For variable-speed cold climate heat pump (CCHP) systems, it’s essential to note that these systems are designed to optimize performance under a wide range of conditions. Here's an outline of the key factors and average ranges that might be seen for return-to-plenum temperature differences, outdoor coil delta T, and auxiliary heat cutover considerations.
1. Delta T Across the Indoor Coil (Return to Plenum)
The delta T (ΔT) across the indoor coil depends on several factors, including indoor airflow rate, system capacity modulation, and outdoor temperature. For variable-speed heat pumps:
- Typical Range: 20°F to 30°F (11°C to 17°C)
- At lower capacities (milder outdoor conditions), the delta T is closer to 15°F to 20°F (8°C to 11°C).
- At higher capacities (colder outdoor conditions), the delta T is closer to 25°F to 30°F (14°C to 17°C).
2. Delta T Across the Outdoor Coil (Heating Mode)
For the outdoor condensing unit in heating mode, the delta T represents the heat absorbed by the refrigerant:
- Typical Range: 20°F to 35°F (11°C to 19°C)
- The exact delta T varies with outdoor air temperature, compressor speed, and system performance.
- At colder outdoor conditions, the delta T may narrow slightly (e.g., 15°F to 25°F) due to reduced heat available in the ambient air.
- In mild conditions, delta T may exceed 30°F as the system operates efficiently at lower speeds.
3. Auxiliary Heat Cutover
In cold climates, many CCHPs are designed to maintain efficiency down to around -10°C to -20°C (14°F to -4°F). Below this point:
- The system may activate auxiliary heat (electric resistance heat or fossil fuel backup) to maintain indoor comfort.
- Factors influencing cutover: design balance point of the home, outdoor temperature, and heat pump capacity at low ambient temperatures.
4. Sequence of Operations
Understanding the normal sequence of operations for defrost and heating is critical:
- Defrost Cycle: Occurs periodically when outdoor temperatures are near or below freezing and humidity is high. Homeowners often mistake this as a malfunction because the system briefly switches to cooling mode to defrost the outdoor coil.
- Duration: Typically lasts 5 to 15 minutes.
- Frequency: Every 30 minutes to 2 hours, depending on conditions.
- Auxiliary Heat Activation: Common during defrost to prevent cold air from being delivered indoors or when the heat pump cannot meet demand.
Typical Operational Table
| Outdoor Temp (°C) | Indoor ΔT (Return to Plenum) (°F) | Outdoor Coil ΔT (°F) | Auxiliary Heat Status |
|---|---|---|---|
| 10°C (50°F) | 15°F to 20°F | 30°F to 35°F | Off |
| 0°C (32°F) | 18°F to 25°F | 25°F to 30°F | Off |
| -10°C (14°F) | 20°F to 30°F | 20°F to 25°F | May engage depending on load |
| -15°C (5°F) | 22°F to 30°F | 15°F to 20°F | Likely engaged |
| -20°C (-4°F) | 25°F to 30°F | 10°F to 15°F | Fully engaged |
Tips for Homeowners
- Defrost Mode:
- "It’s normal for the outdoor unit to occasionally frost up and for the system to defrost itself. During this time, the heat pump may stop producing heat for a few minutes."
- Auxiliary Heat Activation:
- "Auxiliary heat is designed to supplement the heat pump when temperatures drop significantly. This prevents your home from becoming uncomfortable."
- Variable-Speed Operation:
- "The system modulates its output to match the heating demand, which can result in a variable supply temperature. It’s operating efficiently even when you notice small variations."