SCHEDULED MAINTENANCE
29
Before attempting to diagnose an operating fault, run a heating
performance test and apply the results to the Service Problem
Analysis Guide.
To conduct a heating performance test, the BTU input to the
furnace must be calculated.
After the heating cycle has been in operation for at least fteen
minutes and with all other gas appliances turned o, the gas
meter should be clocked.
To nd the BTU input, multiply the number of cubic feet of
gas consumed per hour by the heating value of the gas being
used. (The caloric value of the gas being used is found by
contacting your local utility.)
It is found by the gas meter, that it takes forty
(40) seconds for the hand on the cubic foot dial to make
one complete revolution, with all appliances o, except the
furnace. Take this information and locate it on the gas rate
chart. Observe the forty (40) seconds, locate and read across
to the one (1) cubic foot dial column. There we nd the number
90, which shows that ninety (90) cubic feet of gas will be
consumed in one (1) hour.
Let’s assume the local gas utility has stated that the caloric
value of the gas is 1,025 BTU per cubic foot.
Multiplying the ninety (90) cubic feet by 1,025 BTU per cubic
foot gives us an input of 92,250 BTUH.
Checking the BTU input on the rating plate of the furnace
being tested.
EXAMPLE
92,000 BTU/HR
84,000
Should the gure you calculated not fall within ve (5) percent
of the nameplate rating of the unit, adjust the gas valve
pressure regulator or resize orices.
A
LWAS CNNEC A MNMEER E LE AP A E GAS
VALVE ERE A
D
JSING
E
PRESSRE
REGLA
R
I
N N CASE
S
LD E INAL MANILD
P
RESSRE
VA
R MRE
AN PLS
R
MINS 3 INCES WAER CLMN RM 3 INCES WAER CLMN
R NARAL GAS R 1 INCES WAER CLMN R PRPANE GAS
To adjust the pressure regulator on the gas valve, turn
down (clockwise) to increase pressure and input, and out
(counterclockwise) to decrease pressure and input.
Since normally propane gas is not installed with a gas meter,
clocking will be virtually impossible. The gas orices used
with propane are calculated for 2500 BTU per cubic foot gas
and with proper inlet pressures and correct piping size, full
capacity will be obtained.
With propane gas, no unit gas valve regulator is used;
however, the second stage supply line pressure regulator
should be adjusted to give 10” water column with all other
gas consuming appliances running.
The dissipation of the heat transferred to the heat exchanger is
now controlled by the amount of air circulated over its surface.
The amount (CFM) of air circulated is governed by the external
static pressure in inches of water column of duct work, cooling
coil, registers, etc., applied externally to the unit versus the
motor speed tap (direct drive) or pulley adjustments of the
motor and blower (belt drive).
A properly operating unit must have the BTU per hour input
and CFM of air, within the limits shown to prevent short cycling
of the equipment. As the external static pressure goes up, the
temperature rise will also increase. Consult the proper tables
for temperature rise limitation.
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