SCHEDULED MAINTENANCE
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5. Use this formula to verify the Cubic Feet per Hour (CFH) input determined in step 4 is correct:
(3600 x Gas Meter Dial Size) / Time (seconds) = Cubic Feet per Hour (CFH)
3600 is used as there are 60 seconds in
a minute and 60 minutes in an hour.
60x60=3600
6. Check with your local utility for actual BTU content (caloric value) of natural gas in the area (the average is 1025
BTU’s).
7.
Use this formula to calculate the BTU/HR input (See BTU/HR Calculation Example):
Cubic Feet per Hour (CFH) x BTU content of your natural gas = BTU/HR input
8.
Should the figure you calculated not fall within five (5) percent of the nameplate rating of the unit, adjust the gas valve
pressure regulator or resize orifices. To adjust the pressure regulator on the gas valve, turn downward (clockwise) to
increase pressure and input, and upward (counterclockwise) to decrease pressure and input. 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.
BTU/HR Calculation Example:
The unit being tested takes 40 seconds for the 1 cubic foot dial to make one complete revolution. Using the chart, this trans-
lates to 90 cubic feet per hour. Based upon the assumption that one cubic foot of natural gas has 1,025 BTU’s (Check with
your local utility for actual BTU content), the
calculated input is 92,250 BTU’s per hour.
Furnace Nameplate Input in this example:
90,000 BTU/HR
Calculated Gas Input in this example:
92,250 BTU/HR
This example is within the 5% tolerance input and does not need adjustment.
CAUTION
Always connect a manometer to the outlet 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 (counter-
clockwise) 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 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 sur
-
face.
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.
