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6. With stopwatch, measure time it takes to consume
two cubic feet of gas.
•
If dial is one-half cubic foot per revolution,
measure time for four revolutions.
•
If dial is one cubic foot per revolution, measure
time for two revolutions.
•
If dial is two cubic feet per revolution, measure
time for one revolution.
7. Divide this time by two. This gives average time
for one cubic foot of gas to flow through meter.
Example: If it took 58 seconds for two cubic feet
to flow, it would take 29 seconds for one cubic foot
to flow.
8. Calculate gas input using this formula:
Gas Heating Value (BTU/cu. ft.)
x 3,600 sec/hr
Gas input =
Time (Seconds for one
cubic foot of gas)
Example:
Assume it took 29 seconds for one cubic foot of
gas to flow and heating value of 1,000 BTU/cu. ft.
1,000 x 3,600
Gas input =
BTU
If you left no other pilots on, this is the furnace gas
input.
9. If you left water heater, dryer or range pilots on,
allow for them in calculating correct furnace gas
input. A quick way is to allow 1,000 BTU per
hour for a water heater, 500 BTU per hour for
dryer and 500 BTU per hour for each range burner
pilot.
Example:
If you left gas water heater, dryer, two range
burner pilots and one oven pilot on, allow:
Water heater pilot
Dryer pilot
2 range burner pilots
1 range oven pilot
Subtracting 3,000 BTU per hour from 124,138
BTU per hour measured above equals 121,138
BTU per hour. This would be the correct furnace
gas input after allowing for pilots left on.
= BTU/hour
= 124,138
29
1,000 BTU per hour
500 BTU per hour
1,000 BTU per hour
500 BTU per hour
3,000 BTU per hour
10. Manifold pressure may be adjusted within the
range of 3.2 inches W.C. to 3.8 inches W.C. to get
rated input ± 2 percent. See Section 17, "Manifold
Pressure Adjustment". If you cannot get rated
input with manifold pressure within the allowable
range, you must change orifices.
PROPANE (LP) GAS INPUT RATE
WARNING
Propane (LP) gas installations do not have gas
meters to double check input rate. Measure
manifold pressure adjustment with an accurate
manometer. Failure to accurately adjust
pressure could cause heat exchanger failure,
asphyxiation, fire or explosion, resulting in
damage, injury or death.
1. Make sure you have proper main burner orifices.
2. Gas supply pressure must be between 11 and 13
inches W.C. for propane (LP) gas.
3. Start furnace following "Operating Instructions" on
front door.
4. Let furnace warm up for 6 minutes.
5. Adjust manifold pressure to 10.0 W.C. ± 0.3
inches W.C. See Section 19, "Manifold Pressure
Adjustment".
WARNING
Do not set Propane (LP) manifold pressure at
11.0 inches W.C. It could cause heat
exchanger failure.
21. STANDARD ORIFICE SIZING AND
HIGH ALTITUDE DERATE
This furnace is shipped with fixed gas orifices for use
with Natural Gas and sized for 1000 BTU/cubic foot
gas. Make sure actual gas input does not exceed
rating plate input. You may need to change orifices
to get correct gas input. Whether you do or not
depends on input, and your gas heat value at standard
conditions and elevation. Consult your local gas
supplier for gas heat value and any special derating
requirements. See Section 19 for more information.
Manual 2100-365
Page 21
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