HVAC Relays & Sequencers
HVAC relays are electrically operated switches that control high-voltage components (compressors, fan motors, heat strips) using a low-voltage signal from the thermostat or control board. When a re...
How to Choose the Right HVAC Relay
HVAC systems use various types of relays and switches to control fans, compressors, heat strips, and other high-current loads from low-voltage control signals. Relays act as electrically operated switches: a small 24V signal from the control board energizes the relay coil, which closes contacts that carry higher-voltage or higher-current power to the controlled device. Choosing the correct relay type, voltage, and amp rating is essential for safe and reliable operation.
Fan Relays
Fan relays control the indoor blower motor, switching it between heating speed, cooling speed, and continuous fan operation based on signals from the control board and thermostat. A failed fan relay can cause the blower to not run, run at the wrong speed, or run continuously. Most fan relays are SPDT (single pole, double throw) type with 24V coils and contacts rated for the motor amperage.
Time Delay Relays
Time delay relays add a programmed delay between the control signal and the load activation. In HVAC systems, they are commonly used to delay the blower startup after the burners light (allowing the heat exchanger to warm up) and to keep the blower running after the burners shut off (to extract residual heat). Delay times typically range from 30 seconds to 3 minutes and are often adjustable.
Sequencers
Heat sequencers are specialized time delay relays used in electric furnaces and heat strip packages to stagger the activation of multiple heating elements. Instead of energizing all elements simultaneously (which would cause a massive current spike), the sequencer turns them on one at a time with delays between each stage. Sequencers are rated by the number of elements they control, typically 2 or 3.
Relay Specifications
When selecting a relay, match the coil voltage (typically 24V for HVAC), the contact configuration (SPST, SPDT, or DPDT), and the contact amp rating (must meet or exceed the controlled load). Also verify the mounting style and terminal configuration match your installation. Using an undersized relay will cause overheating, contact welding, and premature failure.
Finding Your Replacement
Check the part number and specifications on your existing relay. Use our Look Up by Model Number tool above to find the exact replacement. Every order placed before 3 PM CST ships same day from National HVAC Parts.
Frequently Asked Questions
What's the difference between a relay vs a contactor?
Both are electrically-operated switches, but contactors handle higher amperage (20-40+ amps) for compressors and condenser fans. Relays handle lower amperage (5-20 amps) for blower motors, fan circuits, and control signals. A contactor has visible contact points you can inspect; a relay is usually a sealed unit. They are NOT interchangeable.
How do I test an HVAC relay?
With the power off, use a multimeter on the coil terminals — you should read the rated coil resistance (typically 10-100 ohms for 24V coils). For the contacts, check continuity between the common and normally-open terminals — should be open with no power, closed when 24V is applied to the coil. A relay that doesn't click when powered has a burned coil.
My blower won't start but the furnace lights — is it the relay?
Likely yes — the fan relay or time delay relay controls the blower. Listen for a click from the relay when the thermostat calls for fan. No click = bad relay coil. Click but no blower = check the relay contacts (may be burned) or the blower motor itself. Also check if the fan switch on the thermostat is set to AUTO vs ON.
What is a heat sequencer and when do I replace it?
A heat sequencer stages electric heat strips on one at a time (with 15-30 second delays) to prevent a massive power surge. Replace when some heat strips work but others don't, or when they all come on simultaneously (causing breaker trips). Test by applying 24V to the control terminals — the sequencer contacts should close in sequence over 30-60 seconds.
Can I use a universal relay or do I need the OEM part?
Universal relays work if the coil voltage (24V, 120V, or 240V), contact configuration (SPST, SPDT, DPDT), and amperage rating match the original. However, mounting style and connector type may differ. OEM relays are plug-and-play with no guesswork. If you're unsure, enter your model number above for the exact match.