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18.0 AIR FLOW
For proper furnace operation, air fl ow over the heat exchanger is of utmost importance. Insuffi cient airfl ow
accelerates metal fatigue and possible failure in the heat exchanger, as well as decreased effi ciency.
Excessive airfl ow promotes accelerated corrosion of the heat exchanger.
IMPORTANT:
DO NOT BYPASS THIS STEP OF THE
START UP PROCEDURES.
18.1
TEMPERATURE RISE CHECK
When the duct system is complete and the air fi lter or fi lters are in place, determine if the airfl ow is correct for both
low and high fi re input rates.
1. Insert a duct thermometer in the supply air duct. The thermometer should be placed as close as practical
to the furnace, but out of the "line of sight" of the heat exchanger (this prevents false readings owing to
radiant heat). Ensure that the thermometer location is within the duct air stream. Avoid locations such as
the inside radius of an elbow, etc.
2. Insert a duct thermometer in the return air duct as close to the furnace as practical. Ensure that the
thermometer location will be unaffected by humidifi er bypass ducts, etc. Choose a location well within the
main air stream.
3. Operate the furnace long enough to obtain steady state conditions at both input rates (High Fire and Low Fire).
4. When the two thermometers have stabilized, usually within 5-8 minutes, compare the two readings.
Subtract the return air temperature from the supply air temperature. The difference is the temperature rise,
also called ∆T.
5. Compare the measured ∆T to the temperature rise range shown on the rating plate.
When adjusting the temperature rise, see Section 18.3. The ideal temperature is approximately mid-range on the
rating plate.
Heat speeds are not meant to be adjusted.
18.2
CALCULATING AIR FLOW
There are circumstances where it may be desirable to know the air fl ow delivery through the duct system,
such as when estimating the amount of air fl ow available for
air conditioning. This can be done by direct measurement with
electronic or sloped manometers and velometers or use the
formula in the next column.
Refer to section 3.1 Model Input and Temperature Rise.
H22.0
H22.1.7
Output
CFM =
∆T
1.085 x
where:
•
CFM is airfl ow in cubic feet per minute;
•
∆T is the temperature rise; and
utput is the furnace output capacity
•
O
from the rating plate
.
H22.2.3B
W415-2379 / B / 09.22.20
47
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