120 VAC INPUT
TWO ZONE SWITCHING RELAY
CIRCULATORS
ZC
INPUT
120 VAC
ZONE 1
ZONE 2
PRIORITY
ZONE1
ZONE2
PRIORITY
THERMOSTATS
(24 VAC)
SR 502-4
FUSE
6 AMP
FUSE
6 AMP
FUSE
6 AMP
X X
R
W R
W
ISOLATED
END
SWITCH
THERMOSTATS
120 VAC
ZR
H
H
N H
N H
N
H
ZONE 2
POWER
ZONE 1
LED
INDICATORS
ZONE 2
PRIORITY
OFF
ON
TO: "TT" ON
BOILER
JUMPER
24 VAC
COM
120
VAC
See
NOTE
120 VAC INPUT
TWO ZONE SWITCHING RELAY
CIRCULATORS
ZC
INPUT
120 VAC
ZONE 1
ZONE 2
PRIORITY
ZONE1
ZONE2
PRIORITY
THERMOSTATS
(24 VAC)
SR 502-4
FUSE
6 AMP
FUSE
6 AMP
FUSE
6 AMP
X X
R
W R
W
ISOLATED
END
SWITCH
THERMOSTATS
120 VAC
ZR
H
H
N H
N H
N
H
ZONE 2
POWER
ZONE 1
LED
INDICATORS
ZONE 2
PRIORITY
OFF
ON
120
VAC
TO: "ZC"
ON BOILER
TO: "ZR"
ON BOILER
24 VAC
COM
See
NOTE
Operation:
When any thermostat calls for heat, the appropriate circu-
lator is energized and the isolated end switch (X and X) will start the
boiler.
Priority Operation:
When zone 2 is switched to the priority setting
and is actuated, all other zones will stop operation until zone 2 is sat-
isfied. When zone 2 is not switched to priority, all zones will operate
independently.
Priority Protection Operation:
When the priority switch is set to ON,
and if the priority zone calls continuously for more than one hour,
power is returned to all the other zones, allowing each zone to function
independently. Once the priority zone is satisfied, the control’s auto-
reset is activated and the priority zone is again allowed to have priority
for up to one hour starting from when it calls next.
Jumper Placement:
The jumper should be placed between terminals
ZC and ZR. Connect the isolated end switch to the aquastat control on
the boiler.
Power Input:
Connect 120 volt ac power input to terminals N and H.
Neutral wire to terminal N. Hot wire to terminal H.
Operation:
When any thermostat calls for heat, the boiler is given a
signal to start. The appropriate circulator is energized only when the
boiler temperature is above the set low limit.
Priority Operation:
When zone 2 is switched to the priority setting
and is actuated, all other zones will stop operation until zone 2 is sat-
isfied. When zone 2 is not switched to priority, all zones will operate
independently.
Priority Protection Operation:
When the priority switch is set to ON,
and if the priority zone calls continuously for more than one hour,
power is returned to all the other zones, allowing each zone to function
independently. Once the priority zone is satisfied, the control’s auto-
reset is activated and the priority zone is again allowed to have priority
for up to one hour starting from when it calls next.
Jumper Placement:
REMOVE the jumper between terminals ZC and
ZR. Connect terminal ZC to ZC on the aquastat control. Connect ter-
minal ZR to ZR on the aquastat control. Confirm polarity is consistent
between boiler aquastat and switching relay.
Power Input:
Connect 120 volt ac power input to terminals N and H.
Neutral wire to terminal N. Hot wire to terminal H.
Cold Start Boiler
Application:
Tankless Coil Boiler Application:
(Alternative Wiring)
Instruction Sheet
SR502-4 Switching Relay
102-377
SUPERSEDES: March 1, 2013
EFFECTIVE: December 20, 2013
Plant ID# 9300-2866
WARNING:
When using Alternative Wiring diagram, wiring instruc-
tions must be followed so power originates from the boiler aquastat.
Failure to follow these wiring instructions may result in a secondary
source of power being connected to the boiler that may activate it
under certain circumstances, causing injury or death.
Specifications:
PRODUCT
NUMBER
INPUT
MAXIMUM
TYPE 1 ENCLOSURE
NUMBER
OF ZONES
VOLTAGE
COMBINED LOAD
WIDTH
HEIGHT
DEPTH
SR502-4
2 with Priority
120/60/1 VAC
15 amps
10
3
/
4
"
7"
2
3
/
4
"
All circulator relay connections, including ZC/ZR, are rated
1
/
3
hp (6 FLA, 36 LRA) at 120 VAC.
End switch connections are rated 24 VAC, 1 amp.
All thermostat connections supply a 24 VAC class 2 output.
This device complies with part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) This device
may not cause harmful interference, and (2) this device must accept any interference received, including interference
that may cause undesired operation.
WARNING:
Wiring connections must be made in accordance with all
applicable electrical codes. Use copper wire only. 120 VAC wiring must have
a minimum temperature rating of 75°C. Failure to follow this instruction
can result in personal injury or death and/or property damage. 12-18 gauge
wire recommended for 120 VAC connections, 14-22 gauge wire for thermo-
stat connections, and 14-22 gauge wire for 24 VAC source connections.
NOTE:
Resistor (1K
Ω
,
1
⁄
2
W) may be needed
between
W
and
C
terminals.
Troubleshooting:
• Problem:
Digital thermostats do not work correctly when connected
to a switching relay.
• Solution:
Some thermostats are a “Power Stealing” type which means
they are powered by the switching relay with just 2 wires (
R
&
W
). A
resistor may be needed in order to have the thermostat work properly.
This resistor should be placed between the
W
&
C
(
common
) termi-
nals of the switching relay. If the thermostat manufacturer does not
supply a resistor, a 1000 ohm ½ watt resistor has proven to work with
most models and is readily available at electronic supply outlets (e.g.
Radio Shack). If the thermostat is battery powered, then check that
the batteries are fresh and installed correctly.
• Problem:
No heat in a zone or room of building.
• Solution:
LED diagnostic lights will help find a component that is
not working properly. The green LED should always be on, indicating
that power is connected and the solid-state fuse is good. When there
is a call for heat, the red LED will come on indicating power to the
zone circulator. This indicates the thermostat is working correctly. If
the red LED does not come on, then check the thermostat and ther-
mostat wiring for errors.
For information on Taco’s Switching Relays (SR)
including catalog sheet, instruction sheets, Visio
stencils and our highly praised Zone Controls
Wiring Guide, scan the QR code to the left or go to
our website: http://www.taco-hvac.com.
