Taco Comfort Solutions SR503-4

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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

/4" 8" 3"

All circulator relay connections, including ZC/ZR, are rated

/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,

⁄

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.

Need help?

Do you have a question about the Taco Comfort Solutions SR503-4 and is the answer not in the manual?

Summary

Cold Start Boiler Application

Cold Start Boiler Operation

Explains circulator energization and boiler start signal for cold start applications.

Cold Start Priority Operation

Details how Zone 3 priority impacts other zones' operation.

Cold Start Priority Protection

Covers auto-reset for priority zones after prolonged calls.

Cold Start Jumper Placement

Instructions for jumper connection between ZC and ZR for boiler control.

Cold Start Power Input

Procedure for connecting 120 VAC power input to N and H terminals.

Cold Start Resistor Note

Advises on the potential need for a resistor between W and C terminals.

Cold Start Wiring Safety

Essential safety precautions and wire gauge recommendations for wiring.

Cold Start Specifications

Lists product details including number, voltage, load, and dimensions.

Tankless Coil Boiler Application

Tankless Coil Boiler Operation

Explains boiler signal based on temperature and circulator energization.

Tankless Coil Priority Operation

Details how Zone 3 priority impacts other zones' operation.

Tankless Coil Priority Protection

Covers auto-reset for priority zones after prolonged calls.

Tankless Coil Jumper Placement

Instructions for jumper removal and connecting ZC/ZR to aquastat.

Tankless Coil Power Input

Procedure for connecting 120 VAC power input to N and H terminals.

Tankless Coil Wiring Warning

Warns about risks of incorrect wiring and secondary power sources.

Tankless Coil Troubleshooting

Addresses digital thermostat issues and zone heating problems.

Limited Warranty Statement

Warranty Details and Exclusions

Covers repair, replacement, responsibilities, exclusions, and liability limitations.

Summary

Cold Start Boiler Application

Cold Start Boiler Operation

Explains circulator energization and boiler start signal for cold start applications.

Cold Start Priority Operation

Details how Zone 3 priority impacts other zones' operation.

Cold Start Priority Protection

Covers auto-reset for priority zones after prolonged calls.

Cold Start Jumper Placement

Instructions for jumper connection between ZC and ZR for boiler control.

Cold Start Power Input

Procedure for connecting 120 VAC power input to N and H terminals.

Cold Start Resistor Note

Advises on the potential need for a resistor between W and C terminals.

Cold Start Wiring Safety

Essential safety precautions and wire gauge recommendations for wiring.

Cold Start Specifications

Lists product details including number, voltage, load, and dimensions.

Tankless Coil Boiler Application

Tankless Coil Boiler Operation

Explains boiler signal based on temperature and circulator energization.

Tankless Coil Priority Operation

Details how Zone 3 priority impacts other zones' operation.

Tankless Coil Priority Protection

Covers auto-reset for priority zones after prolonged calls.

Tankless Coil Jumper Placement

Instructions for jumper removal and connecting ZC/ZR to aquastat.

Tankless Coil Power Input

Procedure for connecting 120 VAC power input to N and H terminals.

Tankless Coil Wiring Warning

Warns about risks of incorrect wiring and secondary power sources.

Tankless Coil Troubleshooting

Addresses digital thermostat issues and zone heating problems.

Limited Warranty Statement

Warranty Details and Exclusions

Covers repair, replacement, responsibilities, exclusions, and liability limitations.

The Taco SR503-4 Switching Relay is a control device designed to manage heating systems, particularly those with multiple zones and either cold start or tankless coil boilers. Its primary function is to energize appropriate circulators and signal the boiler to start when a thermostat calls for heat.

Function Description

The SR503-4 acts as an interface between thermostats, circulators, and the boiler. It features three zones, with one zone designated as a priority zone. When a thermostat in any zone calls for heat, the relay energizes the corresponding circulator. For cold start boiler applications, it also activates an isolated end switch (terminals X and X) to start the boiler. In tankless coil boiler applications, the relay signals the boiler to start via terminals ZC and ZR, but the circulator is only energized when the boiler temperature is above a set low limit.

Priority Operation

The device offers a priority feature for Zone 3. When Zone 3 is set to priority and calls for heat, all other zones will temporarily cease operation until Zone 3's heating demand is satisfied. If Zone 3 is not configured for priority, all zones operate independently, allowing for simultaneous heating in multiple areas.

Priority Protection Operation

To prevent a single priority zone from monopolizing the boiler for extended periods, the SR503-4 includes a priority protection mechanism. If the priority zone (Zone 3) calls for heat continuously for more than one hour, the relay will temporarily override the priority setting and return power to all other zones, allowing them to function independently. Once the priority zone's demand is met, the control's auto-reset feature reactivates, and Zone 3 regains its priority status for up to one hour from its next call for heat. This ensures that other zones are not left without heat for too long.

Usage Features

Cold Start Boiler Application: In this configuration, a jumper must be placed between terminals ZC and ZR. The isolated end switch connects to the aquastat control on the boiler, initiating boiler operation when any zone calls for heat.
Tankless Coil Boiler Application: For tankless coil systems, the jumper between ZC and ZR must be removed. Terminal ZC connects to ZC on the aquastat, and terminal ZR connects to ZR on the aquastat. It is crucial to confirm consistent polarity between the boiler aquastat and the switching relay. In this setup, the boiler is signaled to start, but circulators only activate when the boiler's temperature is above its low limit, optimizing efficiency for domestic hot water production.
LED Indicators: The device includes LED diagnostic lights to aid in troubleshooting. A green LED indicates that power is connected and the solid-state fuse is good. Red LEDs illuminate when a zone circulator receives power, confirming that the thermostat is functioning correctly.
Thermostat Compatibility: The SR503-4 is designed to work with various thermostats. For "Power Stealing" type digital thermostats that draw power from the switching relay via only two wires (R & W), a resistor (1000 ohm, ½ watt) may be necessary between the W and C (common) terminals of the switching relay to ensure proper operation. This is a common solution for compatibility issues with certain digital thermostats. Battery-powered thermostats should have fresh and correctly installed batteries.
Wiring Flexibility: The instruction sheet provides two main wiring diagrams: one for cold start boilers and an alternative wiring diagram for tankless coil boilers. This allows the SR503-4 to be adapted to different heating system types.
Safety Warnings: The manual emphasizes the importance of adhering to all applicable electrical codes and using copper wire only. It specifies minimum temperature ratings for 120 VAC wiring (75°C) and recommends specific wire gauges for 120 VAC, thermostat, and 24 VAC source connections to prevent personal injury, death, or property damage. For tankless coil applications, a critical warning highlights the need to ensure power originates solely from the boiler aquastat to avoid secondary power sources that could lead to dangerous activations.

Maintenance Features

Diagnostic LEDs: The integrated LED lights serve as a primary maintenance feature, allowing users to quickly identify if the device is receiving power (green LED) and if individual zones are calling for heat and energizing their circulators (red LEDs). This visual feedback helps pinpoint issues related to power, fuses, thermostats, or thermostat wiring.
Fuse Replacement: The device uses standard 6 AMP fuses for each zone. In case of a circulator malfunction or electrical fault, a blown fuse can be easily identified and replaced, restoring functionality to the affected zone. The part number for replacement fuses is provided (SR6A-001RP).
Troubleshooting Guide: The manual includes a troubleshooting section that addresses common problems such as digital thermostats not working correctly and a lack of heat in a specific zone. It offers practical solutions, including the use of resistors for power-stealing thermostats and checking LED indicators for diagnostic purposes.
Documentation and Support: Users are directed to Taco's website and provided with a QR code for access to additional resources, including catalog sheets, instruction sheets, Visio stencils, and a wiring guide. This comprehensive support ensures that users have access to the information needed for installation, operation, and maintenance.