SYSTEM OPERATION
37
Constant Fan Speed Menu
This menu allows for adjustment of the multiplier for
constant fan operation in 10% increments. Each furnace
has a “Maximum CFM” determined by motor HP. All
fan operations are based off of multipliers which are
percentages of this number. Max CFM is shown below.
The default constant fan multiplier is 25%.
Model
Max CFM
3 Ton Models
1400
4 Ton Models
1760
5 Ton Models
2200
CFM is 1650
NOTE: *MVM970803 max
Gas Heat Airflow Multiplier Menu
The menu is used to change the gas heat airflow multiplier
for gas heat operation. In most cases the default gas heat
airflow will provide a temperature rise near the middle
of the acceptable range. The multiplier will be expressed
by the 2
nd
& 3
rd
characters of the display as a percentage
of max CFM.
Gas Heat Fan Off Delay Menu
The default setting is 90 seconds. The available
adjustment range is from 30 to 180 seconds in 30 second
increments.
Gas Heat Fan On Delay Menu
The default setting is 30 seconds. The available
adjustment range is from 5 to 30 seconds in 5 second
increments.
Gas Heat Trim Factor Option Menu
Gas Heat airflow may be trimmed from -10% to +10% in
2% increments.
Gas Heat Stage Multiplier Menu (CFM)
This menu allows adjustment of the low fire CFM
multiplier. The default CFM for low fire is 70% of high fire.
Gas Pressure Test Menu
This menu allows 100% firing rate be locked in to check
gas valve pressure.
CAUTION
T
O
PREVENT
UNRELIABLE
OPERATION
OR
EQUIPMENT
DAMAGE
,
THE
INLET
GAS
SUPPLY
PRESSURE
MUST
BE
AS
SPECIFIED
ON
THE
UNIT
RATING
PLATE
WITH
ALL
OTHER
HOUSEHOLD
GAS
FIRED
APPLIANCES
OPERATING
.
Natural Gas Minimum: 4.5" w.c. Maximum: 10.0" w.c.
Propane Gas Minimum: 11.0" w.c. Maximum: 13.0" w.c.
INLET GAS SUPPLY PRESSURE
GAS SUPPLY AND PIPING
The furnace rating plate includes the approved furnace gas
input rating and gas types. The furnace must be equipped
to operate on the type of gas applied. This includes any
conversion kits required for alternate fuels and/or high altitude.
Inlet gas supply pressures must be maintained within the
ranges specified in the following table. The supply pressure
must be constant and available with all other household gas
fired appliances operating. The minimum gas supply pres
-
sure must be maintained to prevent unreliable ignition. The
maximum must not be exceeded to prevent unit overfiring.
HIGH ALTITUDE DERATE
When this furnace is installed at high altitude, the appropriate
High Altitude orifice kit must be applied. This is required due
to the natural reduction in the density of both the gas fuel and
combustion air as altitude increases. The kit will provide the
proper design certified input rate within the specified altitude
range.
High altitude kits are purchased according to the installation
altitude and usage of either natural or propane gas. Consult
the furnace Specification Sheet for appropriate kits.
Do
not
derate the furnace by adjusting the manifold pressure
to a lower pressure than specified on the furnace rating plate.
The combination of the lower air density and a lower manifold
pressure will prohibit the burner orifice from drawing the prop
-
er amount of air into the burner. This may cause incomplete
combustion, flashback, and possible yellow tipping.
In some areas the gas supplier may artificially derate the gas
in an effort to compensate for the effects of altitude. If the
gas is artificially derated, the appropriate orifice size must be
determined based upon the BTU/ft
3
content of the derated
gas and the altitude. Refer to the National Fuel Gas Code,
NFPA 54 / ANSI Z223.1 or CAN / CSA B149.1-15 in Canada,
and information provided by the gas supplier to determine the
proper orifice size.
A different pressure switch may be required at high altitude
regardless of the BTU/ft
3
content of the fuel used. Consult the
furnace Specification Sheet for pressure switch.
