Gas Supplyand Piping; General; High Altitude Derate; Alternate High Altitude Derate - Goodman GMH8 Installation Instructions Manual

IX.GASSUPPLY ANDPIPING
GENERAL
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.
I _
CAUTION
I To PREVENT UNRELIABLE OPERATION OR EQUIPMENT DAMAGE, THE INLET
:lASH :ULP:::HPERRE::Uu::HM:L_: ::t:IRSEF:CAI_IpELDA_:ET::::IR:T:A:NG PLATE
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 pressure must
be maintained to prevent unreliable ignition. The maximum must
not be exceeded to prevent unit overfiring.
Inlet Gas Supply Pressure I
Natural Gas I Minimum: 5.0" w.c. Maximum:10.0" w.c. J
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Propane Gas I Minimum: 11.0" w.c. Maximum:13.0" w.c.
NOTE: Adjusting the minimum supply pressure below the limits
in the above table could lead to unreliable ignition. Gas input to
the burners must not exceed the rated input shown on the rating
plate. Overfiring of the furnace can result in premature heat
exchanger failure. Gas pressures in excess of 13 inches water
column can also cause permanent damage to the gas valve.
At all altitudes, the manifold pressure must be within 0.3 inches
w.c. of that listed in the Product Data Book applicable to your
model for the fuel used. At all altitudes and with either fuel, the air
temperature rise must be within the range listed on the furnace
nameplate. Should this appliance be converted to LP, refer to the
instructions included in the factory authorized LP conversion kit.
HIGH ALTITUDE DERATE
IMPORTANT NOTE: The furnace will naturally derate itself with
altitude. Do not attempt to increase the firing rate by changing
orifices or increasing the manifold pressure. This can cause
poor combustion and equipment failure.
When this furnace is installed at high altitude, the appropriate
High Altitude orifice kit must be applied. The furnace is supplied
with orifices sized for natural gas at altitudes up to 4,500 feet
using a heating value of approximately 1,000 Btu/hr, and should
not normally require change. Most cases will not require a change
at altitudes up to 7,500 feet. For use at altitudes in excess of
4,500 feet, refer to the following table for maximum input rates
based on altitude. Contact your local gas utility for the heating
value of the gas. Refer to National Fuel Gas Code ANSI Z223.1
and the following section to calculate firing rates.
Altitude
(_) 140
5,000 117,000
7,500 105,000
10,000 92,400
Maximium Firing Rates AtAItitude
Model(Kbtu/hr)
45 70 90 115
39,000 58,500 78,000 97,500
35,000 52,500 70,000 87,500
34,000 51,000 68,000 77,000
Firing rates may be lower than listed in table
It is important to check and adjust the input rate of the furnace to
prevent an overfiring situation. Overfiring can cause premature
heat exchanger failure. The input is controlled by the supply
pressure, orifice size, manifold pressure and heating (calorific)
value of the gas.
The supply pressure must be measured with this and all other
gas burning appliances in operation. The supply pressure must
be adjusted to the pressure range stated on the rating plate.
Applications for altitudes in excess of 4,500 feet usually require
an orifice change.
ALTERNATE HIGH ALTITUDE DERATE
Alternately standard derate for altitude from National Fuel Gas
Code ANSI Z223.1 of 4% per 1000' may be taken. Refer to the
most recent version of ASNI Z223.1 for correct gas orifice. The
orifices must be selected using the table below. The furnace
derate is 4% for each 1,000 feet above sea level. This table is
based upon a heating value of approximately 1,000 Btu/ft 3
NAT, GAS
ALTITUDE
ORIFICE SIZE
0-2,000 #43
3,000 #44
4,000 #44
5,000 #45
6,000 #45
7,000 #46
The input to the furnace must be checked AFTER reorificing.
For altitudes above 4,500 feet, refer to appropriate section of the
National Fuel Gas Code, ANSI Z223.1. To calculate the input of
the furnace for installations in altitudes over 7,000 feet, use the
following formula:
Corrected Input = Rating Plate input - (Altitude X .04) X (Rating
Plate Input/ 1000)
Example:
Corrected input for a 90,000 Btu/hr appliance installed at an
altitude of 6,000 ft. utilizing natural gas with a heating value of
1,000 Btu/ft 3 is determined by-
Corrected input = 90,000 - (6,000 X .04) X (90,000 / 1,000)
Corrected input = 90,000 - (240 X 90)
Corrected input = 90,000 - 21,600
Corrected input = 68,400
FIRING RATE
Using the orifices sized as shown in the table for 6,000 feet
(#45), a meter time of 52.6 seconds is measured. The actual
firing rate of the furnace is:
Input = 1,000 (heating value of the gas) X 3600 (constant) / 52.6
(meter time for 1 ft 3 of gas)
Input = 3,600,000 / 52.6
Input = 68,400 Btu/h
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