🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Heat Pumps: Principles, Installation, and Service › Lesson 7
BLD-HVAC07-04 · Lesson 7 · 14 clock hours · theory

Defrost Systems: Controls, Sensors, and Termination Logic

Objective. The student will describe how a demand defrost control board initiates and terminates a defrost cycle and correctly interpret defrost board wiring, sensor placement, and fault indicators using a manufacturer service manual.

Why Heat Pumps Need Defrost

When a heat pump operates in heating mode, the outdoor coil acts as an evaporator. Refrigerant inside the coil is colder than the outdoor air, so the coil absorbs heat. When outdoor air is both cold and humid, moisture from the air freezes onto the coil surface. A light frost coat is normal and does not immediately harm efficiency, but if frost accumulates unchecked it blocks airflow, reduces heat transfer, and can eventually damage the outdoor fan or the compressor. The defrost system exists to melt that frost periodically and return the coil to full efficiency.

During defrost, the heat pump temporarily reverses operation: the reversing valve shifts so the outdoor coil becomes the condenser, hot refrigerant flows through it, and the outdoor fan motor is de-energized so it does not blow cold air across the coil and slow melting. The indoor auxiliary or emergency heat strips may energize to prevent cold air from blowing into the conditioned space while the system is in this reversed mode. When defrost is complete, the reversing valve shifts back, the outdoor fan restarts, and normal heating resumes.

Defrost Initiation Methods

Time/Temperature initiation is the oldest and simplest approach. A timer — often set at fixed intervals such as 30, 60, or 90 minutes of compressor run time — sends a signal to the defrost control board. The board then checks a temperature sensor mounted on the outdoor coil. If the coil temperature is at or below a set threshold (commonly around 32 °F or slightly below), defrost is allowed to start. If the coil is warmer than the threshold, defrost is skipped and the timer resets. This method is dependable but can initiate defrost even when little frost is present, wasting energy.

Pressure-switch initiation monitors the pressure drop across the outdoor coil. As frost builds, it restricts airflow and causes a measurable pressure differential between the inlet and outlet sides of the coil. When the differential reaches a set point, the control board triggers defrost. This method is more responsive to actual frost accumulation than a fixed timer.

Demand defrost (also called adaptive or intelligent defrost) uses electronic sensors — most commonly a combination of outdoor air temperature sensor and coil temperature sensor — along with an algorithm in the control board microprocessor. The board calculates how much frost is likely present based on run time, coil temperature, and outdoor temperature. Some systems also use accelerometer-based ice sensors or measure power consumption. Demand defrost initiates a cycle only when frost accumulation is estimated to be significant, reducing unnecessary defrost cycles and improving seasonal efficiency. Many modern residential heat pumps use demand defrost.

Defrost Termination Logic

Termination is equally important. If defrost ends too early, frost remains and the cycle must repeat sooner. If it runs too long, the system wastes energy and can overheat components.

The primary termination sensor is a defrost termination thermostat or thermistor mounted directly on the outdoor coil tubing. When the coil temperature rises to the termination set point — typically in the range of 57 °F to 65 °F depending on the manufacturer — the board ends defrost: the reversing valve shifts back to heating mode and the outdoor fan restarts. A time-out backup is always included; if the coil sensor never reaches the termination temperature within a maximum period (often 10 minutes), defrost terminates automatically to protect the system. Always consult the specific manufacturer service manual for exact set-point values, because they vary by model.

The Defrost Control Board

The defrost control board is the central coordinator. It receives inputs from:

The board's outputs include:

When reading a board wiring diagram, trace each sensor wire back to its terminal designation on the board. Note polarity for thermistors and verify wire gauge matches the load on output terminals.

Fault Codes and Diagnostics

Modern defrost boards display fault codes through LED flash sequences or, on communicating systems, through a thermostat display. Typical codes flag conditions such as a sensor open or short circuit, a defrost cycle that exceeded the maximum time-out, or a board that received a call for defrost with the outdoor ambient too warm for frost to form. Always retrieve the fault code chart from the manufacturer service manual — codes are not standardized across brands.

When a fault code appears: record it, look it up in the service manual, identify the suspect component the code points to, and perform the listed diagnostic steps before replacing any part.

Safety During Defrost Service

Before working on or near a defrost control board or its wiring:

Key terms9

Defrost control boardThe electronic module that monitors sensor inputs, determines when to initiate and terminate a defrost cycle, and coordinates the reversing valve, outdoor fan, and auxiliary heat outputs.
Demand defrostAn adaptive initiation method in which the control board uses sensors and an algorithm to estimate actual frost accumulation before starting a defrost cycle, reducing unnecessary cycles.
Time/temperature initiationA defrost initiation method that combines a compressor run-time timer with a coil temperature sensor; defrost starts when the timer expires and the coil is at or below a set temperature.
Defrost termination thermostat/thermistorA sensor mounted on the outdoor coil that signals the control board to end the defrost cycle when the coil temperature reaches the manufacturer's specified termination set point.
Reversing valveA four-way refrigerant valve that shifts refrigerant flow direction; during defrost it routes hot discharge gas to the outdoor coil so the coil acts as a condenser and melts frost.
Time-out terminationA safety backup built into the defrost board that ends the defrost cycle after a maximum elapsed time if the coil temperature sensor never reaches the termination set point.
ThermistorA temperature-sensitive resistor used as a sensor on defrost boards; its resistance changes predictably with temperature, allowing the board to calculate coil or ambient temperature.
Pressure-switch initiationA defrost initiation method that monitors the pressure differential across the outdoor coil; when airflow restriction from frost raises the differential to the switch set point, defrost is triggered.
Auxiliary heat lockoutA board output that energizes supplemental electric resistance heat strips during defrost to prevent cold air from being delivered to the conditioned space while the outdoor coil is being defrosted.

Worked examples3

1. A defrost board's service manual states the coil initiation temperature is 28 °F and the termination temperature is 60 °F. A technician probes the outdoor coil and reads 26 °F after 90 minutes of compressor run time. The board does not initiate defrost. Is defrost expected to initiate, and what should the technician check?
  1. Step 1 – Compare measured coil temperature to the initiation threshold. The coil reads 26 °F, which is below the 28 °F threshold, so the temperature condition IS met.
  2. Step 2 – Confirm the timer condition. The manual states 90-minute intervals. If 90 minutes of compressor run time have elapsed, the timer condition is also met.
  3. Step 3 – Since both conditions are met but defrost did not start, suspect the sensor circuit or the board itself. Check the thermistor resistance value against the temperature-resistance chart in the service manual to verify the sensor is reading accurately and is not open or shorted.
  4. Step 4 – Check the wiring between the sensor and the board terminal for loose connections, corrosion, or damaged insulation. If wiring and sensor test good, suspect a faulty board.
Answer: Defrost should have initiated because both the timer interval and the below-threshold coil temperature condition were met. The technician should verify the thermistor resistance matches the chart value for 26 °F, then check wiring continuity before condemning the board.
2. A heat pump defrost board has a maximum time-out of 10 minutes. The coil termination set point is 60 °F. During a defrost cycle, the technician measures the coil temperature rising from 22 °F at initiation. At the 10-minute mark the coil reads 48 °F and defrost terminates automatically. What terminated the cycle, and what does this finding suggest?
  1. Step 1 – Identify which termination event occurred. The coil reached only 48 °F, not the 60 °F termination set point, but 10 minutes elapsed. Therefore the time-out backup terminated the cycle, not the temperature sensor.
  2. Step 2 – Calculate how far below the set point the coil remained: 60 °F − 48 °F = 12 °F short of termination.
  3. Step 3 – Consider what could prevent the coil from reaching 60 °F in 10 minutes: excessive frost load, low refrigerant charge reducing discharge temperature, a partially stuck reversing valve not fully directing hot gas to the outdoor coil, or a refrigerant restriction.
  4. Step 4 – Recommend the next diagnostic step: check system operating pressures and discharge temperature to evaluate refrigerant charge and reversing valve performance.
Answer: The time-out backup terminated the cycle because the coil never reached 60 °F in 10 minutes. This suggests a system problem — such as low refrigerant charge, a partially stuck reversing valve, or a heavy frost load — that is preventing adequate heat delivery to the outdoor coil during defrost.
3. A thermistor on a defrost board reads 15,000 ohms. The service manual table shows that at 32 °F the thermistor should read approximately 16,500 ohms, and at 50 °F it should read approximately 9,800 ohms. What coil temperature range does the measured reading suggest, and is the coil condition consistent with initiating defrost?
  1. Step 1 – Compare the measured 15,000 Ω to the table values. At 32 °F the expected resistance is 16,500 Ω; at 50 °F it is 9,800 Ω. The measured value of 15,000 Ω falls between these two points.
  2. Step 2 – Determine direction: a thermistor (NTC type, which is standard for HVAC sensors) has decreasing resistance as temperature rises. A reading of 15,000 Ω is lower than 16,500 Ω (32 °F) and higher than 9,800 Ω (50 °F), placing the coil temperature between 32 °F and 50 °F.
  3. Step 3 – Assess whether defrost initiation is appropriate. If the initiation threshold is 28 °F or below, and the coil is estimated above 32 °F, the temperature condition is NOT met; defrost should not initiate at this moment.
  4. Step 4 – Conclude: the sensor reading is consistent with a coil that is cold but not yet frost-laden enough to trigger temperature-based initiation.
Answer: The 15,000 Ω reading indicates a coil temperature between 32 °F and 50 °F. Since this is above a typical initiation threshold of around 28 °F, the board would correctly withhold defrost initiation based on the temperature condition.
In class: Study a manufacturer defrost board schematic; identify time/temperature initiation, pressure-switch initiation, and demand defrost methods; fill in a comparison table describing how each method senses frost and triggers defrost. · Read a fault code chart from a defrost control board service manual; given three simulated fault codes, write out what each code means, what component is suspect, and what the next diagnostic step should be. · Simulate a defrost cycle on a heat pump trainer: initiate defrost manually per the service manual procedure, observe reversing valve shift and outdoor fan shutdown, measure outdoor coil temperature at initiation and termination, and record in a lab log.
Materials: Manufacturer defrost control board and service manual, defrost board wiring diagram, heat pump trainer unit, thermometer with coil probe, lab log sheets, NATE Air-to-Air Heat Pump specialty study guide
Check: Lab log reviewed by instructor: student correctly records initiation temperature, termination temperature, and elapsed time, and answers three analysis questions; pass mark 80%.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NATE Air-to-Air Heat Pump — Defrost Systems
1. During a heat pump defrost cycle, what happens to the outdoor fan motor?
HVAC Excellence — Heat Pump Controls
2. Which component provides the primary signal to end a defrost cycle under normal operating conditions?
NATE Air-to-Air Heat Pump — Defrost Systems
3. A demand defrost system differs from a simple time/temperature system primarily because demand defrost:
NATE Air-to-Air Heat Pump — Defrost Systems
4. A heat pump defrost board shows a fault code indicating the defrost cycle terminated on time-out rather than on the coil temperature sensor. Which of the following is the MOST likely cause?
NCCER HVAC — Safety
5. Before touching wiring terminals on a defrost control board, a technician MUST first:
HVAC Excellence — Heat Pump Wiring
6. On a defrost control board wiring diagram, the terminal labeled 'DFT' most likely connects to which component?
NATE Air-to-Air Heat Pump — Diagnostics
7. A technician measures a coil thermistor resistance of 22,000 ohms. The service manual table shows 16,500 ohms at 32 °F and 28,000 ohms at 20 °F. What does this reading indicate about the coil temperature?
NATE Air-to-Air Heat Pump — Defrost Systems
8. During a defrost cycle, why may the indoor auxiliary electric heat strips energize?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →