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Rollout Switch (Manual Reset)
The furnace module 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 igni-
tion 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 furnace module
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.
High Limit Switch
The furnace module is equipped with a fixed-temperature high
limit switch mounted on the vestibule panel. This switch shuts off
gas to the heater through the ignition control module in the event
of reduced circulating airflow over the heat exchanger. Reduced
airflow can be caused by motor failure of the circulating air blow-
er, dirty or blocked filters, or restriction of the air inlet or outlet to
the unit. The high limit switch will automatically reset when the
temperature drops to 30°F (16.7°C) below the set point. Deter-
mine the cause of the reduced airflow and correct.
Induced Draft Air Pressure Switch
An air pressure switch is provided as part of the control system to
verify combustion airflow through the induced-draft fan. The
pressure switch monitors the difference in pressure between the
induced-draft fan and the atmosphere. If sufficient negative pres-
sure is not present, the switch opens, shutting off gas supply
though the ignition control module. The air pressure switches have
fixed settings and are not adjustable.
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 combus-
tion air are not available to the heater, poor combustion and ineffi-
cient operation will result.
For outdoor installations, the heating unit cabinet combustion air
openings should be sized to provide 1 sq in. of free area per 4000
Btuh of input. Combustion air inlet and flue gas outlet must be lo-
cated in the same pressure zone to minimize effects of wind on
burner and heater performance.
UNIT LOCATION AND CLEARANCES
For both outdoor and indoor installations, observe these require-
ments:
• Be sure unit is located with respect to building construction
and other equipment to provide ready access and clearance
to access panels or doors that must be opened to permit
adjustment and servicing of the heating section.
• The heating section needs an ample supply of air for proper
and safe combustion of the fuel gas. Do not block or
obstruct air openings to the area where the heating unit is
installed. Combustion air openings in the cabinet should be
sized to provide 1 sq in. of free area per 4000 Btuh of input.
• Do not install unit where it may exposed to potentially
explosive or flammable vapors.
• Do not locate unit in areas where corrosive vapors (such as
chlorinated, halogenated, or acidic vapors) are present in the
atmosphere or can be mixed with combustion air entering
heater.
Clearance Requirements for Outdoor Installations
Observe the following additional requirements for outdoor
installations:
•
Provide at least 6 feet clearance to side of the unit, where
the combustion air inlet or vent (flue) gas discharge is lo-
cated, from walls, parapets or adjacent buildings or equip-
ment.
•
Unit must be installed with combustion air openings locat-
ed at least one (1) ft above the average snow depth for the
location.
•
Do not locate unit near building ventilators or exhausts, or
areas where corrosive chemical vapors can be drawn into
combustion air supply.
•
Do not install units in locations where flue products can be
drawn in the adjacent building openings such as windows,
fresh air intakes, etc.
•
Be sure that vent discharge for flue gases is directed away
from combustion air inlet and located to prevent flue prod-
ucts from being drawn into combustion air supply. Burner
performance can be adversely affected by recirculation of
flue products.
•
Combustion air inlet and flue gas outlet must be located in
the same pressure zone to minimize effects of wind on
burner and heater performance.
Clearance Requirements for Indoor Installations
Observe the following additional requirements for indoor
installations:
•
Locate unit to ensure an adequate supply of fresh air to re-
place air used in the combustion and ventilation process.
•
When locating units, it is important to consider that ex-
haust vent piping must connected to the outside atmo-
sphere. Location should minimize the number of elbows or
turns in vent piping.
GAS SUPPLY, PIPING, AND CONNECTIONS
Installation of piping must conform with local building codes and
ordinances, or in the absence of local codes with ANSI Z223.1,
the National Fuel Gas Code. In Canada, installation must be in
accordance with CAN/CGA-B149.1 for natural gas and B149.2
for propane units.
Use a pipe sealant resistant to LP gases on gas supply connections
to heater. Gas piping must be sized for the total Btu input of all
furnaces or units serviced by a single supply.
Properly support gas valve with back-up wrench during supply
pipe installation to prevent loosening valve or damage to burner
assembly or manifold.
Gas piping must be sized for the total Btu input of all units
(heaters) serviced by a single supply. A 2-in. NPT main gas header
is provided as standard.
A service regulator must be provided to provide gas to the heater
assembly at the proper pressure. Verify that the regulator installed
is sized and properly orificed for total input of all heaters.
A drip leg (sediment trap) and a manual shut-off valve must be
provided immediately upstream of the gas control on the heating
unit. To facilitate servicing of unit, installation of a union is
recommended. See Fig. 149.
Individual furnaces require a minimum inlet gas pressure as
shown in Table 41.
IMPORTANT: Gas pressure to appliance controls must never
exceed 13.5 in. wg (1/2 psig).
Pressure Testing
A 1/8 in. NPT tap is provided on the inlet side of the gas valve to
the heater. See Fig. 150-152. A fitting suitable for connection to a
pressure gage capable of measuring gas pressure should be
connected to each heater serviced by a single regulator so that gas
pressure at each heater can be measured with all heaters in
operation.
IMPORTANT: When pressure testing at 1/2 psig or less, close
the manual shut-off valve on the appliance before testing. When
pressure testing the gas supply line at 1/2 psig or greater, close
the manual shut-off valve and disconnect the heater from supply
line to be tested. Cap or plug the supply line.
196
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