MAINTENANCE
56
HEATING PERFORMANCE TEST
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
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 calorific
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:
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.
A
LWAYS CONNECT A MONOMETER TO THE OUTLET TAP AT THE GAS
VALVE BEFORE ADJUSTING THE PRESSURE REGULATOR.
I
N 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.
CAUTION
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 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 exchang-
er is now controlled by the amount of air circulated over its
surface.
The amount (CFM) of air circulated is governed by the exter-
nal static pressure in inches of water column of duct work,
cooling coil, registers, etc., applied externally to the unit ver-
sus 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.
