5.3 Defrost Systems: Time-Temperature vs. Demand Defrost Cycles & Sensor Operation

5.3 Defrost Systems: Time-Temperature vs. Demand Defrost Cycles & Sensor Operation

In heating mode, the outdoor coil of an air-source heat pump functions as the evaporator, absorbing thermal energy from the ambient air. When outdoor temperatures drop into the 30°F to 45°F range, the coil surface operating temperature typically runs 10°F to 15°F colder than the surrounding air. Consequently, the coil surface falls below 32°F (0°C). If the ambient air contains moisture (relative humidity > 50%), water vapor freezes upon contact with the aluminum fins and copper tubes, forming frost. An automated defrost system is required to melt this frost and restore heat transfer efficiency.

---

1. The Physics and Penalties of Coil Frosting

Frost formation on an outdoor heat pump coil triggers two detrimental thermodynamic penalties:

1. Thermal Insulation Penalty: Pure copper has a thermal conductivity of $k \approx 223\text{ BTU}/(\text{hr}\cdot\text{ft}\cdot^\circ\text{F})$ and aluminum has $k \approx 118\text{ BTU}/(\text{hr}\cdot\text{ft}\cdot^\circ\text{F})$. In contrast, frost has an extremely low thermal conductivity ($k \approx 0.05\text{ to }0.20\text{ BTU}/(\text{hr}\cdot\text{ft}\cdot^\circ\text{F})$). Frost acts as a thermal insulator, preventing outdoor ambient heat from conducting into the boiling refrigerant.
2. Airflow Restriction Penalty: As frost builds between the closely spaced coil fins (typically 14 to 22 fins per inch), it chokes the free face area. Airflow (CFM) drops, reducing convective heat transfer. This causes outdoor evaporator saturation pressure to plunge, starving the compressor of suction density, dropping COP, and potentially tripping low-pressure safety switches.

                      FROST ACCUMULATION CYCLE

  Outdoor Ambient (35°F–42°F, Humid Air)
                   │
                   ▼
  Coil Surface Drops Below 32°F (e.g., 22°F Saturation)
                   │
                   ▼
  Moisture Condenses & Freezes on Fin Surfaces
                   │
                   ▼
  Airflow Choked ──► Evaporator Pressure Drops ──► Heat Output & COP Collapse
                   │
                   ▼
           [ DEFROST REQUIRED ]

---

2. Defrost Initiation & Termination Methods

Modern heat pump systems manage defrost cycles using two primary control strategies: Time-Temperature Defrost and Demand Defrost.

1. Time-Temperature Defrost Controls

Time-temperature defrost is an electromechanical or solid-state timer method widely used in residential systems:

• Initiation: The defrost control board incorporates a cumulative compressor run timer. A field-selectable jumper pin allows the technician to choose intervals of 30, 60, or 90 minutes of compressor run time. At the end of the selected run interval, the board checks the electrical state of a defrost thermostat (defrost sensor):
  • The defrost thermostat is a sealed bimetal disc switch clamped to the lowest, coldest distributor circuit of the outdoor coil.
  • It closes on temperature fall at approximately ≤ 30°F to 32°F (confirming frost conditions).
  • If the timer interval expires AND the defrost thermostat is closed, the control board initiates a defrost cycle.
  • If the timer interval expires but the thermostat is open (>32°F), the timer resets and no defrost occurs.
• Termination: The defrost cycle terminates via one of two mechanisms:
  1. Temperature Termination (Normal): As hot discharge gas melts the ice, coil temperature rises. When the bimetal switch warms to approximately 65°F to 80°F (depending on manufacturer), the switch opens, signaling the board to end defrost.
  2. Fail-Safe Time Override: If the defrost thermostat fails to open (due to extreme cold, high winds, or a defective switch), an internal fail-safe timer terminates the defrost cycle after 10 to 14 minutes to prevent overheating the compressor and wasting energy.

2. Demand Defrost Controls

Demand defrost systems utilize electronic microprocessors that monitor actual coil performance rather than relying on fixed timers:

• Temperature Differential Method (ΔT): The controller reads two precision thermistors: Outdoor Ambient Temperature Sensor ($T_{\text{ambient}}$) and Outdoor Coil Temperature Sensor ($T_{\text{coil}}$). When the coil is clean, $T_{\text{ambient}} - T_{\text{coil}} \approx 10^\circ\text{F}\text{ to }15^\circ\text{F}$. As frost accumulates, heat transfer degrades and $T_{\text{coil}}$ drops relative to ambient. When ΔT exceeds a calibrated threshold (e.g., >22°F to 28°F differential curve), defrost is triggered.
• Pressure Differential Method: Uses a sensitive diaphragm pressure switch to detect the air pressure drop across the outdoor coil fins. When frost blocks the fins, the air pressure drop across the coil rises, closing the switch to initiate defrost.
• Efficiency Advantage: In dry desert winter climates (such as Tucson or Phoenix where relative humidity is low), an outdoor coil may operate at 28°F for hours without forming frost. A time-temperature system would initiate unnecessary defrost cycles every 60 minutes, wasting energy; a demand defrost system only initiates defrost when frost is physically present.

---

3. Step-by-Step Sequence of Operations During Defrost

When a defrost cycle is initiated, the system executes an automated sequence to melt coil frost rapidly while maintaining indoor comfort:

                    COMPLETE DEFROST SEQUENCE OF OPERATIONS

  1. DEFROST INITIATION
     Timer Interval Met (30/60/90 min) + Defrost Stat Closed (≤30°F)
     ──► Defrost Board Energizes Defrost Relay
                   │
                   ▼
  2. REVERSING VALVE SHIFTS TO COOLING
     4-Way Valve shifts hot discharge gas directly into outdoor coil
                   │
                   ▼
  3. OUTDOOR FAN MOTOR DE-ENERGIZES
     Outdoor fan stops immediately to trap heat in outdoor coil cabinet
                   │
                   ▼
  4. AUXILIARY HEAT ENERGIZES (W2 / Aux Signal)
     Electric strip heat energizes in air handler to temper indoor supply air
     (Prevents "cold blow" into living space during cooling cycle)
                   │
                   ▼
  5. ICE MELTS (Coil Temperature Rises)
     Hot gas condenses in outdoor coil, melting frost from inside out
                   │
                   ▼
  6. DEFROST TERMINATION
     Defrost Stat Opens (65°F–80°F) OR Fail-Safe Timer (10–14 min) expires
                   │
                   ▼
  7. RESTORATION TO HEATING
     - Reversing valve shifts back to heating mode
     - Outdoor fan restarts (often with 5–15 sec delay for pressure relief)
     - Auxiliary strip heat de-energizes
     - Normal heat pump heating resumes

Critical Sequence Elements

• Outdoor Fan Shutdown: Turning off the outdoor fan prevents cold ambient air from blowing across the coil, allowing condensing temperatures to climb rapidly to 100°F–120°F, melting ice in under 3 to 6 minutes.
• Auxiliary Heat Tempering: Because the system is temporarily running in cooling mode during defrost, the indoor coil absorbs heat from the home. Without auxiliary heat, 45°F cold air would blow from the supply registers. The defrost board sends a 24VAC signal to the indoor air handler 'W2' / 'Aux' terminal to energize electric resistance heat strips, tempering the supply air to a neutral 72°F–78°F.

---

4. Defrost Board Testing & Field Troubleshooting

When troubleshooting an outdoor unit that is encased in a solid block of ice ("ice-ball freeze-up"), technicians must methodically isolate the control board, sensors, and reversing valve:

Defrost Board "Test" Pins (Speed-Up Mode)

Defrost control boards feature two metal test pins labeled "TEST" or "SPEED":

1. Ensure the compressor is running in heating mode.
2. Place a flathead screwdriver or jumper wire across the two TEST pins for 3 to 5 seconds.
3. This bypasses the internal timer, accelerating the 30/60/90 minute clock to mere seconds.
4. If Defrost Stat is Closed: The unit should immediately shift into defrost (reversing valve clicks, outdoor fan stops, auxiliary heat energizes). If it does, the board timer and logic are functional.
5. If Defrost Stat is Open (>32°F): You must temporarily jumper the defrost thermostat terminals on the board while shorting the TEST pins to force a manual defrost.

                    DEFROST BOARD TROUBLESHOOTING MATRIX

  Problem: Outdoor Coil Completely Encased in Solid Ice
  ├── Step 1: Check Defrost Thermostat (Bimetal Switch)
  │   └── If coil is iced (<30°F) and switch is OPEN (Infinite Ω) ──► Defrost Stat Failed Open (Replace Stat)
  ├── Step 2: Check Defrost Relay on Board
  │   └── Short TEST pins with stat closed; if fan stays on and valve doesn't shift ──► Defrost Board Defective
  ├── Step 3: Check Reversing Valve Solenoid Coil
  │   └── If board sends 24VAC to valve but valve doesn't shift ──► Solenoid Coil or Valve Slide Stuck
  └── Step 4: Check Outdoor Fan Relay (OFR)
      └── If fan never stops during defrost, board fan relay contacts are welded shut ──► Replace Board

---

5. Worked Field Example: Defrost Sensor Diagnostics

Scenario: A technician in Flagstaff, AZ arrives at a heat pump during a 25°F snowstorm. The outdoor coil is completely choked with 1 inch of solid ice. The homeowner states the unit has been running constantly and blowing cool air.

Diagnostic Measurements

• Outdoor Coil Temperature at Distributor Tube: 18.0°F
• Defrost Thermostat Continuity Test: Technician disconnects one lead of the bimetal defrost thermostat from the board and measures resistance across the thermostat leads with an ohmmeter.
• Ohmmeter Reading: OL (Infinite Resistance / Open Circuit).

Diagnostic Deduction & Repair

1. At a coil temperature of 18°F, a functional defrost thermostat must be closed (0.0 Ω) because its closing setpoint is ≤ 30°F to 32°F.
2. The infinite resistance reading proves the bimetal internal contact is stuck open.
3. Because the thermostat was open, the defrost control board never received the closed-switch signal to initiate defrost when the 60-minute timer elapsed, allowing ice to accumulate until airflow completely stopped.
4. Repair: Replace the defective bimetal defrost thermostat, secure the new sensor firmly to the liquid distributor tube with an approved copper spring clip, wrap with insulating mastic tape, and manually initiate a defrost cycle to clear the ice block.

Exam Trap: Never use an open flame torch to melt ice off an aluminum microchannel or fin-and-tube coil. The intense localized heat will warp fins, melt aluminum tubes, and create severe refrigerant leaks. Always clear ice by forcing an electrical defrost cycle or using warm water.