Johnson Controls RL18 Installation Manual page 32

Measurement of temperature rise
WARNING
The temperature rise, or temperature difference
between the return air and the supply (heated) air
from the furnace, must be within the range shown
on the furnace rating plate and within the application
limitations for specific furnace ratings and physical/
electrical data as specified in these instructions.
The supply air temperature cannot exceed the
"Maximum Supply Air Temperature" as it may cause
premature heat exchanger failure, high levels of
Carbon Monoxide, a fire hazard, personal injury,
property damage, and/or death.
After about 5 min of operation, determine the furnace
temperature rise. Measure the temperature of both
the return air and the heated air in the ducts, about 6 ft
(1.83 m) from the furnace where they are not affected by
radiant heat. Increase the blower speed to decrease the
temperature rise. Decrease the blower speed to increase
the temperature rise.
Figure 29: Furnace control board
32 Installation Manual: RL18 Single-Stage Standard ECM Residential Gas Furnaces (Non-condensing Multi-position Standard
Low NOx)
The blower speed taps are located on the furnace control
board in the blower compartment. See Figure 29 and the
unit wiring label to change the blower speed. To use the
same speed tap for heating and cooling, the heat terminal
and cool terminal must be connected using a jumper
wire and connected to the required motor lead. Place all
unused motor leads on park terminals. One park terminal
is provided.
Adjustment of blower control settings
This furnace is equipped with a time-on/time-off heating
blower control. The blower-on delay is fixed at 30 s. The
blower-off delay has four settings (60 s, 90 s, 120 s, and
180 s). The blower-off delay is factory-set to 120 s. The
blower-off setting must be long enough to adequately
cool the furnace, but not so long that cold air is blown into
the heated space. The blower-off timing can be adjusted
by positioning the dip switches as shown in Table 13. See
Figure 29 for the dip switch location on the control board.
Table 13: Dip switch settings for blower-off timing
Dip switch setting
00
01
10
11
Johnson Controls Ducted Systems
Blower-off delay (s)
60
90
120
180