Lennox MPGA 050 B3, Application Manual

Page: 22 / 37

Burner Orifice Sizing:
The furnace is supplied with standard orifices for the
gas shown on the rating plate. Table 6 shows
combinations of heating values and specific gravities for
various gases, from which proper input can be obtained.
If changing orifices is required, remove the manifold
from the furnace (following the instructions found on page
23) and replace orifices as required by Table 6, the
altitude derating section of this instruction or as local code
dictates.
TABLE 6
Burner Orifice Selection
Type of Gas@Manifold Press.
(Heating Value-Specific Gravity)
Btu per Cu. Ft.
Orifice
Size
(Drill #)
Natural Manifold Press.= 3.5"w.c.
800-0.6
900-0.6
1000-0.6
1100-0.6
40
41
42
43
Propane Manifold Press.= 10"w.c.
2500-1.53 54
After securing the manifold assembly, replace all
other components and/or wiring, being sure that all
connections and screws are tightened properly.
Altitude Derating:
The following information is provided as guidelines for
altitude derating and is not meant to supersede any state
or local codes. Local codes have priority over any others
and in some case might limit your options in dealing with
an altitude derate situation.
NOTE: In Canada for altitudes up to 4500 ft. (1372 m)
see the rating label on this furnace for proper
manifold pressure and orifice size. Certification
for installations at altitudes over 4500 ft. (1372 m)
is the jurisdiction of local authorities.
Check with your local gas company to find out if the
gas supply in your area is derated. Gas deration negates
the necessity of performing any adjustment on the
furnace.
If your gas supply is not derated, and regardless of
the type of gas used, installation of this furnace at
elevations above 2,000 ft. requires an input reduction at
the rate of four percent (4%) for each 1,000 ft. above sea
level.
Unless an orifice change is specified by an applicable
code, or the furnace is to be installed above 6,999 feet,
the recommended method of altitude derating this furnace
is to appropriately lower your manifold pressure. The
appropriate manifold pressures based on the elevation
and the heating value can be found in Table 7.
TABLE 7
High Altitude Manifold Pressure Derate
(with standard 42 orifice Natural / 54 orifice LP sizes)
Altitude
(Feet)
*Heating Value of Natural Gas
(BTU/FT3)
LP
Propane
900 950 1000 1050 1100 2500
0-999 4.32 3.88 3.50 3.16 2.84 10
1000-1999 4.32 3.88 3.50 3.16 2.84 10
2000-2999 3.67 3.29 2.97 2.68 2.41 8.46
3000-3999 3.38 3.04 2.74 2.47 2.22 7.74
4000-4999 3.11 2.79 2.52 2.27 2.04 7.05
5000-5999 2.88 2.58 2.33 2.10 1.89 6.40
6000-6999 2.64 2.37 2.14 1.93 1.73 5.77
* Heating-Value based on atmospheric pressure of 30 inhg and 60°F temperature.
If local codes require an orifices change or if the
furnace installation is above 6,999 feet. The appropriate
orifice size based on the elevation and the heating value
can be found in Table 8. Sizing of the orifice must be
based on the previously mentioned 4% derate for each
1,000 feet for installations at/or above 2,000 feet rule and
the orifices must be drilled in such a way as to assure
concentricity.
Hand drilling of orifices is unacceptable.
TABLE 8
High Altitude Orifice Size Derate
Altitude
(Feet)
*Heating Value of Natural Gas
(BTU/FT3)
LP
Propane
900 950 1000 1050 1100 2500
2000-2999 N.C. N.C. 43 43 44 N.C.
3000-3999 N.C. N.C. 43 44 44 N.C.
4000-4999 43 43 44 44 45 55
5000-5999 43 44 44 45 46 55
6000-6999 44 44 45 46 47 55
7000-7999 44 45 46 47 48 56
8000-8999 45 46 47 48 48 56
9000-9999 46 47 48 48 49 56
10000-10999 47 48 49 49 50 57
* Heating-Value based on atmospheric pressure of 30 inhg and 60°F temperature.
Hand drilling of orifices is never acceptable since it
could lead to delayed ignition, overfiring, improper
combustion, flashback and flame rollout. All these
conditions could lead to a fire hazard and bodily
harm, or loss of life.
Blower Adjustment Checkout:
Prior to any blower adjustment, electrical service must
be turned OFF.
This furnace is equipped with a 3 speed direct drive
motor to deliver a temperature rise within the range
specified on the rating label, between the return and
supply plenums, at the external duct static pressure noted
on the rating label.
20541501 Issue 0240 Page 20 of 26