Time Delay Sequencer PTC Technology Applications & Case Study

What Is a Time Delay Sequencer?

    A time delay sequencer is an electromechanical control device used primarily in HVAC systems to manage the staggered startup of heating elements or fans. Unlike simple relays that respond instantly, a sequencer introduces a deliberate delay—typically ranging from a few seconds to several minutes—ensuring that multiple loads do not activate simultaneously, which could cause electrical overload or mechanical stress.

         

    

 

Key Features at a Glance

    1. Primary Function: Controls delayed operation of heating elements or fans in electric furnaces, heat pumps, and gas furnaces

    2. Timing Technology: Solid-state Positive Temperature Coefficient (PTC) heater

    3. Contact Options: Single, double, or three independent timing contacts

    4. Certifications: UL and CUL approved

    5. Series Compatibility: Similar to TOD Series 12S 145 155

    6. Typical Applications: Electric furnaces, heat pumps, gas furnaces, air handlers

 

Technical Deep Dive: How PTC Heating Enables Precision Timing

The PTC Advantage

    A PTC heater is a thermistor with a unique property: its electrical resistance increases dramatically as its temperature rises. This characteristic is harnessed to create a predictable and repeatable timing mechanism.

    1. Energization: When control voltage is applied, the PTC heater begins to draw current and generate heat.

    2. Heat Accumulation: The heat is transferred to a bimetal element (or similar thermal actuator). 

    3. Contact Closure: Once the temperature threshold is reached, the bimetal snaps or bends, closing the timing contacts and energizing the connected load

    4. Self-Regulation: As the PTC reaches its set temperature, its resistance increases, reducing power consumption—an inherent energy-saving feature.

 

Comparing Timing Technologies

Technology	Advantages	Limitations
PTC Thermal Sequencer	Rugged, immune to voltage fluctuations, long service life, no electronic components susceptible to surge damage	Fixed timing curves (not field-adjustable)
Electronic Timer (RC)	Adjustable timing, compact	Sensitive to temperature extremes and voltage spikes
Mechanical Motor Timer	Precise timing, visible dial	Moving parts wear out, larger footprint, higher cost

 

 

Flexible Contact Configurations for Varied System Needs

    Different HVAC systems require different levels of control. This sequencer series offers three contact configurations to match application requirements:

    Single Timing Contact: Ideal for basic systems requiring a simple delay—such as a single-stage electric furnace or a fan delay in a basic air handler.

    Double Timing Contacts: Suited for two-stage heating systems, allowing sequential activation of first-stage and second-stage heating elements.

    Three Independent Timing Contacts: Designed for complex systems such as multi-stage electric furnaces, heat pumps with auxiliary heat, or systems requiring coordinated control of multiple fans and heating elements.

 

Key Applications

    1. Electric Furnaces

    In electric furnaces, sequencers are essential for staging heating elements. A typical 10 kW or 15 kW furnace contains multiple heating elements. Without a sequencer, all elements would energize simultaneously when the thermostat calls for heat, creating a massive inrush current that can trip breakers, dim lights, and shorten component life.

    2. Heat Pumps

    In heat pump systems, sequencers manage auxiliary heat strips (electric resistance heat) that supplement the heat pump during defrost cycles or extremely cold weather. Proper sequencing ensures that the auxiliary heat does not activate simultaneously with the compressor, preventing excessive current draw. Additionally, sequencers often control fan delay—keeping the blower running briefly after the compressor stops to capture residual heat or cold from the coil, improving system efficiency.

    3. Gas Furnaces

    While gas furnaces primarily rely on integrated furnace controls, sequencers are sometimes used in add-on applications or in older systems to manage blower timing. They ensure the blower does not start until the heat exchanger has warmed up (preventing cold air delivery) and continues running after the gas valve closes to extract residual heat.

 

 

Customer Case Study: Reducing Callbacks for an HVAC Contractor

    The Challenge: A regional HVAC contractor was experiencing an unusually high rate of service callbacks on new electric furnace installations. The primary complaint was tripped breakers during initial startup and, in some cases, premature failure of heating elements.

    The Investigation: Upon inspection, the contractor discovered that the standard sequencers they had been using were allowing multiple heating elements to energize nearly simultaneously, creating inrush currents exceeding circuit breaker ratings.

    The Solution: The contractor switched to a three-contact PTC-based sequencer with staggered timing intervals: 10 seconds for the first stage, 25 seconds for the second, and 45 seconds for the third. This spread the electrical load across a longer startup period.

    The Result: Breaker trips were eliminated, and element failures dropped by over 40% within the first year. The contractor now specifies this sequencer as the standard for all multi-stage electric furnace installations.