Boiler Operation:
The supply water temperature from a boiler can be controlled by
cycling the boiler on and off. Modulation of the boiler’s operating
temperature in hot water heating systems not only provides more
comfort but also offers significant energy savings. The cooler the
boiler runs, the more efficient it is due to less heat losses up the flue
and reduced boiler jacket losses.
Differential:
An on / off boiler must be operated with a differential in order to pre-
vent short cycling. When the supply water temperature drops below
the bottom rail of the differential, the boiler is turned on. The boiler is
then kept on until the supply water temperature rises above the top
rail of the differential. If the differential is too wide, there can be large
supply water temperature swings; however, if the differential is too
narrow, the boiler short cycles and operates inefficiently. This control
can be set for two different boiler differentials, 10°F (6°C) or 20°F
(12°C). This setting will help to achieve an appropriate balance
between temperature swings and boiler efficiency.
Minimum Boiler Supply:
Most boilers require a minimum supply water temperature in order to
prevent corrosion from flue gas condensation. The control should
therefore only modulate the boiler supply water temperature down to
the boiler manufacturer’s minimum recommended operating temper-
ature. Some boilers are designed to condense and should be oper-
ated at low water temperatures as much as possible for maximum
efficiency.
S
U
P
P
L
Y
W
A
T
E
R
T
E
M
P
E
R
A
T
U
R
E
TIME
Differential = 10
°F (5 °C)
165
°F(74 °C)
155°F (68 °C)
160°F (7 1°C)
BOILER ON
BOILER ON
BOILER OFF
BOILER OFF
OUTDOOR AIR TEMPERATURE
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
90
(32)
WWSD
TEMPERATURE
°
F (
°
C)
SU
PP
LY
W
AT
ER
T
EM
PE
RA
TU
RE
110 (43)
70 (21)
90 (32)
210 (99)
170 (77)
150 (65)
130 (54)
190 (88)
RESET
RATIO
BOILER
MINIMUM
SEQUENCE OF OPERATION:
Powering Up the Control:
After the SR501-OR is powered up, the green power LED light will be on.
Heating Demand:
The SR501-OR receives a heating demand from thermostat or zone
control end switch closure on the R and W thermostat terminals. The
thermostat red LED light will also be turned on. The control can be
setup for boiler temperature reset or ON – OFF boiler operation.
Reset Demand:
When the SR501-OR receives a heating demand the controls enable
the boiler based on the reset curve. The Boiler Relay red LED light
will be turned on when the boiler relay is energized.
DHW Demand:
When the SR501-OR receives a DHW demand the controls enable
the boiler and ignore the boiler reset curve. The DHW Override red
LED light will also be turned on.
Warm Weather Shut Down (WWSD):
When the outdoor temperature rises above 70°F (21°C), the control
turns off the boiler and blinks the Thermostat LED light. This function
only applies for space heating and does not affect DHW operation.
Boiler Operation:
When the SR501-OR has a reset demand, the system supply water
temperature is controlled by turning the boiler on and off. The control
calculates the target supply water temperature based on the outdoor
temperature, and the settings that were entered. The boiler operates
around a differential that is manually set by dip switch number 2. The
control includes a minimum on and off time for the boiler, in order to
minimize short cycling.
Maximum Boiler Supply:
The SR501-OR does not allow the boiler target temperature to
exceed the boiler design temperature setting plus 10°F (6°C) up to a
maximum of 225°F (107°C).
Minimum Boiler Supply:
The SR501-OR has a 140°F minimum boiler setting which is used in
order to prevent corrosion from flue gas condensation. During mild
outdoor conditions, the boiler cycles around the minimum boiler set-
ting. This programmed function can be set to the OFF setting for
condensing and electric boilers.
INSTALLATION:
Step One:
Check the contents of this package. If any of the contents listed are
missing or damaged, please contact your sales representative for
assistance. The SR501-OR includes:
SR501-OR Boiler Reset Control and Switching Relay Combination
One Outdoor Sensor
One Strap on Sensor
Instruction Sheet
Replacement sensors are available from Taco, Inc.:
Strap on Sensor – 9300-2044RP
Outdoor Sensor – 9300-2052RP
Note:
Carefully read the details of the Sequence of Operation section
in this brochure to ensure that you have chosen the proper control for
your application.
Step Two:
Installing the Control:
The enclosure is to be mounted flush onto a wall or any ridge sur-
face.
• The mounting holes in the enclosure accept #6 screws.
• Line voltage to enter from bottom of control and low voltage to
enter from top of control.
Installing the Outdoor Sensor:
Note:
The temperature sensor (thermistor) is built into the enclosure.
• Use one round or pan head screw to attach the base of the sen-
sor to the wall.
• The sensor is wall mounted and the wiring enters through the
bottom of the enclosure. The hole for the cable entry must face
downward in order to prevent water from entering and filling the
enclosure.
CAUTION: Improper installation and operation of this control
could result in damage to the equipment and possibly even
personal injury. It is your responsibility to ensure that this con-
trol is safely installed according to all applicable codes and
standards. This electronic control is not intended for use as a
primary limit control. Other controls that are intended and cer-
tified as safety limits must be part of the control circuit.
