SERVICING
55
Rev. 0
TEST EQUIPMENT
Proper test equipment for accurate diagnosis is as essen-
tial as regulator hand tools.
The following is a must for every service technician and
service shop.
1. Dial type thermometers or thermocouple meter (op-
tional) - to measure dry bulb temperature.
2. Amprobe - to measure amperage and voltage.
3. Volt-Ohm Meter - testing continuity, capacitors, and mo-
tor windings.
4. Inclined Manometer - to measure static pressure, pres-
sure drop across coils, filters, and draft.
5. Water Manometer (12") - to test gas inlet and manifold
pressure.
Other recording type instruments can be essential in solv-
ing abnormal problems, however, in many instances they
may be rented from local sources.
Proper equipment promotes faster, more efficient service
and accurate repairs resulting in fewer call backs.
HEATING PERFORMANCE TEST
Before attempting to diagnose an operating fault, run a
heating performance test and apply the results to the Ser-
vice 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
fifteen minutes and with all other gas appliances turned off,
the gas meter should be clocked.
To find the BTU input, multiply the number of cubic feet of
gas consumed per hour by the heating value of the gas
being used. (The calorific value of the gas being used is
found by contacting your local utility.)
Example: 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 off, 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 find
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 calo-
rific value of the gas is 1025 BTU.
Multiplying the ninety (90) cubic feet by 1025 BTU gives us
an input of 92,250 BTUH.
Checking the BTU input on the rating plate of the furnace
being tested.
EXAMPLE:
GUCA090AX40
INPUT: 92,000 BTU/HR
OUTPUT CAP: 84,000
Should the figure you calculated not fall within five (5) per-
cent of the nameplate rating of the unit, adjust the gas valve
pressure regulator or resize orifices.
CAUTION
ALWAYS CONNECT A MANOMETER TO THE 1/8" PIPE
TAP AT THE GAS VALVE BEFORE ADJUSTING THE
PRESSURE REGULATOR. IN NO CASE SHOULD THE
FINAL MANIFOLD PRESSURE VARY MORE THAN
PLUS OR MINUS .3 INCHES WATER COLUMN FROM
3.5 INCHES WATER COLUMN FOR NATURAL GAS OR
10 INCHES WATER COLUMN FOR PROPANE GAS.
To adjust the pressure regulator on the gas valve, turn down
(clockwise) to increase pressure and input, and out (coun-
terclockwise) to decrease pressure and input.
Since normally propane gas is not installed with a gas meter,
clocking will be virtually impossible. The gas orifices used
with propane are calculated for 2500 BTU 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; how-
ever, the second stage supply line pressure regulator should
be adjusted to give 11" 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 ex-
ternal static pressure in inches of water column of duct work,
cooling coil, registers and etc., applied externally to the unit
versus the motor speed tap (direct drive) or pulley adjust-
ments of the motor and blower (belt drive).
A properly operating unit must have the BTU 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.
