Heat pumps represent the future of building decarbonization, moving facilities away from fossil fuel dependency toward electrified heating and cooling. Building Automation Systems must properly manage heat pump operation to maximize efficiency, maintain comfort, and protect equipment.
| Temperature Range | System Status | COP | Control Strategy |
|---|---|---|---|
| +15C to +45C (Cool) | Cooling, low aux load | 3.5-4.5 | Compressor variable load, free cooling possible |
| +5C to +15C (Mild) | Heating, minimal aux | 3.0-3.5 | Heat pump primary, monitor for defrost |
| -5C to +5C (Cold) | Heating, aux staged | 2.0-3.0 | Heat pump primary + electric/gas backup |
| Below -5C (Very Cold) | Heating, all aux | 1.2-2.0 | Balance point reached, fossil fuel backup |
Water temperature maintained 20-30C by central boiler/cooler or cooling tower. Consistent high COP (typically 4.5-6.0) even at outdoor extremes. Heat from internal zones reused to heat cold zones rather than rejected outdoors.
Constant source temperature 10-15C regardless of season. Excellent COP range 4.5-7.0. No defrost cycles needed. Installation cost $15,000-30,000 per unit.
IF OutdoorTemp > 21C THEN SystemMode = Cooling
ELSE IF OutdoorTemp < 15C THEN SystemMode = Heating
ELSE
// Dead band: follow last mode, prevent hunting
IF LastMode = Heating THEN stay Heating until OutdoorTemp > 21C
IF LastMode = Cooling THEN stay Cooling until OutdoorTemp < 15C
Changeover dead band: Typically 5-10C to prevent rapid switching.
When outdoor coil temperature drops to 3-7C AND humidity is high (>60%), frost accumulates on coil fins, blocking airflow. The heat pump must periodically reverse to melt frost.
| Trigger Type | Monitoring Point | Typical Value | Notes |
|---|---|---|---|
| Time-based | Wall clock timer | Every 90 minutes | Simplest, least efficient |
| Temperature-based | Outdoor coil temp | < 5C + 10 min runtime | More efficient |
| Demand-based | Pressure/capacity drop | >20% capacity loss | Most efficient, complex |
| Hybrid | Time + temp + pressure | Combination | Industry best practice |
1. DEFROST START:
- Reverse refrigerant flow
- Stop outdoor fan
- Enable supplemental electric heat
- Begin timing cycle
2. DEFROST ACTIVE (5-15 minutes):
- Monitor outdoor coil outlet temp
- When outlet temp exceeds 15-18C, frost melted
- Or timeout after 15 minutes
3. DEFROST END:
- Resume normal heating mode
- Restart outdoor fan
- Log defrost event with timestamp, duration
- Allow 10 minute cool-down before next defrost
During defrost, indoor temperature may drop 2-5C. Mitigation: size supplemental heat to maintain setpoint, avoid defrost during peak occupied hours, display Defrost Cycle Active on controls.
Balance Point = Indoor Setpoint - (Building Heat Loss / Heat Pump Capacity)
Example:
- Indoor setpoint: 21C
- Building heat loss at -10C: 150 kW
- Heat pump capacity at -10C: 40 kW
- Auxiliary capacity: 100 kW
Balance Point = 21 - (150 - 40) / 100 = approximately 0C
Below 0C outdoor, stage auxiliary heat
| Outdoor Temp | HP Capacity | Control Action |
|---|---|---|
| > 5C | 100% | Heat pump only |
| -5C to +5C | 60-100% | Heat pump + Stage 1 aux as needed |
| -10C to -5C | 40-60% | Heat pump + Stage 1 + Stage 2 aux |
| < -15C | < 40% | Heat pump + All aux stages |
// Heat pump capacity derating based on outdoor temp
IF outdoor_temp > 5:
hp_max_capacity = 50 kW
ELSE IF outdoor_temp > 0:
hp_max_capacity = 50 * (5 + outdoor_temp) / 5
ELSE IF outdoor_temp > -10:
hp_max_capacity = 50 * (outdoor_temp + 10) / 10
ELSE:
hp_max_capacity = 10 kW (minimum floor)
// Enable/disable auxiliary stages
aux_stage_1 = (required_heat > hp_max_capacity * 0.7)
aux_stage_2 = (required_heat > hp_max_capacity * 1.2)
COP = Thermal Energy Output / Electrical Energy Input
| Point | Unit | Purpose |
|---|---|---|
| Heat pump electrical power | kW | Input for COP |
| Thermal output (calculated) | kW | Output for COP |
| COP (calculated) | ratio | Efficiency metric |
| Outdoor source temp | C | COP reference |
| Compressor runtime | hours/day | Utilization |
| Defrost events | count/day | Stress indicator |
| Auxiliary heat runtime | hours/day | Balance point proximity |
IF COP < NominalCOP * 0.75 THEN
Alert: Low COP - Check for fouling, refrigerant leak, or outdoor coil freeze
Control Logic:
| Alarm | Set Point | Action |
|---|---|---|
| Low outdoor coil temp | 0C | Initiate defrost |
| High discharge temp | 80C | Reduce load |
| Low refrigerant pressure | 3 bar | Possible leak, disable compressor |
| High refrigerant pressure | 35 bar | Reduce load, check condenser |
| Compressor high current | 15 A | Mechanical problem, disable |
| Defrost timeout exceeded | 15 minutes | Sensor failure, escalate |
| Aux heat excessive runtime | 8 hrs/day | Balance point issue |
| Low loop temp | 5C water | Freeze risk, disable |
| Load Condition | Stage 1 | Stage 2 | Benefit |
|---|---|---|---|
| 0-30% | Off | Off | Power off |
| 30-60% | Variable speed | Off | Single unit efficient |
| 60-85% | Max speed | Low speed | Both for better efficiency |
| 85-100% | Max speed | Max speed | Maximum capacity |
| Requirement | Implementation |
|---|---|
| HP lockout above economizer setpoint | Disable cooling compressor if outdoor < 21C |
| Aux heater lockout to balance point | Disable aux above temp where HP meets load |
| Defrost frequency | Time-based max 90 min or on-demand with sensors |
| COP verification | Trend efficiency for 12 months post-commissioning |
| Supply temperature control | Maintain within +/-3C of target |
| Occupancy scheduling | Reduce heating setpoint to 15C unoccupied |