Instruction Sheet
FuelMizer
®
SR501-OR-4 Switching Relay with Outdoor Reset
A Taco Family Company
102-431
SUPERSEDES: March 1, 2013
EFFECTIVE: December 20, 2013
Plant ID# 9300-2871
MULTI-ZONE INSTALLATION WITH ZONE CONTROL
AND WITH INDIRECT DHW
ONE ZONE
SWITCHING RELAY
WITH OUTDOOR RESET
S
R
5
0
1
O
R
-4
LED
INDICATORS
POWER
TSTAT
DHW
OVERRIDE
BOILER
RELAY
N H
INPUT
120 VAC
ZONE 1
120 VAC
N H
6
AMP
X
R
BOILER
TSTAT
W
C R
W
DHW
OUTDOOR
SENSOR
SUPPLY
SENSOR
X
DESIGN OUTDOOR
TEMPERATURE
FOR 180°F SUPPLY
TEMPERATURE
OFF
-30
-20
0
-10
10
20
40
30
ON
SW1
OFF
SYSTEM
CIRCULATOR
(OPTIONAL)
N
H
POWER
INPUT
(120 VAC)
BOILER
CONTROL
OUTDOOR
SENSOR
BOILER
SUPPLY
SENSOR
POT1
1 2 3 4
ZONE 1
1 2 3 4
ZONE 2
1 2 3 4
ZONE 3
EXISTING
ZONE VALVE CONTROL
P
O
W
E
R
I
N
2
4
V
A
C
N/O COM N/C
PUMP END
SWITCH
FU
S
E
5
A
M
P
ZONE 3 PUMP
END SWITCH
THERMOSTATS
(24VAC)
T STAT 1
VALVE 1
T STAT 2
VALVE 2
T STAT 3
VALVE 3
POWER
LED
INDICATORS
ZONE 1 ZONE 2 ZONE 3
ISOLATED
X
X
R W
R W
R W
END SWITCH
THERMOSTATS
PRIORITY
24 VAC ZONE VALVES
PRIORITY
X
X
ZONE 3
PRIORITY
OFF
ON
C
C
C
2 WIRE ZONE VALVE
(NO END SWITCH)
MUST USE JUMPER
TACO 3 WIRE
ZONE VALVE
1
2
3
JUMPER
3 & 4
TACO 3 WIRE
ZONE VALVE
1
2
3
DHW
AQUASTAT
A
CONTROL STRATEGY:
Outdoor Reset:
In order to properly control a hot water heating system, the heat sup-
plied to the building must equal the heat lost by the building.
• The heat supplied to a building is proportional to the temperature of
the supply water and the surface area of the heating element. A
small surface area such as baseboard radiators requires a higher
water temperature than a larger surface area such as radiant floors.
• The heat lost from a building is dependent on the outdoor tem-
perature, as well as other factors. As the outdoor temperature
drops, the building heat loss increases.
Reset Ratio:
Operation of a hot water heating system can generally be improved by
modulating the supply water temperature as the outdoor temperature
changes. Using this approach, the heat lost from the building is
matched by the heat input to the building. The SR501-OR boiler reset
control utilizes a reset ratio to set the relationship between outdoor tem-
perature and supply water temperature. The reset ratio determines the
amount the supply water temperature is raised for every 1° drop in out-
door air temperature, and it is determined from the starting point and
the system design conditions. In order for the control to automatically
determine the reset ratio, outdoor design conditions must be estab-
lished. This is usually the typical coldest outdoor temperature or it can
be set by using ASHARE standards for Design Outdoor Temperature
for specific geographic locations.
APPLICATION:
The SR501-OR is a microprocessor-based control designed to regulate
the supply water temperature of a single boiler, based on the outdoor
temperature. The SR501-OR is a boiler reset control and switching relay
in one unit. The SR501-OR is ideal for retrofit application, but may also
be used in new installations as well.
The SR501-OR includes functions such as automatic reset ratio calcu-
lation, warm weather shut down (WWSD), minimum boiler temperature
setting, and boiler differential setting. The control also has boiler sensor
and outdoor sensor error checking.
40
30
20
10
0
-10
-20
-30
70
-20
90
130
110
150
170
180
190
210
90
80
70
60
50
40
30
20
10
0
-10
OUTDOOR TEMPERATURE (°F)
BOILER SUPPLY TEMPERATURE vs. OUTDOOR TEMPERATURE (°F)
FOR OUTDOOR DESIGN TEMPERATURES
B
O
IL
E
R
S
U
P
P
LY
T
E
M
P
E
R
A
T
U
R
E
(
°F
)
For information on Taco’s
Switching Relays (SR)
including catalog sheet,
instruction sheets, Visio sten-
cils and our highly praised
Zone Controls Wiring Guide,
scan the QR code above or
go to our website:
http://www.TacoComfort.com.
ONE ZONE INSTALLATION W/O INDIRECT DHW
ONE ZONE
SWITCHING RELAY
WITH OUTDOOR RESET
S
R
5
0
1
O
R
-4
LED
INDICATORS
POWER
TSTAT
DHW
OVERRIDE
BOILER
RELAY
N H
INPUT
120 VAC
ZONE 1
120 VAC
N H
6
AMP
X
R
BOILER
TSTAT
W C R
DHW
OUTDOOR
SENSOR
SUPPLY
SENSOR
X
DESIGN OUTDOOR
TEMPERATURE
FOR 180°F SUPPLY
TEMPERATURE
OFF
-30
-20
0
-10
10
20
40
30
ON
SW1
OFF
SYSTEM
CIRCULATOR
(OPTIONAL)
N
H
POWER
INPUT
(120 VAC)
24 VAC
THERMOSTAT
BOILER
CONTROL
OUTDOOR
SENSOR
BOILER
SUPPLY
SENSOR
POT1
See
NOTE
NOTE:
Resistor
(1K
Ω
,
1
⁄
2
W)
may be needed
between
W
and
C
terminals.
W
