127408-YIM-B-0606
46
Unitary Products Group
3.
Open gas heat access panel.
4.
Turn gas valve clockwise to “OFF” position (See Figure 29).
MANIFOLD GAS PRESSURE ADJUSTMENT
This gas furnace has two heat stages. Therefore, the gas
valve has two adjustment screws located under a plastic pro-
tective cover. The second stage (100% input) adjustment
screw is adjacent to the “HI” marking on the valve and the
first stage (60% input) adjustment screw is located adjacent
to the “LO” marking on the valve (See Figure 29).
Manifold pressure adjustment procedure.
Adjust second stage (100% input) pressure first, then adjust
first stage (60% input) pressure.
1.
Turn off all power to the unit.
2.
Using the outlet pressure port on the gas valve, connect
a manometer to monitor the manifold pressure.
3.
Remove plastic cap covering HI and LO pressure adjust-
ment screws.
4.
Turn on power to the unit.
5.
Set thermostat to call for second stage heat and start fur-
nace.
6.
If necessary, using a screwdriver, turn the
second stage
adjustment screw (adjacent to the “HI” marking on the
valve) clockwise to increase manifold pressure or coun-
terclockwise to decrease manifold pressure.
Be sure
not to over-fire the unit on second stage
.
7.
After the high manifold pressure has been checked,
adjust the thermostat to call for first stage heat.
8.
If necessary, using a screwdriver, turn the first stage
adjustment screw (adjacent to the “LO” marking on the
valve) clockwise to increase manifold pressure or
counterclockwise to decrease manifold pressure.
Be
sure not to under-fire the unit on first stage
.
9.
Once pressure has been checked, replace the plastic
cap covering the HI and LO pressure adjustment screws.
NOTE:
When using natural gas, the manifold pressure for
second stage (100% input) should be 3.5 IWG ±
0.3. The manifold pressure for first stage (60%
input) when using natural gas should be 1.5 IWG ±
0.3.
CHECKING GAS INPUT
NATURAL GAS
This unit has two stages of gas heat. The first stage is 60% of
the full fire input and is considered the minimum input for the
furnace. The intended input for each furnace is shown in the
table below. The following Table applies to units operating on
60 Hz power only.
To determine the rate of gas flow (Second Stage).
1.
Turn off all other gas appliances connected to the gas
meter.
2.
Turn on the furnace and make sure the thermostat is
calling for Second stage (100% input) heat.
3.
Measure the time needed for one revolution of the hand
on the smallest dial on the meter. A typical gas meter
has a 1/2 or a 1 cubic foot test dial.
4.
Using the number of seconds it takes for one revolution
of the dial, calculate the cubic feet of gas consumed per
hour. (See example below).
5.
If necessary, adjust the high pressure regulator as
discussed in the section “Manifold Gas Pressure
Adjustment”.
Be sure not to over-fire
the furnace on
Second stage. If in doubt, it is better to leave the Second
stage of the furnace slightly under-fired. Repeat Steps 1-
5.
To determine the rate of gas flow (First Stage)
1.
Turn off all other gas appliances connected to the gas
meter.
2.
Turn on the furnace and make sure the thermostat is
calling for first stage (60% input) heat.
3.
Even when the thermostat is calling for first stage heat,
the unit will light on second stage and will run on Second
stage for 1 minute. Allow this one-minute time period to
expire and be certain the unit is running on first stage.
4.
Measure the time needed for one revolution of the hand
on the smallest dial on the meter. A typical gas meter
has a 1/2 or a 1 cubic foot test dial.
5.
Using the number of seconds it takes for one revolution
of the dial, calculate the cubic feet of gas consumed per
hour (See example below).
6.
If necessary, adjust the low pressure regulator as dis-
cussed in the section “Manifold Gas Pressure Adjust-
ment”.
Be sure not to under-fire
the furnace on first
stage. If in doubt, it is better to leave the first stage of the
furnace slightly over-fired (greater than 60% input).
Repeat Steps 1-6.
TABLE 48: GAS HEAT STAGES
# of Burner Tubes
2nd Stage
Input (100%
Btuh)
1st Stage
Input (60%)
Btuh
4
120,000
72,000
6
175,000
105,000
