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.
Priority Oper tion: When the priority dip switch is set to ON and the
priority zone is actuated, all other zones will stop operation until pri-
ority zone is satisfied. When not switched to priority, all zones will
operate independently.
Mode Oper tion: When the dip switch is set to NORMAL, the end
switch relay will be energized if any zone is in operation. When the switch
is set to RESET, the end switch relay will only be energized if the priority
zone is in operation, or through the operation of a plug-in reset control.
Prim ry Pump Oper tion: When the dip switch is set to OFF, the pri-
mary circulating pump output will energize when any zone calls for heat,
except the priority zone. When the dip switch is set to ON, the primary
circulating pump output will energize when any zone calls for heat.
Post Purge Oper tion: When the dip switch is set to ON, the priority
zone output will stay energized for 2 minutes after its thermostat or
aquastat is satisfied, but not operate the boiler.
Priority Protection Oper tion: When the dip 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.
Pump Exercise Oper tion: When the dip switch is set to ON, the
solid state timer cycles all the circulating pumps that are attached to
the Expandable Switching Relay at the selected time interval. The time
interval can be set for the pumps to run for either 30 seconds every 2
weeks or for 4 minutes every 24 hours.
Low Limit (ZC) Oper tion: When the dip switch is set to ON and the
boiler drops below the set low limit (terminal ZC connected to boiler),
all zone circulating pumps will stop. When the boiler rises above the
set low limit, the zone circulating pumps are allowed to operate.
End Switches (Dry Cont cts): The main end switch closes when any
zone thermostat calls for heat and the mode switch is set to NORMAL.
The main end switch also closes when the mode switch is set to RESET
and a PC Series boiler reset power control is calling for heat. The priority
end switch closes only when the priority zone thermostat or aquastat is
calling for heat.
Exp nsion Connections: Set the expansion switch to MASTER on the
switching relay that has the designated priority zone or is utilizing the
PC Series plug-in option. Set all other daisy chained controls to SLAVE.
Using thermostat wire (18-22 gauge) connect between terminals A, B,
C on the master control to the corresponding A, B, C on the SLAVE
control(s). Controls may be daisy chained up to 20 zoning panels using
any combination of -EXP controls (120 zones if all are 6 zone panels).
Thermost t Input (24 v c):
R Hot side of transformer. Connect to Ron thermostat.
W Switched R signal from thermostat. Connect to W on thermostat.
C Common side of transformer. Connect to COM on
thermostat (optional).
NET Network terminals 1 & 2 are tied together for wiring convenience
when using communicating style thermostats (optional).
120 VAC Connections (N is Neutr l, H is Hot):
Power Input Connect 120 Volt AC power
Prim ry Primary Pump (optional)
Zone 1-3 Circulator Zones
Priority Zone 4 Priority Zone (if enabled) or Zone 4
N/C Zone 4 Normally closed terminals for the Priority Zone.
Will deactivate on a Priority Zone call.
Cold St rt Boiler
Applic tion:
T nkless Coil Boiler
Applic tion: (Altern tive Wiring)
Instruction Sheet
SR504-EXP-4 Switching Relay
102-389
SUPERSEDES: December 20, 2013 EFFECTIVE: January 18, 2017
Plant ID# 9300-2874
For Both Cold St rt Boiler Applic tion nd T nkless Coil Boiler Applic tion (Altern tive Wiring)
For Cold St rt Boiler Applic tion
Oper tion: When any thermostat calls for heat, the appropriate circu-
lating pump is energized and the isolated end switch (X and X) will
start the boiler.
For T nkless Coil Boiler Applic tion (Altern tive Wiring)
Oper tion: When any thermostat calls for heat, the boiler will be enabled
and appropriate circulating pump is energized when the boiler tempera-
ture is above the set low limit and low limit (ZC) dip switch is set to on.
ZC nd ZR Termin ls: Connect terminal ZC to ZC terminal on the aqua-
stat control. Connect ZR to ZR terminal on the aquastat control. Confirm
polarity is consistent between boiler aquastat and switching relay.
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.
Specific tions:
P ODUCT NUMBE INPUT MAXIMUM TYPE 1 ENCLOSU E
NUMBE OF ZONES VOLTAGE COMBINED LOAD WIDTH HEIGHT DEPTH
S 504-EXP-4 4 with Priority 120/60/1 VAC 20 amps 121/4" 8" 3"
All circulator relay connections, including ZC/Z , are rated 1/3hp (6 FLA, 36 L A) at 120 VAC.
End switch connections are rated 24 VAC, 1 amp.
All thermostat connections supply a 24 VAC class 2 output.
For more wiring diagrams, visit www.taco-hvac.com.
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.
