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For Cold Start Boiler Application

Operation:

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 Tankless Coil Boiler Application (Alternative Wiring)

Operation:

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 and ZR Terminals:

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.

120 VAC INPUT

TO:  "TT" ON BOILER

PRIMARY CIRCULATOR

(OPTIONAL)

TO:  "TT" ON BOILER WITH
        DHW "TT" INPUT

SIX ZONE

EXPANDABLE SWITCHING RELAY

POWER

CONTROLS

120 VAC CIRCULATORS

ZC   

INPUT

120 VAC

ZONE 1 

ZONE 2 

 

ZONE 3 

 

ZONE 4 

ZONE 5 

ZONE 6

ZONE1  ZONE2  ZONE3  ZONE4  ZONE5  

THERMOSTATS

SR 506-EXP-4

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

NET

EXPANSION

MAIN PRIORITY

XX XX

AB CR W CR W CR W CR W CR W CR W C 12

END SWITCH

THERMOSTATS (24 VAC)

PRIORITY

N/C

PRIMARY

120 VAC

ZR

H

H

N

H

N

H

NH

NH

NH

NH

NH

NH

N

H

SLAVE

PRIORITY ON

RESET

PRIMARY PUMP ON

POST PURGE ON

PRIORITY PROTECT ON

PUMP EXERCISE ON

30 SEC/2 WK

MASTER

OFF

OFF

OFF

OFF

OFF

NORMAL

LOW LIMIT (ZC) ON

OFF

4 MIN/24 HOUR

ZONE6 ZONE6

PRIORITY

ACCESSORY

PORT

ZONE 2

POWER

ZONE 1

ZONE 3

ZONE 4

ZONE 5

ZONE 6

LED

INDICATORS

SIX ZONE

EXPANDABLE SWITCHING RELAY

POWER

CONTROLS

120 VAC CIRCULATORS

ZC   

INPUT

120 VAC

120 VAC INPUT

TO:  "ZC" ON BOILER

ZONE 1 

ZONE 2 

 

ZONE 3 

 

ZONE 4 

ZONE 5 

ZONE 6

ZONE1  ZONE2  ZONE3  ZONE4  ZONE5  

THERMOSTATS

SR 506-EXP-4

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

FUSE

6 AMP

NET

EXPANSION

MAIN PRIORITY

XX XX

AB CR W CR W CR W CR W CR W CR W C 12

END SWITCH

THERMOSTATS (24 VAC)

PRIORITY

N/C

PRIMARY

120 VAC

ZR

H

H

N

H

N

H

NH

NH

NH

NH

NH

NH

N

H

SLAVE

PRIORITY ON

RESET

PRIMARY PUMP ON

POST PURGE ON

PRIORITY PROTECT ON

PUMP EXERCISE ON

30 SEC/2 WK

MASTER

OFF

OFF

OFF

OFF

OFF

NORMAL

LOW LIMIT (ZC) ON

OFF

4 MIN/24 HOUR

TO:  "ZR" ON BOILER

ZONE6 ZONE6

PRIORITY

ACCESSORY

PORT

ZONE 2

POWER

ZONE 1

ZONE 3

ZONE 4

ZONE 5

ZONE 6

LED

INDICATORS

Priority Operation:

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 Operation:

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.

Primary Pump Operation:

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 prima-
ry circulating pump output will energize when any zone calls for heat.

Post Purge Operation:

When the dip switch is set to ON, the priori-

ty zone output will stay energized for 2 minutes after its thermostat or
aquastat is satisfied, but not operate the boiler.

Priority Protection Operation:

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 inde-
pendently. 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 Operation:

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) Operation:

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 Contacts):

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 priori-
ty end switch closes only when the priority zone thermostat or aquastat
is calling for heat.

Expansion 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).

Thermostat Input (24 vac):

R

Hot side of transformer. Connect to 

R

on thermostat.

W

Switched 

signal from thermostat. Connect to 

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 Neutral, H is Hot):

Power Input

Connect 120 Volt AC power

Primary

Primary Pump (optional)

Zone 1-5

Circulator Zones

Priority Zone 6

Priority Zone (if enabled) or Zone 6

N/C Zone 6

Normally closed terminals for the Priority Zone.
Will deactivate on a Priority Zone call.

Cold Start Boiler 
Application

Tankless Coil Boiler 
Application
(Alternative Wiring)

Specifications:

PRODUCT

NUMBER

INPUT

MAXIMUM

TYPE 1 ENCLOSURE

NUMBER

OF ZONES

VOLTAGE

COMBINED LOAD

WIDTH

HEIGHT

DEPTH

SR506-EXP-4

6 with Priority

120/60/1 VAC

20 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.

Instruction Sheet

SR506-EXP-4 Switching Relay

102-365

SUPERSEDES: New

EFFECTIVE: May 1, 2010

Plant ID# 9300-2875

For Both Cold Start Boiler Application and Tankless Coil Boiler Application (Alternative Wiring)

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.

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 ther-
mostat connections, and 14-22 gauge wire for 24 VAC source connections.

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