Operating instructions

EXAMPLE: (0—2000 ft altitude)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 45
Manifold pressure 3.6-in. wc
* Furnace is shipped with No. 45 orifices. In this example
all main burner orifices are the correct size and do not need
to be changed to obtain the proper input rate.
Check and verify burner orifice size in furnace. NEVER ASSUME
ORIFICE SIZE; ALWAYS CHECK AND VERIFY.
2. Adjust manifold pressure to obtain input rate.
NOTE: Manifold pressure must always be measured with burner
enclosure front REMOVED. Gas meter must always be clocked
with burner enclosure front INSTALLED.
a. Remove burner enclosure front.
b. Remove cap that conceals adjustment screw for gas
valve regulator. (See Fig. 44.)
c. Turn adjusting screw, counterclockwise (out) to decrease
manifold pressure or clockwise (in) to increase manifold
pressure.
NOTE: This furnace has been approved for a manifold pressure
of 3.2 in. wc to 3.8 in. wc when installed at altitudes up to 2000 ft.
For altitudes above 2000 ft, the manifold pressure can be adjusted
from 2.0 in. wc to 3.8 in. wc.
CAUTION: DO NOT bottom out gas valve regulator
adjusting screw. This can result in unregulated manifold
pressure and result in excess overfire and heat exchanger
failures.
NOTE: If orifice hole appears damaged or it is suspected to have
been redrilled, check orifice hole with a numbered drill bit of
correct size. Never redrill an orifice. A burr-free and squarely
aligned orifice hole is essential for proper flame characteristics.
d. Replace gas valve regulator adjustment screw cap.
e. Replace burner enclosure front and verify adjusted gas
input rate using method outlined in item 3.
f. Look through sight glass in burner enclosure and check
burner flame. Burner flame should be clear blue, almost
transparent. (See Fig. 45.)
TABLE 7—MODEL 340MAV ORIFICE SIZE AND MANIFOLD PRESSURE FOR CORRECT INPUT Continued
(TABULATED DATA BASED ON 20,000 BTUH PER BURNER, DERATED 2% FOR EACH 1000 FT ABOVE SEA LEVEL)
ALTITUDE
RANGE
(FT)
AVG GAS
HEAT VALUE
AT ALTITUDE
(BTU/CU FT)
SPECIFIC GRAVITY OF NATURAL GAS
0.58 0.60 0.62 0.64 0.66
Orifice
No.
Manifold
Pressure
Orifice
No.
Manifold
Pressure
Orifice
No.
Manifold
Pressure
Orifice
No.
Manifold
Pressure
Orifice
No.
Manifold
Pressure
U.S.A. Only
575 43 3.8 42 3.2 42 3.3 42 3.5 42 3.6
600 43 3.5 43 3.6 43 3.7 42 3.2 42 3.3
9001 625 44 3.7 44 3.8 43 3.5 43 3.6 43 3.7
650 44 3.4 44 3.5 44 3.7 44 3.8 43 3.4
to 675 45 3.8 44 3.3 44 3.4 44 3.5 44 3.6
700 45 3.6 45 3.7 45 3.8 44 3.3 44 3.4
10,000 725 45 3.3 45 3.4 45 3.6 45 3.7 45 3.8
750 45 3.1 45 3.2 45 3.3 45 3.4 45 3.5
775 45 2.9 45 3.0 45 3.1 45 3.2 45 3.3
Fig. 44—Redundant Automatic Gas Valve
A95622
MANIFOLD
PRESSURE TAP
GAS
PRESSURE
REGULATOR
ADJUSTMENT
BURNER
ENCLOSURE
REFERENCE
PRESSURE
TAP
INLET
PRESSURE TAP
ON AND
OFF SWITCH
CAUTION: DO NOT redrill orifices. Improper drilling
(burrs, out-of-round holes, etc.) can cause excessive
burner noise and misdirection of burner flames. This can
result in flame impingement of burners and heat exchang-
ers causing failures.
A93059
BURNER 
ORIFICE
—35—