Instructions / Assembly

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 3 is switched to the priority setting
and is actuated, all other zones will stop operation until zone 3 is sat-
isfied. When zone 3 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 3 is switched to the priority setting
and is actuated, all other zones will stop operation until zone 3 is sat-
isfied. When zone 3 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: REMO
VE 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
SR503-4 Switching Relay
102-379
SUPERSEDES: December 20, 2013 EFFECTIVE: January 18, 2017
Plant ID# 9300-2867
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
SR503-4 3 with Priority 120/60/1 VAC 15 amps 12
1
/4" 8" 3"
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.
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.
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.
NOTE: Resistor (1KΩ,
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 work-
ing properly. The green LED should always be on, indicating that power is con-
nected 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 thermostat 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.