LAARS Rheos+ RHHV - Install and Operating Manual - Manual (Page 27)

LAARS Heating Systems Page 26
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 120°F (49°C) and
the offset is 5°F (3°C), then the unit’s throttling range
will begin at 115°F (46°C). Between 115°F (46°C) and
120°F (49°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 120°F, with a 20°F differential, a 5°F
offset, and a throttling range of 10°F, the unit will
come on at 100°F and shut off at 120°F. The offset
will set the top of the throttling range 5°F under the
setpoint, which is 115°F in this example. Therefore,
the throttling range will cause the Rheos+ to modulate
between 25% and 100% of full fire between 115°F and
105°F. Between 115°F and 120°F, the unit will remain
at low fire. Between 100°F and 105°F, the unit will be
at full fire.
Example, Metric units: Using a temperature
setpoint of 49°C, with a 11°C differential, a 3°C offset,
and a throttling range of 5°C, the unit will come on at
38°C and shut off at 49°C. The offset will set the top
of the throttling range 3°C under the setpoint, which is
46°C in this example. Therefore, the throttling range
will cause the Rheos+ to modulate between 25% and
100% of full fire between 46°C and 49°C. Between
100°F
38°C 41°C
105°F
43°C
110°F
46°C
115°F
49°C
120°F
100%
75%
50%
0%
25%
Throttling Range
Differential
Offset
Control Parameters:
Setpoint = 120°F / 49°C
Differential = 20°F / 11°C
Offset = 5°F / 3°C
Throttling Range = 10°F / 5°F
Minimum Output = 25% (Factory Setting)
Note: Graph depicts a rise in Rheos temperature after a call for heat. After achieving 120°F/49°C, the Rheos
will not re-energize until the temperature falls below 100°F/38°C.
Rheos+ Inlet Temperature
%
of
Full
Fire
Figure 14 - Control Parameter Graph