– 19 –
When shipped from the factory, all gas fired units are
equipped for the average BTU of the gas stamped on
the rating plate.
Since the BTU content of gases varies
in many localities, the input must be checked after
installation of the unit. If the unit is overfired, the
overheating will substantially shorten the life of the
heat exchanger. Never exceed the input on the rating
plate.
The input may be checked by either the meter method
or the pressure of the gas in the manifold. A 1/8 inch
pipe tap is available on the body of the gas valve.
Never overfire the unit heater, as this
may cause unsatisfactory operation or shorten the
life of the heater.
METER METHOD OF CHECKING INPUT:
1.
Obtain the heating value of the gas from the l o c a l
utility. This should be in BTU per cubic foot
(MJ/m
3
).
2.
Determine the cubic feet per minute of the gas as
shown in the following example. The rating plate
input should be taken from the unit as stamped on
the plate.
EXAMPLE: Assume this is a unit having an input of
250,000 BTU/hr. and the heating value of the gas to
be used in the furnace is 1000 BTU/hr.
GAS INPUT ADJUSTMENT
Before starting the rooftop gas heating unit, use the
“Installation Check Sheet” (found at the end of this
manual) in conjunction with the procedures outlined
below to ensure that the unit is completely and properly
installed and ready for start-up.
1.
Inspect all wiring connections; connections should
be clean and tight.
2.
Trace circuits to ensure that actual wiring agrees
with the “as wired” diagrams provided with the unit.
Information in the title block of the wiring diagram(s)
should match the data appearing on the unit
nameplate.
3.
Verify that the system switch is in the OFF position.
4.
Check unit supply voltage to ensure that it is within
the utilization range.
5.
Inspect the interior of the unit; remove any debris or
tools which may be present.
START-UP
STARTING UNIT IN HEATING MODE
1.
Close the unit disconnect switch(s) that provide
current to the unit control panel.
High voltage is present in some
areas of the control panel(s) with the unit
disconnect switch closed. Failure to exercise
caution when working around energized
electrical components may result in injury or
death from electrical shock.
2.
Set the room thermostat/switching subbase as
indicated below:
a. position the heating system switch at either
HEAT or AUTO;
b. set the fan switch at AUTO; and,
c. adjust the temperature control setting to some
point above room temperature.
3.
Place the system switch in the ON position.
With the thermostat calling for heating, unit operation is
automatic.
250,000 BTU/hr.
INPUT=
1000 BTU/cu. ft. x 60 min/hr
= 41.7 cu ft. per min.
Before checking the unit input, all other gas
appliances connected to the same meter must be
turned off.
3.
Fire the unit according to instructions.
4.
Observe on the meter dial the cubic feet of gas for a
period of five minutes. This should be 41.7 cu ft. per
min. x 5 minutes, or 20.8 cu. feet. Minor input
adjustments can be made by moving the regulator
screw clockwise for increased input and counter-
clockwise for decreased input. Any appreciable
adjustment in input should be made by re-orificing.
MANIFOLD PRESSURE METHOD
OF CHECKING INPUT
1.
Close the manual gas valve on the unit.
2.
Install a 1/8 inch pipe connection in the tapped hold
provided on the valve body and a “U” tube or
manometer by means or a rubber hose.
3.
Fire the unit and observe the pressure.
4.
Small variations in gas pressure adjustment can be
made by means of the pressure regulator. Remove
the cap from the regulator. Turning the screw
clockwise will increase the input, and turning it
counterclockwise will decrease the input. The
adjusted manifold pressure should not vary more
than 10% from the pressures specified in chart 5.
