181
• Furnace is installed in orientation marked on vestibule. Ori
-
entation is specific to airflow direction through the heating
section of the unit.
• There is an adequate supply of fresh air for the combustion
and ventilation process. Combustion air openings in the
cabinet should be sized to provide 1 sq in. of free area per
4000 Btuh of input.
• A properly designed vent system is connected to the heating
unit to convey the products of combustion (flue gases) out
-
side the building. For outdoor applications be sure the flue
gases are directed away from any combustion air inlets.
• Duct furnace assembly is installed in a non-combustible
duct or cabinet on the positive pressure side of the circulat
-
ing air blower.
• An air flow proving switch is installed and wired to prove
operation of the system circulating air blower.
• An auxiliary manual reset limit switch is installed to shut off
furnace in the event of low airflow conditions due to filter
blockage, coil blockage and or damper failure.
• A drain tube is installed for disposal of condensate, if the
heating unit is equipped with modulating controls or is
located downstream of cooling system.
• Equipment access panels and doors are sized and located to
provide easy access for servicing, adjustment and mainte
-
nance of the heating units installed.
CONDENSATE DRAINS
In furnace applications operating with modulating controls, tem
-
perature rises below 40°F (4.4°C), or with outside make-up air,
some condensation may occur in the heating cycle. In these appli
-
cations connection of a condensate drain line is required to avoid
condensate build-up and possible heat exchanger damage.
If heating section is located downstream of a refrigeration system
or cooling coil, condensation can occur during operation of the air
conditioning, resulting in condensation from warm, moist air in
the heat exchanger tubes and flue collector. This condensate is not
harmful to the heat exchanger provided it is drained continuously.
For these applications a ¼-in. NPT connection is provided for at
-
tachment of condensate drain line to remove condensate from heat
exchanger.
A P-trap is recommended as the system operates under a negative
pressure. The use of a “Tee” fitting is recommended to allow for
priming and cleaning the trap. Use a plug in the cleanout opening.
See Fig. 143.
FURNACE COMPONENT IDENTIFICATION
See Fig. 144 for single-unit gas furnace components.
OPERATING AND SAFETY CONTROLS
The following controls are provided for the duct furnace:
Combustion Air Pressure Switch — An air pressure switch is
provided as part of the control system to verify combustion air
-
flow through induced-draft fan (ID fan) by monitoring the differ
-
ence in pressure between the ID fan and the atmosphere. If suffi
-
cient negative pressure is not present, indicating lack of proper air
movement through heat exchanger, the switch opens shutting off
gas supply though the ignition control module. On units with two-
speed draft inducer operation, a dual air pressure switch is used,
monitoring high and low speed pressures. The air pressure switch
-
es have fixed settings and are not adjustable.
Rollout Switch (Manual Reset) — The duct furnace is equipped
with manual reset rollout switch(es) in the event of burner flame
rollout. The switch will open on temperature rise and shut off gas
supply through the ignition control module. Flame rollout can be
caused by insufficient airflow for the burner firing rate (high gas
pressure), blockage of the vent system or in the heat exchanger.
The duct furnace should not be placed back in operation until the
cause of rollout condition is identified. The rollout switch can be
reset by pressing the button on the top of the switch.
Primary High Limit Switch — To prevent operation of the duct
furnace under low airflow conditions, the unit is equipped with a
fixed temperature high limit switch mounted on the vestibule pan
-
el. This switch will shut off gas to the heater through the ignition
control module before the air temperature reaches 250°F
(121.1°C). Reduced airflow may be caused by restrictions up
-
stream or downstream of the circulating air blower, such as dirty
or blocked filters or restriction of the air inlet or outlet to the unit.
The high limit switch will shut off the gas when the temperature
reaches its set point and then automatically reset when the tem
-
perature drops to 30°F (16.7°C) below the set point, initiating a
furnace ignition. The furnace will continue to cycle on limit until
the cause of the reduced airflow is corrected.
Ignition Control Module — Monitors furnace function and pro
-
vides safety interlocks. Ignition control modules are available hav
-
ing a number of different operating functions. Refer to Sequence
of Operation and Control Diagnostic data sheets provided in the
instruction package for a detailed description of the control fea
-
tures, operation and troubleshooting for the model control in
-
stalled. Check the furnace section for this information, or contact
your Carrier service representative.
COMBUSTION AIR SUPPLY
All gas-fired furnaces need an ample supply of air for proper and
safe combustion of the fuel gas. If sufficient quantities of
CAUTION
The presence of chlorine vapors in the combustion air sup
-
plied to gas-fired heaters presents a substantial corrosion
hazard.
WARNING
Gas-fired furnaces are not designed for use in hazardous
atmospheres containing flammable vapors or combustible
dust, in atmospheres containing chlorinated or halogenated
hydrocarbons, or in applications with airborne substances
containing silicone. Use in these environments may result
in damage to equipment and personal injury.
CAUTION
Do not use copper tubing for condensate drain lines. Flue
gas condensate is slightly acidic and may corrode lines.
4.00"
2.00"
TEE FITTING
PLUG
COLLECTOR BOX
Fig. 143 —
Condensate Drain
a39-4394
