Reznor RPBL Unit Installation Manual - Manual (Page 49)

Form I-SSCBL/RPBL, P/N 149159 R7, Page 49
Sensor Location -
Options AG39 and
AG41
Calculate the Setpoints with Two Furnace Sections
- Use the following formulas to calculate the controller
settings for Heat Stage 2 and Heat Stage 3 in a system
with two furnace sections.
T
SP
= Setpoints of Heat Stage Controllers (T
SP2
and T
SP3
)
T
SA
= Desired Supply Air Temperature
T
D
= Design (minimum) Entering Air Temperature
Formulas for two furnace sections:
Setpoint for Heat
Stage 2: T
SP2
= T
SA
- 0.46 (T
SA
- T
D
)
Setpoint for Heat Stage 3: T
SP3
= T
SA
- 0.73 (T
SA
- T
D
)
Set Heat Stage
Controllers - applies
to Options AG41 and
AG42 only
For the convenience of the installer, the duct temperature sensor is factory installed
in the cabinet leg (See FIGURE 49, page 47). Although the sensor has a mixing tube,
at this distance from the discharge it does not receive a true mix, so the temperature
read by the sensor will be slightly higher than the actual air entering the ductwork. The
system will provide comfort level heat if the selector is set slightly lower to compensate
for this reading. The offset temperature will vary with the application. If a direct correla-
tion of these two temperatures is required, move the duct sensor to a location in the
ductwork about 10-12 feet (3 - 3.7M) from the furnace discharge.
Systems with Option AG41 and AG42 have “heat stage controllers” that control opera-
tion of the "two-stage" furnace based on outside air temperature setpoints. Proper
setpoints are important to ensure the modulating furnace ("Heat Stage 1") is always in
control and avoids cycling . The proper setpoint for each controller must be determined
from basic design information. Sizes 500, 600, and 800 with two furnace sections have
two heat stage controllers; Size 1200 with three furnace sections has four heat stage
controllers.
Follow the steps and example to determine appropriate setpoints. Follow the instruc-
tions to locate and set the controllers.
EXAMPLE : 3600 CFM, Power vented, 100%
Outside Air, -10°F Outdoor Winter Design, 75°F
Desired Supply Air
T
SP2
= 75 - [.46 x (75 - (-10))] = 75 - (.46 x 85) =
35.9
T
SP3
= 75 - [.73 x (75 - (-10))] = 75 - (.73 x 85) =
12.9
Set Stage Heat #2 Controller to 36°F
Set Stage Heat #3 Controller to 13°F
Calculate the Setpoints with Three Furnace Sections
- Use the following formulas to calculate the controller
settings for Heat Stage 2, Heat Stage 3, Heat Stage
4, and Heat Stage 5 in a system with three furnace
sections.
T
SP
= Setpoints of Heat Stage Controllers (T
SP2
, T
SP3
,
T
SP4
, T
SP5
)
T
SA
= Desired Supply Air Temperature
T
D
= Design (minimum) Entering Air Temperature
Formulas for three furnace sections:
Setpoint for
Heat Stage 2: T
SP2
= T
SA
- 0.30 (T
SA
- T
D
)
Setpoint for Heat Stage 3: T
SP3
= T
SA
- 0.49 (T
SA
- T
D
)
Setpoint for Heat Stage 4: T
SP4
= T
SA
- 0.65 (T
SA
- T
D
)
Setpoint for Heat Stage 5: T
SP5
= T
SA
- 0.82 (T
SA
- T
D
)
EXAMPLE : 8850 CFM, Power vented, 100% Outside
Air, -10°F Outdoor Winter Design, 75°F Desired
Supply Air
T
SP2
= 75 - [.30 x (75 - (-10))] = 75 - (.30 x 85) = 49.5
T
SP3
= 75 - [.49 x (75 - (-10))] = 75 - (.49 x 85) = 33.3
T
SP4
= 75 - [.65 x (75 - (-10))] = 75 - (.65 x 85) = 19.8
T
SP5
= 75 - [.82 x (75 - (-10))] = 75 - (.82 x 85) = 5.3
Set Stage Heat #2 Controller to 49°F
Set Stage Heat #3 Controller to 33°F
Set Stage Heat #4 Controller to 20°F
Set Stage Heat #5 Controller to 5°F
Locate and Set the Heat
Stage Controllers
1. In the inlet air section, locate the heat stage controllers. The same type of
controller may also used as the optional high ambient limit control (Option BN2)
and as the mixed air controller that is part of certain air control options (Options
AR12, AR13, AR15, and AR16). Identify the controllers marked Heat #2 and Heat
#3 or Heat #2, Heat #3, Heat #4, and Heat #5.
2 Furnaces - Identify controllers marked Heat #2 and Heat #3; set as
calculated above.
3 Furnaces - Identify controllers marked Heat #2, Heat #3, Heat #4, and
Heat #5; set as calculated above.
2. Adjust each controller to the setpoint as determined in the calculation.