LAARS Rheos RHCV - Install and Operating Manual - Manual (Page 24)

LAARS Heating Systems Page 24
5.5.1 Temperature Setpoint
The temperature setpoint is the point at which the
temperature control relay will be de-energized and the
unit’s call for heat will end. The temperature setpoint
is adjusted with the setpoint dial on the A350 control
(see Figure 13). The temperature control’s sensor is in
the inlet of the unit.
5.5.2 Differential Setting
The differential is the difference in temperature
settings when the temperature control’s relay is
energized and de-energized. For instance, if the
temperature setpoint is 180°F (82°C) and the
differential is 20°F (11°C), the temperature control
will call the unit to energize at 160°F (71°C). (The
differential is the setpoint minus the differential
setting) The call for heat will end when the unit
reaches its setpoint of 180°F (82°C). The Rheos
differential is adjustable from 1°F to 30°F (1°C to
17°C). In general, the differential setting for a water
heater system will be low, so that when a storage tank
is drawing down, the heater will react to sustain a
minimum tank temperature. Boilers generally have
higher differentials than water heaters, to minimize
short cycling (see throttling range). The differential
is set by removing the cover of the A350 control
and adjusting the potentiometer marked “DIFF” (see
Figure 13).
5.5.3 Offset
The offset determines the temperature below the
controller’s setpoint where the throttling range begins.
It offsets the throttling range.
For instance, if the setpoint is 180°F (82°C) and
the offset is 5°F (3°C), then the unit’s throttling range
will begin at 175°F (79°C). Between 175°F (79°C) and
180°F (82°C), the Rheos will be in low fire.
The offset can be adjusted between 0°F and 30°F
(0°C and 17°C) by removing the cover of the S350P
control and adjusting the Offset dial (see Figure 14).
5.5.4 Throttling Range
The throttling range allows the Rheos to match
the BTU demands of varying systems, by allowing the
user to choose the temperature range through which
the unit will modulate. This will tell the controller how
fast to respond to a change in temperature. A small
throttling range keeps the unit in high fire more, and
quickens the response of the system. A larger throttling
range allows the Rheos to spread its modulating range
over a larger temperature range.
Example, Imperial units: Using a temperature
setpoint of 180°F, with a 20°F differential, a 5°F
offset, and a throttling range of 10°F, the unit will
come on at 160°F and shut off at 180°F. The offset
will set the top of the throttling range 5°F under the
setpoint, which is 175°F in this example. Therefore,
the throttling range will cause the Rheos to modulate
between 50% and 100% of full fire between 175°F and
165°F. Between 175°F and 180°F, the unit will remain
at low fire. Between 160°F and 165°F, the unit will be
at full fire.
Example, Metric units: Using a temperature
setpoint of 82°C, with a 11°C differential, a 3°C offset,
and a throttling range of 5°C, the unit will come on at
71°C and shut off at 82°C. The offset will set the top
of the throttling range 3°C under the setpoint, which is
79°C in this example. Therefore, the throttling range
will cause the Rheos to modulate between 50% and
100% of full fire between 79°C and 74°C. Between
79°C and 82°C, the unit will remain at l02 fire.
Between 71°C and 74°C, the unit will be a t full fire.
The throttling range is adjustable between 2°F
and 30°F (1°C to 17°C). The range can be set by
removing the cover of the S350P control and adjusting
the “THROT RANGE” dial (see Figure 14).
5.5.5 Minimum Output Setting
The minimum output setting of the controller is
0% of full fire. However, the minimum output of the
Rheos is limited to 50% of full fire by the fan speed
controller, to ensure proper operation of the Rheos.
If a minimum desired output is higher than 50%, the
setting can be adjusted up by removing the cover of
the S350P control and adjusting the Min Output dial
(see Figure 14). Turning the dial clockwise increases
the minimum output. In most cases, this setting should
be left as shipped from the factory, with the arrow
pointing to the 5 o'clock position.
5.5.6 Integration Constant
The proportional control used on the Rheos has
internal circuitry that helps to minimize overshoot that
can sometimes be associated with proportional plus
integral (PI) controls. There are three field-selectable
integration constants (slow, medium and fast), and
an off position on the proportional control, which
are selected with the dipswitches on the control (see
Figure 14). In most cases, this setting will not need to
be changed
Off: Switch 1 to ON position and all others to
OFF. Setting the control to the “Off” position puts the
control into proportional only operation. This is not a
recommended setting for the Rheos.
Slow: Switch 2 to ON position and all others
to OFF. This is the slowest integration constant. It
allows the Rheos the most time to reach its setpoint.
In most cases, this is too slow for Rheos installations.
Medium: Switch 3 to ON position and all others
to OFF. The integration constant is set to “Medium”
at the factory, and in most cases, will not need to be
changed. This setting will be the best suited to most
Rheos installations.
Fast: Switch 4 to ON position and all others
to OFF. If the rate of system recovery to setpoint is