Power Relays in HVAC Design

­Power relays act as the essential switching elements that allow a thermostat or controller to direct real power flow. Each time a compressor engages, a fan motor starts, a heating strip energizes, or a heat pump shifts modes, the relay must execute a clean, consistent, and safe state transition (Figure 1). Because HVAC equipment operates under high inrush currents, frequent duty cycles, wide ambient temperature ranges, and significant moisture exposure, relay selection has a major impact on comfort, energy efficiency, and long-term reliability. With the increased deployment of cooling systems and heat pumps—driven by hotter summers, electrification initiatives, and retrofit programs—system volumes are growing alongside tighter reliability expectations. Efficiency standards, such as SEER, HSPF, and regional energy codes, also place pressure on designers to minimize standby losses, reduce board-level power dissipation, and limit contact degradation so that performance remains stable over time.

Regional variation must be considered early in the design phase. HVAC systems are shipped into markets with different mains voltages, line frequencies, control methods, environmental ratings, and regulatory requirements. Instead of reviewing every jurisdiction, it is more practical to highlight the common variations that influence relay specification and PCB layout. For products intended for multiple regions, it is best to design to the most demanding set of requirements, so that redesigns are avoided late in the development cycle.

An Overview of Relays in HVAC

Relays function as the interface between low-power control circuits and high-power loads. In HVAC applications, this typically means panel-mounted contactors for large loads such as compressors, and PCB-mounted relays for indoor fans, heaters, and smaller motors. Regardless of type, these devices must operate consistently over millions of cycles, often in challenging environmental conditions.

Control voltage schemes vary by region and system architecture. In North America, 24-volt AC control loops from step-down transformers remain common, while modern inverter-driven designs often use low-voltage DC logic (5 V or 12 V) to drive coils directly from the control board. Other markets follow different conventions, requiring engineers to validate coil voltage ratings, pickup and dropout thresholds, and brownout tolerances. Coil type, contact material, and package design may appear secondary, but each has a direct impact on efficiency, maintainability, and long-term system reliability.

A major stress factor for HVAC relays is motor inrush current. Compressors, condenser fans, and indoor blowers can demand five to eight times their full-load current at startup. Relay contacts must close under these surge conditions, which generate arcs across partially closed contacts. These arcs erode surfaces, increase resistance, and accelerate heat buildup, all of which shorten service life (Figure 2). Contact bounce further worsens arcing intensity and wear. To mitigate these effects, robust contact materials—such as silver tin oxide—are commonly specified for HVAC relays, given their resistance to welding and erosion under inductive loads. Same Sky offers a variety of power relays at different switching voltages with silver tin oxide for HVAC and other applications that expect these unfavorable conditions.

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Figure 2: Arcing generated by inductive loads accelerates contact wear and reduces relay service life

In fixed-speed compressor systems, inrush current is present at every startup, making relay endurance under these surge conditions a key design requirement. Inverter-driven compressors mitigate this stress by ramping up gradually, resulting in lower inrush currents and reduced loading on the main contactor. However, the focus shifts to relays managing secondary loads—such as reversing valves, heaters, and pumps—that operate with high switching frequency. Even for smaller relays, careful selection is essential, as inductive loads can still produce damaging voltage transients unless suppression components such as RC snubbers or varistors are incorporated.

Where Relays Are Applied in HVAC Systems

Relays are distributed across HVAC equipment well beyond the primary compressor contactor. Condenser fans and indoor blower motors are commonly switched through relays, which must withstand both frequent mechanical cycling and the electrical stress imposed by inductive loads. While these motors draw less power than the compressor, their repeated operation makes contact durability and coil life critical to long-term reliability.

In heat pump systems, relays also drive the reversing-valve solenoid—a relatively small load that plays a vital role in directing refrigerant flow between heating and cooling modes. Accurate timing and clean switching are essential here to prevent pressure spikes that can damage the system.

Relays further control resistive heating elements, such as crankcase heaters that protect against refrigerant migration and auxiliary heat strips that supplement heating during low outdoor temperatures. Since these heat strips are often staged in multi-kilowatt increments, the associated relays must safely carry high continuous currents without exceeding thermal limits.

Finally, relays support additional system functions such as condensate pumps and humidifiers. These examples underscore how integral relays are across HVAC applications, spanning major loads to smaller but equally critical auxiliary components.

Relay Design Considerations and Concerns for HVAC

The primary design factor for HVAC relays is electrical performance. Devices must be rated not only for continuous operating current but also for the surge currents inherent to motor-driven loads. Beyond inrush, designers must account for locked rotor amperage (LRA), which occurs if a motor is mechanically stalled. These extended high-current conditions can heavily stress conductors, electronics, and relay contacts. Additionally, the break event—when contacts open—often imposes the most severe stress due to arcing, making proper contact material and geometry selection essential.

Environmental factors are equally critical. HVAC systems operate in conditions ranging from rooftop summer heat to freezing winter exposure. Relays must maintain functionality across ambient temperatures from below –40°C to above 60°C, often in the presence of moisture, condensation, or airborne particulates. Insulation quality, housing seals, and material durability are therefore major design priorities.

Control voltage conventions vary by system architecture. While 24-volt AC control loops remain common in North America, low-voltage DC coils are increasingly used, and some systems even energize relays directly from mains-level logic. Each approach influences coil characteristics, pickup and dropout performance, and overall efficiency.

Reliability under frequent cycling is a constant concern, especially with inductive loads that accelerate mechanical wear and contact degradation. Finally, all relay selections must comply with regulatory standards such as UL and IEC, which define safety margins, creepage and clearance requirements, and material flammability ratings. Aligning designs with these frameworks early helps prevent compliance issues later in the development process.

Design Challenges for Air-Source Heat Pumps

Air-source heat pumps place added demands on relays due to their operating complexity. A primary task is driving the reversing valve, which alternates refrigerant flow between heating and cooling modes. Each transition must be carefully sequenced with compressors and fans, making switching coordination more critical than in standard HVAC designs.

Auxiliary electric strip heaters are another common element, activating when outdoor temperatures are too low for the refrigerant cycle to provide sufficient heating. Relays controlling these elements must reliably manage high continuous currents, often in the tens of amps range, without excessive heating or contact degradation.

The defrost cycle introduces further switching stress. In cold conditions, the outdoor coil may accumulate frost, requiring the system to temporarily reverse into cooling mode to melt it. This process increases the number of state changes, with relays sequentially switching compressors, fans, and heaters to balance indoor comfort while ensuring the coil is cleared. Cold weather also amplifies compressor inrush currents in fixed-speed systems, adding strain to relay contacts.

To address modern efficiency requirements, many heat pump designs now employ low-power coil relays or latching relays to minimize standby draw at the control board. However, it is important to note that while latching relays reduce coil power consumption, they do not eliminate the continuous hold power required by downstream devices such as the reversing-valve solenoid.

Final Checklist for Relays in HVAC

While HVAC relay specifications can be complex, a clear checklist provides a practical framework for guiding the design and selection process.

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Table 1: Relay selection checklist for HVAC

Conclusion

Relay selection represents just one part of HVAC system design, but it plays a pivotal role. The relays chosen directly influence overall energy efficiency, regulatory compliance, and long-term serviceability. As HVAC adoption continues to expand globally and efficiency requirements become increasingly strict, relays must be treated as key design elements rather than peripheral components. Proper selection leads to systems that operate quietly, reliably, and efficiently while withstanding years of demanding service. Same Sky’s wide selection of different power relays provides an array of high-quality choices that thrive in this growing, yet demanding field.

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