7.3 Electric Resistance Heating and Heat Pump Operation in Heating & Defrost Modes

Electric Resistance Heating Fundamentals

Electric resistance heating generates thermal energy by passing electric current through high-resistance conductor elements (Joule heating). In Texas residential and commercial air handlers, electric heat packages serve as emergency heating or auxiliary supplemental heat for heat pumps.

Resistance Heating Element Mechanics

• Material: Open-wire heating elements are constructed from Nichrome alloy (80% Nickel, 20% Chromium). Nichrome resists high-temperature oxidation and exhibits steady electrical resistance across high thermal operating ranges.
• Insulation: Coiled Nichrome elements are suspended inside the air handler chassis on high-grade ceramic insulator bushings to isolate high voltage ($240\text{V AC}$) from the grounded sheet metal cabinet.
• Power & Thermal Conversion Formula: Electric power in kilowatts (kW) converts directly into thermal output in BTUs per hour:

$1.0 \text{ kW} = 3,413 \text{ BTU/hr}$

$\text{Total Heating Capacity (BTU/hr)} = \text{kW Rating} \times 3,413$

Sample Field Conversion Calculation

An air handler is fitted with a 15 kW supplemental electric heat strip kit operating at $240\text{V}$:

$\text{Thermal Capacity} = 15 \text{ kW} \times 3,413 \text{ BTU/kW} = 51,195 \text{ BTU/hr}$

To determine electrical current draw under full load:

$\text{Current (Amperes)} = \frac{\text{Power (Watts)}}{\text{Voltage (Volts)}} = \frac{15,000 \text{ W}}{240 \text{ V}} = 62.5 \text{ Amperes}$

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Sequencers and Thermal Safety Limit Controls

Electric heat packages utilize specialized switching devices and multi-stage limit controls to protect building wiring and prevent equipment overheating.

Electric Heat Sequencers

If a $15 \text{ kW}$ or $20 \text{ kW}$ electric heat package energized simultaneously upon a thermostat call for heat, the sudden $60 - 80 \text{ Amp}$ current spike would trip main circuit breakers and cause severe voltage drops across the building panel. Furthermore, blowing cold air over cold elements creates initial cold air drafts.

• Operation: A sequencer is a solid-state or bimetallic disc relay containing a 24V PTC (Positive Temperature Coefficient) heater disc.
• Staging Delay: Upon a call for heat ($W_1 / W_2$), the sequencer internal heater warms a bimetallic disc. After a $20 - 60 \text{ second}$ delay, the disc snaps, closing high-voltage contacts to energize Element Bank 1 and start the indoor blower fan. Successive banks energize sequentially in stages ($20 - 60 \text{ seconds}$ apart). Upon thermostatic satisfaction, elements de-energize sequentially, allowing the blower fan to purge residual heat into the ductwork.

Limit Switches and Thermal Cutouts

1. Primary High-Limit Switch: A normally-closed bimetallic thermal switch mounted adjacent to the heat strips in the main air stream. Wired in series with the $24\text{V}$ sequencer coil or control circuit. If airflow drops due to a clogged air filter, broken blower belt, or failed blower motor, air temperatures inside the cabinet rise. At setpoint (typically $160^\circ\text{F} - 180^\circ\text{F}$), the primary limit opens, de-energizing the heating elements while keeping the blower running. Once cooled, it automatically resets.
2. Secondary Thermal Cutout (Thermal Fuse): A one-time fusible link wired directly in series with the $240\text{V}$ high-voltage line feeding each element bank. If the primary limit switch fails or sticks closed during a severe no-airflow event, temperatures rise rapidly. At safety limit (typically $250^\circ\text{F} - 300^\circ\text{F}$), the alloy link melts, permanently opening high-voltage power to prevent a structural building fire. Thermal fuses cannot be reset and must be replaced.

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Heat Pump Reverse Refrigeration Cycle and 4-Way Reversing Valve

A heat pump provides year-round climate control by reversing the flow of refrigerant using a 4-way reversing valve.

Cooling Mode:
Compressor Discharge ---> Reversing Valve ---> Outdoor Coil (Condenser) ---> TXV ---> Indoor Coil (Evaporator) ---> Reversing Valve ---> Compressor Suction

Heating Mode:
Compressor Discharge ---> Reversing Valve ---> Indoor Coil (Condenser) ---> TXV ---> Outdoor Coil (Evaporator) ---> Reversing Valve ---> Compressor Suction

Reversing Valve Construction and Thermodynamics

• Internal Slide Block: The reversing valve contains a Teflon-faced D-slide block inside a brass body. High-pressure discharge gas pushes the slide block to redirect refrigerant vapor flow.
• Pilot Solenoid Valve: A 24V AC electromagnetic pilot solenoid controls small capillary tubes connected to the main valve body. Shifting the pilot solenoid creates a pressure differential between the left and right ends of the slide block, forcing the slide to shift positions.
• Thermostat Terminal Conventions (O vs. B):

Technician Diagnostic Tip: If a thermostat is misconfigured (setting reversing valve to 'B' on a Carrier system requiring 'O'), the heat pump will operate in heating mode when cooling is called, blowing hot air into the structure during Texas summer months.

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Heat Pump Efficiency Metrics and Balance Point Dynamics

Heat pump efficiency is rated by comparing energy heat output to electrical energy input.

Coefficient of Performance (COP)

$\text{COP} = \frac{\text{Thermal Energy Output (Watts or BTU/hr)}}{\text{Electrical Energy Input (Watts or BTU/hr Equivalent)}}$

• Electric Resistance Heat: Converts 1 Watt of electricity into 1 Watt of heat ($\text{COP} = \mathbf{1.0}$).
• Heat Pump Performance: At mild outdoor temperatures ($47^\circ\text{F}$), a modern heat pump moves 3 to 4 units of heat energy for every unit of electrical energy consumed ($\text{COP} = \mathbf{3.0 - 4.5}$). As outdoor temperature falls, air density drops and heat capacity decreases, lowering COP toward 1.5 - 2.0.

Heating Seasonal Performance Factor (HSPF / HSPF2)

• HSPF: Total heating output in BTUs during the annual heating season divided by total watt-hours consumed.
• HSPF2: Updated efficiency metric under M1 testing standards (incorporating higher external static pressure test conditions). Higher numbers indicate greater seasonal efficiency.

Thermal vs. Economic Balance Point

Heat Output / Load (BTU/hr)
  ^
  |   / Building Heat Loss Line
  |  / 
  | /     Heat Pump Capacity Line
  |/      \
  +--------X-------------------------> Outdoor Ambient Temp (°F)
           Thermal Balance Point (approx. 30°F - 35°F)

1. Thermal Balance Point: The outdoor ambient temperature at which the heat pump's maximum heating capacity exactly equals the building's structural heat loss. Below the thermal balance point, the heat pump alone cannot maintain the indoor setpoint, requiring auxiliary electric heat strips (or gas furnace in dual-fuel systems) to energize and supplement the deficit.
2. Economic Balance Point: The outdoor ambient temperature at which operating supplemental electric heat or a secondary gas furnace becomes cheaper than operating the heat pump, based on local utility electric rates (cents/kWh) versus natural gas rates (dollars/therm).

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Heat Pump Defrost Controls and Sequence of Operation

When a heat pump operates in heating mode with outdoor ambient temperatures below $45^\circ\text{F}$, the outdoor coil surface temperature drops below $32^\circ\text{F}$. Moisture in outdoor air condenses and freezes on the outdoor coil fins. Accumulated frost acts as a thermal insulator and blocks airflow, causing suction pressure to collapse.

Defrost Initiation Logic

• Time-Temperature Defrost: Uses an electronic timer module (selectable 30, 60, or 90 minute intervals) combined with an outdoor coil bimetal thermostat. If the coil thermostat senses temperature below freezing ($\approx 30^\circ\text{F}$) when the timer interval expires, defrost initiates.
• Demand Defrost: Microprocessor boards measure the temperature differential ($\Delta T$) between outdoor ambient air and outdoor coil refrigerant temperature, or measure air pressure drop across the outdoor coil face. Defrost initiates only when actual frost accumulation restricts performance, eliminating unnecessary defrost cycles and saving significant energy.

Four Concurrent Steps of a Defrost Cycle

When defrost initiates, the control board executes four simultaneous control actions:

Defrost Initiation:
1. Reversing Valve Shifts to COOLING (Hot gas melts outdoor ice)
2. Outdoor Fan De-energizes (Traps heat at outdoor coil)
3. Auxiliary Heat Energizes (Prevents cold indoor draft)
4. Compressor Continues Running (Drives thermodynamic cycle)

1. Reversing Valve Shifts to Cooling Mode: The 4-way valve solenoid shifts refrigerant flow, directing high-pressure, high-temperature discharge gas into the outdoor coil to rapidly melt ice.
2. Outdoor Fan Motor Shuts Off: De-energizing the outdoor fan prevents freezing outdoor ambient air from blowing across the coil, trapping heat inside the outdoor cabinet to accelerate ice melt.
3. Auxiliary Electric Heat Energizes (W2/E Terminal): Because the heat pump is operating in cooling mode during defrost, cold air would dump into the living space. The defrost board energizes indoor electric heat strips to temper the air stream.
4. Compressor Continues Running: The compressor remains operational to pump hot gas through the outdoor coil.

Defrost Termination

Defrost terminates when either:

• The outdoor coil bimetal sensor/thermistor warms up to its termination setpoint (typically $55^\circ\text{F} - 60^\circ\text{F}$), proving all frost has melted, or
• The maximum override safety time limit (usually 10 to 12 minutes) is reached.