55
Step 5 -- Adjustments
SET GAS INPUT RATE
Furnace gas input rate on rating plate is for installations at
altitudes up to 2000 ft (609.6 M).
In the U.S.A., the input ratings for altitudes above 2000 ft (609.6
M) must be reduced by 2 percent for each 1000 ft (305 M) above
sea level.
In Canada, the input ratings must be derated by 5 percent for
altitudes of 2001 ft to 4500 ft (610 to 1372 M) above sea level.
Adjust manifold pressure to obtain input rate.
Furnace input rate must be within
±
2 percent of input on furnace
rating plate adjusted for altitude.
1. Determine Natural Gas Orifice Size And Manifold
Pressure For Correct Input.
a. Obtain average gas heat value (at installed altitude)
from local gas supplier.
b. Obtain average gas specific gravity from local gas
supplier.
c. Verify furnace model and size. Table 15 can only be
used for model 58MEC furnaces with heating inputs of
20,000/13,000 (High/Low) Btuh per burner.
d. Find installation altitude in Table 15.
NOTE
: For Canada altitudes of 2001 to 4500 ft, (610 to 1372 M)
use U.S.A. altitudes of 2001 to 3000 ft (610 to 914 M) in Table
12.
e. Find closest natural gas heat value and specific gravity
on Table15.
f. Follow heat value and specific gravity lines to point of
intersection to find orifice size and manifold pressure
settings for proper operation.
EXAMPLE:
(0 -- 2000 ft (0 to 610 M) altitude using Table 15)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 45
Manifold pressure 3.6-IN. WC for high heat
1.5-IN. WC for low heat
* 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.
g. Check and verify burner orifice size in furnace.
NEVER ASSUME ORIFICE SIZE; ALWAYS CHECK
AND VERIFY.
2. Adjust Manifold Pressure To Obtain Input Rate.
a. Remove burner enclosure front.
NOTE
: Manifold pressure MUST always be measured with
burner enclosure front REMOVED.
b. Remove regulator seal caps that conceal adjustment
screws for low-and high-heat gas valve pressure
regulators. (See Fig. 58.)
c. Move setup switch SW-1 on control center to ON
position. This keeps furnace locked in low-heat
operation.
d. Jumper R and W/W1 thermostat connections on
control to start furnace.
e. Turn low-heat adjusting screw counterclockwise (out)
to decrease input rate or clock-wise (in) to increase
input rate.
NOTE
: DO NOT set low-heat manifold pressure less than
1.3-IN. WC or more than 1.7-IN. WC for natural gas. If manifold
pressure is outside this range, change main burner orifices to
obtain pressure in this range.
UNIT DAMAGE HAZARD
Failure to follow this caution may result in reduced
furnace life.
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.
CAUTION
!
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.
f. Move setup switch SW-1 to OFF position after
completing low-heat adjustment.
g. Jumper R and W1 and W2 thermostat connections on
control. (See Fig. 37.) This keeps furnace locked in
high-heat operation.
h. Turn high-heat adjusting screw counterclockwise (out)
to decrease input rate or clockwise (in) to increase rate.
NOTE
: DO NOT set high-heat manifold pressure less than
3.2-IN. WC or more than 3.8-IN. WC for natural gas. If manifold
pressure is outside this range, change main burner orifices to
obtain pressure in this range.
i. When correct input is obtained, replace caps that
conceal gas valve regulator adjustment screws. Main
burner flame should be clear blue, almost transparent.
(See Fig. 59.)
j. Remove jumper R-to-W1 and W2.
UNIT DAMAGE HAZARD
Failure to follow this caution may result in unit component
damage.
DO NOT redrill orifices. Improper drilling (burrs,
out-of-round holes, etc.) can cause excessive burner noise and
misdirection of burner flames. (See Fig. 60.)
CAUTION
!
58MEC
