5835254-YIM-A-1119
48
Johnson Controls Ducted Systems
Calculating the cubic feet of gas consumed per hour
1.
To find the BTU input, multiply the number of cubic feet of
gas consumed per hour by the BTU content of the gas in
your particular locality.
Note:
The BTU content of gas varies widely from area to
area, contact your gas company for this information.
By actual measurement, it takes 19 seconds for the hand
on a 1 cubic foot dial to make a revolution with a 192,000
BTU/h furnace running.
2.
To determine rotations per minute, divide 60 by 19 = 3.16.
3.
To calculate rotations per hour, multiply 3.16
60 = 189.6.
4.
Multiply 189.6
1 (0.5 if using a 1/2 cubic foot dial) = 189.6.
5.
Multiply 78
(the BTU rating of the gas). For this example,
assume the gas has a BTU rating of 1050 BTU/ft.
3
.
The result of 199,000 BTU/h is within 5% of the 192,000
BTU/h rating of the furnace.
Adjusting the manifold gas pressure
This gas furnace has two heat stages. The gas valve has two
adjustment screws located under a plastic protective cover. The
second stage (100% input) adjustment screw is adjacent to the
HI marking on the valve. The first stage (60% input) adjustment
screw is located adjacent to the LO marking on the valve (see
Figure 30).
Adjust the second stage (100% input) pressure first, then adjust
first stage (60% input) pressure.
1.
Turn off electrical power to the unit.
2.
Using the outlet pressure port on the gas valve, connect a
manometer to monitor the manifold pressure.
3.
Remove the plastic cap that covers the HI and LO pressure
adjustment screws.
4.
Turn on electrical power to the unit.
5.
Set the thermostat to call for second stage heat and start
the furnace.
6.
If necessary, use a screwdriver to turn the
second stage
adjustment screw clockwise to increase manifold pressure
or counterclockwise to decrease manifold pressure.
Note:
Do not over-fire the unit on second stage.
7.
After you check the high manifold pressure, adjust the
thermostat to call for first stage heat.
8.
If necessary, use a screwdriver to turn the first stage
adjustment screw clockwise to increase manifold pressure
or counterclockwise to decrease manifold pressure.
Note:
Do not under-fire the unit on first stage.
9.
After you check the pressure, 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) is 3.5 IWG ± 0.3. The
manifold pressure for first stage (60% input) when
using natural gas is 1.5 IWG ± 0.3.
Adjusting the temperature rise
The temperature rise is the difference of temperature between
the return air and the heated air from the furnace. The
temperature rise must lie within the range shown on the CSA
rating plate and the data in Table 11.
1.
After about 20 minutes of operation, determine the furnace
temperature rise. Take readings of both the return air and
the heated air in the ducts (about 6 feet from the furnace)
where they are not affected by radiant heat.
2.
After you determine the temperature rise, calculate the
CFM according to the following formula.
3.
Increase the blower CFM to decrease the temperature rise.
Decrease the blower CFM to increase the rise (see
Note:
Each gas heat exchanger size has a minimum
allowable CFM. Below this CFM, the limit opens.
Inspecting and servicing burners and orifices
1.
Open the union fitting just upstream of the unit gas valve
and downstream from the main manual shut-off valve in
the gas supply line.
2.
Remove the screws that hold each end of the manifold to
the manifold supports.
3.
Disconnect the wiring to the gas valves and spark igniters.
4.
Remove the manifold and gas valve assembly. Inspect the
orifices and replace them if required.
56
32
64
58
31
62
60
30
60
Table 28: Gas rate cubic feet per hour
Seconds for one
rev.
Size of test dial
1/2 cu. ft.
1 cu. ft.
Table 29: Gas heat stages
No. of burner tubes
2nd stage input
(100% BTU/h)
1st stage input
(60% BTU/h)
4
120,000
72,000
6
180,000
108,000
8
240,000
144,000
Before you check or change burners, pilot, or orifices,
close the main manuals shut-off valve and turn off all
electrical power to the unit.
