MAINTENANCE
15
Before attempting to diagnose an operating fault, run a heat-
ing performance test and apply the results to the
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 be-
ing 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.
92,000 BTU/HR
84,000
Should the gure you calculated not fall within ve (5) per-
cent of the nameplate rating of the unit, adjust the gas valve
pressure regulator or resize orices.
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 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; how-
ever, the second stage supply line pressure regulator should
be adjusted to give 11” water column with all other gas con-
suming 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 cy-
cling of the equipment. As the external static pressure goes
up, the temperature rise will also increase. Consult the prop-
er tables for temperature rise limitation.
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