9.2 Defrost Controls (Time-Temperature, Demand Defrost), Sensors, and Sequences

9.2 Defrost Controls, Sensors, and Sequences of Operation

When an air-source heat pump operates in heating mode, the outdoor coil functions as the evaporator, absorbing thermal energy from the ambient air. In ambient temperatures between 20°F and 45°F (-6.7°C to 7.2°C) with elevated relative humidity, moisture in the outdoor air condenses and freezes onto the outdoor coil surfaces. Uncontrolled frost accumulation acts as a thermal insulator and blocks air passage between fin surfaces, drastically reducing heat transfer, lowering system suction pressure, and causing potential compressor damage. Defrost controls detect this frost buildup and initiate an automatic sequence to melt the ice quickly and return to normal heating.

1. The Physics of Outdoor Coil Frost Formation

A common misconception among building owners is that frost only forms when outdoor ambient air is below freezing (32°F / 0°C). In reality, frost frequently accumulates when ambient temperatures are as high as 42°F to 45°F (5.5°C to 7.2°C).

Why Frost Forms Above 32°F Ambient

• To absorb heat from outdoor air, the refrigerant inside the outdoor coil must boil at a saturation temperature 10°F to 20°F below the ambient air temperature.
• At 40°F (4.4°C) outdoor ambient, the refrigerant evaporating inside the coil is operating at 20°F to 30°F (-6.7°C to -1.1°C).
• Consequently, the metal surface of the aluminum fins and copper tubing falls well below the 32°F freezing point of water.
• If the outdoor air's dew point temperature is above the coil surface temperature, water vapor condenses onto the metal and immediately freezes into frost crystals.

Consequences of Unchecked Frost Accumulation

1. Thermal Resistance: Frost has low thermal conductivity relative to aluminum ($k_{\text{frost}} \approx 0.1\text{--}0.3\text{ W/m}\cdot\text{K}$ vs $k_{\text{aluminum}} \approx 205\text{ W/m}\cdot\text{K}$), creating an insulating blanket that inhibits heat transfer from ambient air to refrigerant.
2. Airflow Restriction: Frost restricts the narrow gaps between coil fins (typically 14 to 22 fins per inch). This reduces CFM airflow, starving the evaporator.
3. Pressure Drop & Suction Starvation: Reduced heat transfer causes evaporating pressure and suction temperature to plummet. This reduces refrigerant vapor density, decreasing mass flow rate, dropping indoor heating capacity, and elevating compression ratios.

2. Defrost Initiation and Control Strategies

Modern heat pumps utilize either Time-Temperature Defrost or Electronic Demand Defrost systems to manage coil clearing.

┌─────────────────────────────────────────────────────────────────────────────┐
│                     DEFROST CONTROL CLASSIFICATION                          │
├──────────────────────────────────────┬──────────────────────────────────────┤
│       TIME-TEMPERATURE DEFROST       │        DEMAND DEFROST (SMART)        │
├──────────────────────────────────────┼──────────────────────────────────────┤
│ • Fixed timer (30, 60, or 90 min)    │ • Dual thermistors (Ambient vs Coil) │
│ • Bimetal snap switch on coil (≤30°F)│ • Microprocessor monitors ΔT split   │
│ • Simple, low initial equipment cost │ • Initiates only when frost is real  │
│ • Initiates 'dry' unnecessary cycles │ • High seasonal efficiency (HSPF)    │
└──────────────────────────────────────┴──────────────────────────────────────┘

Method 1: Time-Temperature Defrost Controls

Time-temperature controls are electromechanical or solid-state timer boards paired with a bimetal temperature sensor (snap-disc thermostat) clamped to the outdoor coil:

• The Timer Module: Accumulates running time only when the compressor is energized (via the Y signal). The board features field-selectable jumper pins for 30, 60, or 90 minutes.
  • 30-minute setting: Humid, wet coastal climates or frequent winter fog.
  • 60-minute setting: Standard temperate climates (factory default on most units).
  • 90-minute setting: Dry, arid winter regions with low ambient humidity.
• The Defrost Thermostat (Sensor): Clamped to the lowest, coldest liquid circuit or return bend of the outdoor coil.
  • Closing Temperature: Typically closes when coil temperature drops to 30°F ± 3°F (-1.1°C ± 1.7°C).
  • Opening Temperature: Opens when coil temperature warms to 55°F to 65°F (12.8°C to 18.3°C).
• Operating Logic: When the accumulated run timer expires (e.g., 60 minutes), the board checks the status of the defrost thermostat. If the thermostat is closed (coil is $\le 30^\circ\text{F}$), defrost initiates. If the thermostat is open (coil is warm), the timer resets for another 60 minutes without defrosting.

Method 2: Electronic Demand Defrost Controls

Demand defrost systems use a microprocessor and electronic thermistors to eliminate unnecessary "dry" defrost cycles, improving seasonal efficiency (HSPF) by 5% to 10%:

• Dual Sensor Differential Algorithm:
  • Outdoor Ambient Sensor ($T_{\text{ambient}}$): Measures outdoor dry-bulb air temperature.
  • Coil Temperature Sensor ($T_{\text{coil}}$): Measures outdoor coil refrigerant saturation temperature.
  • On a clean, unfrosted coil, the temperature differential $\Delta T = T_{\text{ambient}} - T_{\text{coil}}$ remains relatively constant (e.g., $10^\circ\text{F}\text{ to }15^\circ\text{F}$).
  • As frost insulates the coil and chokes airflow, the coil temperature drops lower to extract heat, causing the split $\Delta T$ to widen to $22^\circ\text{F}\text{ to }28^\circ\text{F}$.
  • When the microprocessor detects this critical temperature differential for a predetermined time window, it initiates defrost.
• Air Pressure Differential Method: An alternative demand method uses an air pressure differential switch sensing static pressure drop across the outdoor coil. When ice bridges the fins, static pressure rises across the coil face, closing the switch and triggering defrost.

3. Step-by-Step Defrost Sequence of Operation

When defrost initiation criteria are satisfied, the system executes a tightly choreographed electromechanical sequence:

1. Step 1: Reversing Valve Shifts to Cooling Mode
   • The defrost control board energizes the reversing valve solenoid (on O-type systems) or de-energizes it (on B-type systems).
   • This immediately redirects superheated, high-pressure discharge gas (160°F–200°F / 71°C–93°C) from the compressor directly into the frozen outdoor coil.
2. Step 2: Outdoor Fan Motor is De-energized
   • The defrost board opens the normally closed (NC) contacts of the outdoor fan relay, cutting power to the outdoor fan motor.
   • Critical Engineering Rationale: Stopping the fan prevents cold ambient air from blowing across the outdoor coil. This retains all heat within the coil structure, rapidly elevates condensing temperature and head pressure (300 to 450 psig on R-410A), and melts the ice within 2 to 6 minutes.
3. Step 3: Auxiliary Electric Heat is Energized
   • Because the system is operating in cooling mode during defrost, the indoor coil becomes the evaporator, absorbing heat and chilling the indoor airflow to 45°F–50°F (7°C–10°C).
   • To prevent blowing cold air into the occupied home ("cold blow"), the defrost board sends 24 VAC to the W (or W2) terminal, energizing indoor electric resistance heating elements to temper the air stream to 85°F–95°F.
4. Step 4: Ice Melts
   • The high-pressure hot gas converts the ice into water runoff. Water drains through the outdoor coil base pan drain slots.
5. Step 5: Defrost Termination
   • As the last ice melts, the temperature of the outdoor coil rises rapidly.
   • When the coil temperature sensor reaches its termination setpoint (55°F to 65°F / 12.8°C to 18.3°C), the defrost thermostat opens, signaling complete ice removal.
6. Step 6: Fail-Safe Override Timer
   • If the coil sensor fails open, is dislodged, or extreme winds prevent the coil from reaching 55°F, an internal fail-safe timer (typically 10 to 14 minutes) automatically terminates the defrost cycle to prevent compressor overheating and excessive electrical consumption.
7. Step 7: Resumption of Normal Heating
   • The reversing valve shifts back to Heating mode.
   • The outdoor fan restarts.
   • The auxiliary electric heat de-energizes.
   • The defrost timer resets to zero.

4. Defrost Sensor Testing and Board Diagnostics

When diagnosing heat pump freeze-up issues, technicians must systematically verify each component of the defrost circuit.

Defrost Thermostat (Bimetal Snap Disc) Testing

1. Disconnect sensor leads from the control board.
2. When warm (above 65°F), an ohmmeter placed across the switch leads must read infinite resistance (Open Circuit / O.L.).
3. Submerge the sensor in an ice bath or spray with freeze spray until its temperature falls below 28°F (-2.2°C). The switch must snap closed, reading 0.0 to 0.5 ohms (Continuity).
4. Warm the sensor; it must open cleanly between 55°F and 65°F.

10kΩ NTC Thermistor Testing (Demand Systems)

Electronic demand boards use Negative Temperature Coefficient (NTC) thermistors (resistance decreases as temperature increases). Technicians measure sensor resistance with an ohmmeter and compare with the manufacturer's temperature-resistance chart:

Forcing a Test Defrost Cycle (Speed-Up Pins)

Most control boards feature two metal pins labeled TEST or SPEED:

1. Ensure the compressor is running in heating mode.
2. Place a flathead screwdriver or jumper across the TEST pins for 2 to 5 seconds.
3. On solid-state timer boards, shorting the test pins accelerates the 30/60/90-minute clock to mere seconds (e.g., 1 second of clock time = 1 minute of run time).
4. If the defrost thermostat is closed, the board immediately initiates defrost: reversing valve shifts, outdoor fan stops, and auxiliary heat turns on.