Table of Contents
Advertisement
• Gas supply provided matches the Gas Type marked on the
duct furnace Rating Plate.
• 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.
The presence of chlorine vapors in the combustion air sup-
plied to gas-fired heaters presents a substantial corrosion
hazard.
• 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.
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.
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 1/4-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.
CAUTION
Do not use copper tubing for condensate drain lines. Flue
gas condensate is slightly acidic and may corrode lines.
CAUTION
4.00"
Fig. 143 — Condensate Drain
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 airflow through induced-draft fan (ID fan) by
monitoring the difference in pressure between the ID fan and the
atmosphere. If sufficient negative pressure is not present, indicat-
ing 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 pres-
sures. The air pressure switches have fixed settings and are not ad-
justable.
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 tem-
perature 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 condi-
tions, the unit is equipped with a fixed temperature high limit
switch mounted on the vestibule panel. 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 upstream 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 temperature drops to 30°F (16.7°C) below the set
point, initiating a furnace ignition. The furnace will continue to cy-
cle on limit until the cause of the reduced airflow is corrected.
Ignition Control Module
Monitors furnace function and provides safety interlocks. Ignition
control modules are available having 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 features, operation and
troubleshooting for the model control installed. 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
188
COLLECTOR BOX
PLUG
TEE FITTING
2.00"
a39-4394
Advertisement
Table of Contents
