HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Heat Pump Technology for HVAC Engineers

Heat Pump Technology for HVAC Engineers

Heat pumps transfer heat from cold sources to warm spaces, providing heating with COP of 2.5-4.5 (250-450% efficiency). Understanding performance degradation at low temperatures enables proper sizing and supplemental heat integration.

Heat Pump Types

Air-Source Heat Pumps (ASHP)

Extract heat from outdoor air.

Advantages:

  • Lower first cost
  • Easy installation
  • No ground loop required

Disadvantages:

  • Capacity degrades at low outdoor temperatures
  • Requires defrost cycle
  • COP drops significantly below 32°F

Ground-Source Heat Pumps (GSHP)

Extract heat from ground via buried loops.

Advantages:

  • Stable ground temperature (50-60°F year-round)
  • Higher COP (3.0-5.0)
  • No defrost cycle
  • Longer life (25+ years)

Disadvantages:

  • High first cost ($6,000-$12,000/ton installed)
  • Requires land area or deep bores
  • Complex ground loop design

Performance Metrics

Heating COP:

$$COP_h = \frac{Q_h}{W_{comp}} = \frac{h_2 - h_3}{h_2 - h_1}$$

Cooling EER:

$$EER = \frac{Q_c \text{ (Btu/h)}}{W_{comp} \text{ (W)}}$$

HSPF (Heating Seasonal Performance Factor): Seasonal heating efficiency including defrost and cycling losses.

$$HSPF = \frac{\sum Q_h}{\sum W_{total}}$$

Units: Btu/W·h (higher is better)

Minimum standards:

  • Federal minimum: HSPF 8.2 (Northern), 8.8 (Southern)
  • ENERGY STAR: HSPF 9.0+

Balance Point Analysis

Balance point: outdoor temperature where heat pump capacity equals building heat loss.

$$T_{balance} = T_{indoor} - \frac{Q_{design}}{UA_{building}}$$

Below balance point: Supplemental heat required

Sizing strategies:

  1. Size for balance point: No supplemental heat above balance point (common for ASHP)
  2. Size for full load: Heat pump handles entire load (expensive for ASHP, typical for GSHP)
  3. Dual fuel: Switch to gas furnace below economic balance point

Defrost Control

Frost formation: When evaporator coil < 32°F and humid outdoor air

Defrost methods:

  • Reverse cycle: Switches to cooling mode, hot gas melts frost (most common)
  • Hot gas bypass: Diverts compressor discharge to outdoor coil
  • Electric resistance: Heats outdoor coil (inefficient)

Defrost initiation:

  • Time-temperature: Every 30-90 min if coil < 32°F
  • Demand: Pressure differential or coil temperature triggers

Energy penalty: 5-15% capacity loss during heating season

Practical Applications

  1. Cold climates: Consider GSHP or dual-fuel ASHP
  2. Moderate climates: ASHP cost-effective
  3. Cooling-dominant: Heat pump ideal (high cooling efficiency)

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 9: Air-Source Heat Pumps
  • IGSHPA Design and Installation Standards