An off delay relay (specifically a True Off-Delay or TOFF relay) maintains its energized contact state for a precise, user-adjustable time after the control voltage is removed from the coil. Once the timing interval expires, the contacts return to their normal de-energized state. This is the critical component for HVAC blower run-on cycles, motor cooling fan delays, and sump pump run-dry protection.
If you are sizing one today for a standard 24VAC/VDC control circuit driving a fractional horsepower blower or cooling fan, the default, no-brainer pick is the Omron H3CR-H8L (24VAC/DC). It handles up to 10A at 250VAC and features dedicated motor ratings. Below is the exact engineering framework to verify if this part works for your specific load, how to wire it safely, and how to test it on the bench.
What is a True Off-Delay (TOFF) Sequence?
Many hobbyists and junior technicians confuse an "off-delay" with an "interval" timer. The distinction dictates whether your circuit will function or fail catastrophically on startup.
- Apply voltage to the coil (A1/A2). The output contacts change state instantly.
- Remove voltage from the coil. The internal timing circuit (powered by an internal capacitor or retained line voltage) starts counting.
- When the preset time expires, the contacts revert to their normal state.
True Off-Delay relays require continuous power to the coil to keep the contacts in the "timed" state. If your application requires the timer to start when a momentary switch is pressed and power is removed, you need a TOFF. If you need the timer to start when power is applied and run for a set duration regardless of whether power remains, you need an Interval (ON-Delay) timer.
Decoding the Rating Table: Which Column Governs Your Load?
The most common point of failure in electromechanical timing relays is contact welding due to misreading the ampacity table. A relay rated for "10A" is almost never 10A across all load types. The lowest applicable utilization category governs your load.
| Rating Parameter | Resistive (AC-1) | Inductive (AC-15) | Motor (AC-3 / HP) | Breaking Capacity |
|---|---|---|---|---|
| Typical 10A Relay | 10A @ 250VAC | 3A @ 250VAC | 1/3 HP @ 120VAC 1/2 HP @ 240VAC |
Make: 40A Break: 10A |
| Governing Rule | Use for heaters, incandescent lighting. | Use for solenoids, contactor coils, valves. | Use for compressors, blowers, fans. | Inductive loads generate arcs on break; contacts must withstand the thermal spike. |
Which rating column governs this load? If you are switching an HVAC blower motor, the AC-3 (Motor) or Horsepower (HP) column governs, not the 10A resistive column. A 1/3 HP motor at 120VAC draws roughly 3.5A to 4A at full load, but the Locked Rotor Amps (LRA) inrush can hit 25A. If your relay lacks a specific HP or AC-3 rating, the inrush current will pit and eventually weld the contacts shut.
Coil vs. Contact Wiring: Power, Protection, and Flyback Diodes
An off-delay relay features two completely isolated circuits: the Coil (Control) and the Contacts (Load).
The Coil Side (A1 / A2)
The coil dictates the control voltage (e.g., 24VAC, 24VDC, 120VAC). When wiring DC coils driven by a PLC transistor output or a microcontroller, you must install a flyback diode.
When a DC coil is de-energized, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback) that will instantly destroy a PLC transistor or Arduino GPIO pin. Always wire a 1N4007 rectifier diode in reverse bias across A1 (cathode/stripe) and A2 (anode). AC coils do not require flyback diodes, as the alternating zero-crossings naturally collapse the field, though an RC snubber may be used to prevent EMI.
The Contact Side (COM / NO / NC)
Standard SPDT (Single Pole Double Throw) relays use terminals 15 (Common), 16 (Normally Closed), and 18 (Normally Open). For an off-delay blower run-on, wire your line voltage to 15, and your blower motor to 18. When the thermostat calls for cooling, the coil energizes, closing 15-18 instantly. When the thermostat drops the call, the coil loses power, but 15-18 remains closed until the timer expires.
Note on Overcurrent Protection: Never treat standard fuses and thermal-magnetic breakers as interchangeable for motor loads. A standard breaker will trip on motor inrush. You must use a time-delay (dual-element) fuse or a motor-protection circuit breaker with a magnetic trip curve specifically designed to tolerate the 6x inrush current of an AC-3 load.
Selection Decision Path: From Load Type to Exact Part Number
Use this decision tree to select the correct configuration and finalize your part number.
| Load Type | Load Characteristics | Action Required | Concrete Part Pick |
|---|---|---|---|
| Resistive | Strip heaters, incandescent lamps (< 10A total). | Standard 10A SPDT relay is sufficient. No arc suppression needed. | Omron H3CR-H8L (24VAC/DC) |
| Inductive | Solenoid valves, large contactor coils. | Derate contact capacity by 50%. Add an RC snubber across the load. | Omron H3CR-H8L + RC Snubber |
| Motor (Direct) | HVAC blowers, exhaust fans (< 1/2 HP, < 10A FLA). | Verify AC-3 / HP rating on datasheet. Use magnetic-curve breaker. | Macromatic TR-60822 (120VAC control) or Omron H3CR-H8L (24V control) |
| Motor (Indirect) | Compressors, large pumps (> 1/2 HP or > 10A). | Do NOT switch motor directly. Use relay to pilot a contactor. | Omron H3CR-H8L piloting a Schneider LC1D09 Contactor |
The Default Recommendation: For 90% of industrial and commercial HVAC run-on applications operating on a 24V control circuit, buy the Omron H3CR-H8L (24VAC/DC). It costs roughly $50, fits a standard 35mm DIN rail, features a multi-range time dial (0.1s to 12s), and carries explicit 1/3 HP @ 120VAC and 1/2 HP @ 240VAC motor ratings. If your control voltage is strictly 120VAC line-level, pivot to the Macromatic TR-60822.
Testing Dead and Live: Bench Verification and Troubleshooting
Before installing the relay in a panel, verify its health on the bench. Here is exactly how to test it dead and live, and how to decide if it needs replacing.
Dead Testing (Multimeter in Ohms/Continuity)
- Coil Integrity: Place probes across A1 and A2. A healthy 24VDC coil will read between 150Ω and 400Ω. A 120VAC coil will read much higher (often 2kΩ - 5kΩ). If it reads OL (Open Line), the internal coil wire is broken. If it reads 0.1Ω, it is shorted.
- Contact State: With no power applied, probe 15 (COM) and 16 (NC). It should read < 1Ω (continuity). Probe 15 and 18 (NO). It should read OL.
Live Testing (Powered Bench Test)
- Apply the rated coil voltage (e.g., 24VDC) to A1/A2. You should hear an audible click. The NO contact (15-18) should now show continuity.
- Connect a multimeter in continuity/beep mode across 15 and 18.
- Set the time dial to a known value (e.g., 5 seconds).
- The Drop Test: Remove the 24VDC power from the coil entirely. Start a stopwatch. The multimeter should continue to beep (contacts held closed) for exactly 5 seconds, then stop (contacts open). If the contacts drop out instantly, the internal timing capacitor has failed.
When to Repair vs. Replace
Never attempt to repair the internal timing IC, potentiometer, or sealed relay contacts. Electromechanical timers are potted or sealed to prevent dust and arc-carbon ingress. If the dead test shows an open coil, or the live test shows a failed timing sequence, replace the entire unit. The cost of a $50 replacement is negligible compared to the labor cost of troubleshooting a flooded basement or a burnt-out compressor motor.
The only acceptable field repair on an off-delay relay is replacing an externally mounted, burnt flyback diode or re-crimping a loose terminal lug that has suffered thermal degradation due to improper torque. For the relay itself, treat it as a sealed, replaceable module.
