factory-adjusted for NG fuel use. Adjust as
required to obtain best flame characteristic (see Table 7).
LIQUID PROPANE
Accessory packages are available for field-installation that will
convert the 48FC unit to operate with liquid propane (LP) fuels.
These kits include new orifice spuds, new springs for gas valves
and a supply line low pressure switch. See High Altitude Gas
Conversion Kit Gas Heating/Electric Cooling 3-15 Ton Small
Rooftop Units Accessory LP (Liquid Propane) Installation In-
structions for details on orifice size selections.
Fuel line pressure entering unit gas valve must remain within
specified range (see Table 8).
Manifold pressure for LP fuel use must be adjusted to specified
range (see Table 9). Follow instructions in the accessory kit to
make initial readjustment.
Induced-
Draft
Motor
Mounting
Plate
Induced-
Draft Motor
Manifold
Pressure
Tap
Vestibule
Plate
Flue
Exhaust
See Detail
Blower
Housing
Gas
Valve
Burner
Section
Pressure Switch
Rollout Switch
Switch Mount Detail
Heat Exchanger
Cover Panel
Gas Limit Switch
Unit Center Post
Table 6 — Natural Gas Supply Line Pressure Ranges
UNIT MODEL UNIT SIZE MIN. MAX.
48FC(D/E/F/S/R/T) 08, 09, 12, 14
4.0 in. wg
(996 Pa)
13.0 in. wg
(3240 Pa)
48FC(D/E/F/S/R/T) 16
5.0 in. wg
(1250 Pa)
13.0 in. wg
(3240 Pa)
Table 7 — Natural Gas Manifold Pressure Ranges
UNIT MODEL UNIT SIZE HIGH FIRE LOW FIRE
a
NOTE(S):
a. LOW FIRE, 1.7 in. wg (423 Pa), applies to the following units only: 
48FC(D/S)*08 and 48FC(D/S)*09.
48FC(D/E/F/S/R/T) 08, 09, 12, 14, 16
3.5 in. wg
(872 Pa)
2.0 in. wg
(498 Pa)
Table 8 — Liquid Propane Supply Line Pressure
Ranges
UNIT MODEL UNIT SIZE MIN. MAX.
48FC(D/E/F/S/R/T) 08, 09, 12, 14, 16
11.0 in. wg
(2740 Pa)
13.0 in. wg
(3240 Pa)
Table 9 — Liquid Propane Manifold Pressure Ranges
UNIT MODEL UNIT SIZE HIGH FIRE LOW FIRE
a
NOTE(S):
a. LOW FIRE, 5.0 in. wg (1244 Pa), applies to the following units only: 
48FC(D/S)*08 and 48FC(D/S)*09.
48FC(D/E/F/S/R/T) 08, 09, 12, 14
10.0 in. wg
(2490 Pa)
5.7 in. wg
(1418 Pa)
48FC(D/E/F/S/R/T) 16
10.0 in. wg
(2490 Pa)
6.6 in. wg
(1644 Pa)