Relays
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Honeywell Protectorelay Oil Burner Relay with Lock Out Timing (MPN: R8184G4009/U)SKU: 1691$145.97 12% off $165.22 -
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Honeywell Universal Digital Electronic Oil Primary, 120 to 240 VAC (MPN: R7284U1004/U)SKU: 1914469$103.83 18% off $126.26 -
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Honeywell Switching Relay with Internal Transformer DPST Line Voltage Relays (MPN: R845A1030/U)SKU: 11872$125.00 15% off $146.80 -
Honeywell Fan Center with SPDT Switching Action, 40VAC (MPN: R8239A1052/U)SKU: 1674$71.81 14% off $83.95 -
Honeywell Protectorelay Oil Burner Relay with Lock Out Timing, 4" x 4" Junction Box (MPN: R8184M1051/U)SKU: 1692$248.20 19% off $304.66 -
Honeywell General Purpose Relay with DPDT Switching, 24 VAC (MPN: R8222D1014/U)SKU: 27102$27.03 18% off $32.80 -
Honeywell Single-Zone Switching Relay with Internal Transformer, 1 SPST Line Relay, 120VAC (MPN: RA89A1074/U)SKU: 1694$104.63 12% off $118.79 -
Honeywell General Purpose Relay with SPDT switching, 24 VAC (MPN: R8222B1067/U)SKU: 129928$25.24 14% off $29.19 -
Honeywell Two-Zone Switching Relay with DPST Switching, 120VAC (MPN: RA832A1066/U)SKU: 26754$126.77 13% off $146.55 -
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Relays
A relay is one of the most essential electronic components in modern heating, HVAC, and building automation systems. It functions as an electrically operated switch, using a low-current signal to control a high-current or high-voltage load circuit with precision and built-in safety.
Whether you are activating a single circulator pump from a thermostat output or orchestrating a multi-zone radiant heating network, a relay serves as the critical interface between a low-voltage sensor or control signal and the line-voltage devices that deliver actual heat or airflow. At Plumbing Supply and More, we carry a professional selection of relays from Honeywell, Taco, Argo, Mars, and Uponor — engineered for residential and commercial hydronic heating and HVAC applications.
How a Relay Works: Electromagnet, Coil Voltage & Contact Switching
Understanding the internal mechanics of a relay is essential to selecting and applying one correctly. Every electromechanical relay contains an electromagnet, a set of mechanical contacts, and a spring-return mechanism housed within a compact enclosure.
When alternating current (AC) or direct current (DC) is applied to the relay coil, it generates a magnetic field that pulls the armature physically to close or open the contact circuit. Common coil voltages in HVAC and hydronic applications include 24 VAC, 30 VAC, and 120 VAC, with some pilot relays accepting both 30 VAC and 12 VDC simultaneously for flexible integration into mixed-voltage control panels.
The switching contact side of the relay is electrically isolated from the coil side, providing galvanic isolation between the control signal and the load circuit. This separation protects sensitive thermostat outputs, programmable logic controller (PLC) terminals, and microcontroller pins from damaging back-EMF and inrush current spikes generated by the load.
The inductor effect of the relay coil also means that a diode or snubber circuit is commonly applied across DC coil terminals to suppress voltage transients when the coil de-energizes. Correctly specifying coil voltage, contact rating, and protection components ensures both the relay and the broader printed circuit board or panel hardware remain protected over years of cycling service.
Switching Relay Types: SPDT, DPDT, SPST & SPNO Configurations
The contact configuration of a relay is the single most important specification after coil voltage. Contact form determines how many independent circuits a relay can switch and whether each circuit is normally open or normally closed when the coil is de-energized.
SPDT (Single-Pole Double-Throw) relays have one common terminal, one normally open terminal, and one normally closed terminal. This configuration allows the relay to route a signal to one of two possible outputs depending on the energized state, making SPDT relays versatile for heating, fan control, and circuit diversion. The Honeywell General Purpose Relay with SPDT Switching, 24 VAC and the Mars General Purpose Relay with SPDT Switch are compact, field-proven examples of this contact form.
DPDT (Double-Pole Double-Throw) relays operate two entirely independent SPDT switching actions simultaneously from a single coil. This makes them ideal when two separate line-voltage circuits must switch in sync. The Taco SR503 Switching Relay is rated at 15 A, 120 VAC with a DPDT configuration, suitable for switching both a circulator pump and an auxiliary load simultaneously — a requirement common in zone-controlled hydronic systems.
SPST (Single-Pole Single-Throw) relays are the simplest form, switching a single circuit open or closed. The Honeywell Single-Zone Switching Relay with Internal Transformer uses an SPST line relay within its architecture, providing a straightforward method of engaging a single heating zone circulator from a 120 VAC supply.
SPNO (Single-Pole Normally Open) contacts remain open when the coil is de-energized and close only when energized. The Mars General Purpose Relay with SPNO Switch exemplifies this form for pump and load switching where fail-safe open behavior during a power loss is operationally required. Knowing your required contact form before selection prevents incorrect wiring, ensures correct failure-mode behavior, and eliminates unnecessary troubleshooting time.
Zone Switching Relays for Hydronic Heating & Radiant Systems
Zone switching relays are the command center of any multi-zone hydronic heating or radiant heat system. These relays integrate low-voltage thermostat inputs with line-voltage circulator outputs, enabling independent temperature control in each zone without running separate power wiring to each individual thermostat.
Taco's SR Series represents the industry benchmark for zone relay performance. The Taco SR501-4 Single-Zone Switching Relay, SR502-4 Two-Zone Switching Relay with Priority, SR503-4 Three-Zone Switching Relay, SR504-5 Four-Zone Switching Relay with Priority rated at 20A, and the SR506-5 Six-Zone Switching Relay with Priority form a complete family covering single-zone simplicity up to large residential or light commercial multi-zone systems.
All SR Series relays operate at 120 VAC and accept standard 24 VAC thermostat signals on the low-voltage side. Priority zoning logic on the SR502-4, SR504-5, and SR506-5 gives a designated priority zone — typically domestic hot water — immediate and uncontested control of the heat source. This prevents simultaneous competing demand across zones, a critical functional requirement in boiler-based hydronic systems with a domestic hot water priority circuit.
Argo's Z Series offers equivalent multi-zone coverage with the Z140C Single-Zone Switching Relay, Z213 Three-Zone Expandable Switching Relay, Z214 Four-Zone Switching Relay, and Z216 Six-Zone Switching Relay, all rated at 120 VAC. The Z213's expandable architecture deserves particular attention — additional zone modules can be added to the base panel without replacing the entire unit.
This field-expandable design is a significant advantage in phased construction or renovation projects where heating zones are added incrementally. The Honeywell Two-Zone Switching Relay with DPST Switching, 120VAC and the Honeywell Switching Relay with Internal Transformer with DPST Line Voltage Relays incorporate a built-in step-down transformer that eliminates the need for a separate 24 VAC power supply in the panel. The Uponor Single-Zone Pump Relay, 120VAC rounds out the single-zone category with a dedicated design optimized for radiant heat circulator activation in Uponor manifold-based systems.
Oil Burner Relays & Electronic Oil Primary Controls
Oil burner relays — also called primary controls or protectorelays — are safety-critical devices that govern ignition sequencing and flame supervision in oil-fired heating systems. These relays are not interchangeable with standard switching relays and carry specific UL and ANSI certifications for oil burner safety control duty.
The Honeywell Protectorelay R8184G4009/U is the widely recognized industry standard for residential oil burner primary control. It interfaces with the cad cell (cadmium sulfide photocell) to confirm flame presence, executes a calibrated trial-for-ignition period, and locks out the burner if flame is not established within the specified time window. This lockout function prevents unburned fuel accumulation — a serious safety hazard in any oil-fired appliance.
The Honeywell Protectorelay R8184M1051/U delivers the same flame supervision and lockout logic but is pre-mounted in a 4" x 4" junction box format. This self-contained package simplifies field wiring in tight utility spaces and older boiler rooms where surface-mounting a standalone primary control is impractical.
The Honeywell R7284U1004/U Universal Digital Electronic Oil Primary extends the platform with 120 to 240 VAC universal input capability, integrated digital diagnostics, and expanded control logic. Its diagnostic feedback significantly reduces troubleshooting time during service calls, making it a preferred choice for oil burner retrofits and new oil-fired boiler installations.
Correct selection of an oil burner relay requires matching lockout timing, cad cell sensitivity range, and voltage compatibility to the specific burner and heat exchanger. Installing an incorrect primary control can cause nuisance lockouts, ignition failure, or — most critically — unsafe fuel delivery during a failed ignition sequence.
General Purpose Relays, Pilot Relays & Fan Center Relays
General purpose relays, pilot relays, and fan center relays are the precision workhorses of low-voltage HVAC and hydronic control wiring. Each type serves a distinct role: amplifying control signals, bridging voltage levels, or activating auxiliary devices based on thermostat or sensor commands from the broader control system.
Honeywell's R8222B1067/U General Purpose Relay with SPDT Switching and R8222D1014/U General Purpose Relay with DPDT Switching operate at 24 VAC coil voltage and are among the most commonly installed relays in residential and light commercial HVAC control panels. Their compact footprint, plug-in terminal connections, and UL-listed ratings make them fast to install and straightforward to replace during maintenance calls.
Pilot relays provide moderate current switching capacity between a small control signal and a load that exceeds what a thermostat or controller terminal can drive directly. The Pilot Relay 10 Amp DPDT, 30VAC/12VDC and the Enclosed Pilot Relay 10 Amp SPDT, 30VAC/12VDC both accept dual-voltage coil inputs, operating correctly on either 30 VAC or 12 VDC control signals.
This dual-voltage flexibility makes pilot relays especially valuable in zone controllers, boiler management systems, and hydronic control panels where both AC and DC control voltages coexist on the same terminal block. The enclosed version provides additional protection in environments with airborne dust or moisture.
The Honeywell Fan Center with SPDT Switching Action, 40VAC (R8239A1052/U) is a dedicated relay for forced-air heating blower motor control. It manages the fan independently of the heat source, ensuring correct fan-on and fan-off delay timing for occupant comfort and heat exchanger protection. The Argo Replacement Plug-In Relay, 24VAC (R35C) is a direct maintenance-replacement relay engineered specifically for Argo zone relay panels, restoring switching function without requiring a complete panel replacement or field re-wiring.
Contact Amperage, Inrush Current & Coil Voltage: Specifying Relays Correctly
Three core specifications define every relay selection decision: contact configuration, coil voltage, and contact amperage rating. An error in any one of these three parameters results in relay failure, equipment damage, or persistent control system faults that are difficult to diagnose.
Coil voltage must match the available control circuit voltage precisely. Most HVAC thermostats and zone controllers output 24 VAC low-voltage signals, but line-voltage thermostats, fan centers, and oil burner primary controls typically operate at 120 VAC or higher. Applying a 24 VAC relay coil to a 120 VAC control source immediately burns the coil winding.
Contact amperage must meet or exceed the full load amperage (FLA) of the connected load, with meaningful headroom reserved for motor inrush current at each startup cycle. Hydronic circulator pumps typically draw 1 to 5 amps at running load, but startup inrush can reach 3 to 6 times the running current for a brief period. Selecting a relay rated at the absolute minimum amperage for an application accelerates contact wear and creates intermittent failures over time.
Contact configuration — SPDT, DPDT, SPST, or SPNO — must match the wiring diagram of the application. An SPDT relay can substitute for SPST duty by leaving the normally closed terminal unconnected, but an SPST relay cannot serve an application where the normally closed path is functionally required in the control sequence. Taking a few minutes to confirm all three specifications before ordering eliminates costly field returns and project delays.
Why Shop Relays at Plumbing Supply & More?
Plumbing Supply and More has supplied contractors and professionals with wholesale-priced HVAC controls and plumbing products for over 75 years. Our relay inventory brings together trusted brands — Honeywell, Taco, Argo, Mars, and Uponor — to cover every application from single-zone residential heating to complex multi-zone hydronic systems and oil burner primary control.
Every relay we carry is sourced directly to ensure product authenticity and full manufacturer warranty coverage. We offer free same-day shipping on most orders over $150, and our knowledgeable customer care team is available to help you match coil voltage, contact form, and amperage rating to your specific application. Browse our full relay selection above and find the right control component for your heating or HVAC system today.
