32
AIR FLOW
The importance of proper air flow over
the heat exchanger cannot be over
emphasized.
IT IS IMPORTANT THAT EACH DUCT
SYSTEM BE SIZED AND INSTALLED
FOR THE SPECIFIC APPLICATION
BY PROPERLY APPLYING THE
APPROPRIATE INDUSTRY
ACCEPTED STANDARD. IF LESS
THAN MINIMUM STANDARDS ARE
APPLIED, THE EQUIPMENT USER
COULD EXPECT TO EXPERIENCE
HIGHER UTILITY BILLS, MAJOR
COMPONENT FAILURE, VARYING
DEGREES OF AIR NOISE OR OTHER
UNSATISFACTORY ISSUES, OVER
WHICH THE MANUFACTURER HAS
NO CONTROL.
One of the most common causes of
heat exchanger failure is overheating
due to low air flow. An air flow table is
located inside the blower door and on
the following pages.
TEMPERATURE RISE CHECK
To determine if the air flow is correct,
make a temperature rise check.
1. Insert a thermometer in the supply
air duct as close to the furnace as
possible yet out of a direct line from
the heat exchanger. See Figure 21.
2. Insert a thermometer in the return
air duct as close to the furnace as
possible.
3. Operate the furnace.
4. When the thermometer in the
supply air duct stops rising
(approximately five minutes),
subtract the return air temperature
from the supply air temperature.
The difference is the temperature
rise.
5. Compare the measured
temperature rise to the approved
temperature rise range listed on
the furnace name plate. See Figure
22.
If the measured temperature rise is
above the approved range, either the air
flow is too low or the manifold pressure
needs to be adjusted. More air must be
moved by speeding up the blower, by
removing restrictions in the duct
system, or by adding more supply or
return air duct. If the measured
temperature rise is below the approved
range, either the air flow is too much or
the manifold pressure needs to be
adjusted. Use lower speed tap on the
multi-speed blower.
IMPORTANT:
The measured
temperature rise should be in the
middle of the range.
IMPORTANT:
Some high-efficiency
filters have a greater than normal
resistance to airflow. This can adversely
affect furnace operation. BE SURE TO
CHECK AIRFLOW if using any filter
other than factory-provided filter.
FIGURE 21
TEMPERATURE RISE MEASUREMENT
LUBRICATION
The indoor blower motor and induced
draft motor are prelubricated by the
motor manufacturer and do not require
further attention.
The motors must be cleaned
periodically by a qualified installer,
FIGURE 22
TYPICAL FURNACE NAME PLATE
service agency, or the gas supplier to
prevent the possibility of overheating
due to an accumulation of dust and dirt
on the windings or on the motor exterior.
Air filters should be kept clean. Dirty
filters can restrict airflow and results in
motor overheating.
CAUTION
!
AIR CONDITIONING DIVISION
FORT SMITH, ARKANSAS
Summary of Contents for GMDA
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