On/Off Control in Heat Pump Control System

On/off control is the most basic heat pump control method. The controller reads a temperature signal, compares it with a setpoint, and switches the heat pump fully on or fully off. In control terminology, this happens inside a deadband or hysteresis window. ASHRAE defines dead band as the range within which a sensed variable can vary without initiating a change in the controlled process.

What Is On/Off Control?

On/off control is the simplest form of heat pump regulation. It operates the compressor in one of two states: fully on or fully off. No intermediate power level exists between these two states.

This control method uses a binary signal to start or stop heat pump operation. When space temperature deviates from the setpoint, the controller sends an activation signal. When the setpoint is reached, the controller sends a stop signal.

On/off control functions as the foundational layer of heat pump regulation. All other control strategies—modulating, variable-speed, cascade—build upon or replace this fundamental switching logic.

Purpose of On/off Control in Heat Pump Controls

On/off control maintains indoor temperature within a defined band around a target setpoint. The controller monitors space temperature continuously. It activates the heat pump when temperature falls below the lower threshold and deactivates it when temperature exceeds the upper threshold.

This process is known as thermostat-based switching. The acceptable temperature range between activation and deactivation is called the dead band or hysteresis band. A correctly configured dead band prevents excessive compressor cycling while maintaining acceptable comfort levels.

Why On/Off Control Is Needed

The Core Problem: Managing Thermal Demand Without Continuous Regulation

Buildings require consistent thermal conditioning. Heating and cooling loads vary constantly due to occupancy, solar gain, ventilation, and external temperature. A control mechanism must match heat pump output to these changing loads.

On/off control addresses this need through time-proportioning. The system runs at full capacity when active and rests when demand is satisfied. The ratio of run time to total cycle time reflects the actual thermal load.

Why Simpler Control Is Often the Right Choice

Advanced modulating controls add cost and complexity. Not every application requires or benefits from variable-speed regulation. On/off control delivers adequate performance in:

• Low-load residential heating and cooling applications
• Systems where thermal mass buffers temperature swings
• Buildings with consistent and predictable heat loads
• Legacy installations with existing binary control infrastructure

Regulatory Context

Energy performance standards influence control selection. EN 14825 (European testing standard for heat pumps) and ErP Directive 2009/125/EC define minimum seasonal performance requirements. AHRI Standard 210/240 governs residential equipment in North America. On/off systems must meet minimum efficiency benchmarks under these standards. Modulating systems often achieve higher seasonal coefficients of performance (SCOP/SEER), but on/off systems remain compliant in many application categories.

Key Features of On/Off Control

Binary Switching Logic

Definition: The control signal has exactly two states—on and off. No partial output levels exist.

Purpose: Binary switching eliminates the need for complex signal conditioning or proportional output hardware. The controller output directly drives a contactor or relay.

Benefits:

• Low-cost control hardware
• High reliability due to minimal electronic complexity
• Compatibility with standard thermostat wiring
• Easy fault diagnosis

Practical application: A room thermostat connected to a 24 V control circuit activates the compressor contactor when temperature drops 0.5 °C below setpoint. The contactor de-energises when setpoint is restored.

Hysteresis Band (Dead Band)

Definition: The hysteresis band is the temperature differential between the switch-on and switch-off points. It defines how far the temperature must deviate before a state change occurs.

Purpose: The dead band prevents the system from switching continuously at the setpoint temperature. Without hysteresis, a perfectly accurate controller would cycle the compressor thousands of times per hour.

Benefits:

• Reduces compressor short-cycling
• Extends compressor service life
• Reduces electrical inrush stress on contactors and windings
• Lowers mechanical wear on reversing valves in heat pump configurations

Practical application: A system set to 21 °C with a ±0.5 °C dead band activates at 20.5 °C and deactivates at 21.5 °C. This creates a 1 K hysteresis band.

Thermostat Integration

Definition: A thermostat is the sensing and switching device that provides the on/off signal to the heat pump controller.

Purpose: The thermostat converts ambient temperature into a control signal. It acts as the primary feedback element in on/off control loops.

Benefits:

• Widely available and standardised
• Low installation cost
• Compatible with building management systems (BMS) via dry contacts
• Simple commissioning and replacement

Practical application: A bimetal room thermostat closes a circuit at 19 °C in heating mode. This energises the heat pump controller input, starting the compressor.

Thermostat types used in on/off heat pump control:

• Bimetal mechanical thermostat (legacy installations)
• Electronic NTC-based thermostat (residential and light commercial)
• Programmable room thermostat (scheduled control)
• Smart thermostat with connectivity (demand-response integration)

Minimum Run Time and Minimum Off Time

Definition: Minimum run time is the shortest duration the compressor must stay on after activation. Minimum off time is the mandatory rest period after deactivation before the next start.

Purpose: These timers protect the compressor from thermal and electrical stress caused by frequent short cycles.

Benefits:

• Prevents compressor overheating from insufficient oil circulation
• Protects against high-pressure surges during rapid restarts
• Reduces electrical demand spikes from repeated motor starts
• Extends refrigerant circuit component life

Practical application: A controller sets minimum run time to 3 minutes and minimum off time to 5 minutes. Even if the thermostat calls for heat immediately after shutdown, the compressor waits 5 minutes before restarting.

Cycle Rate (Starts Per Hour)

Definition: Cycle rate is the number of compressor start-stop cycles per hour. It is a direct measure of on/off control activity intensity.

Purpose: Monitoring cycle rate identifies whether the system is correctly sized and configured. Excessive cycling indicates oversizing or an incorrectly set dead band.

Benefits of controlled cycle rate:

• Predictable compressor wear profiles
• Accurate maintenance scheduling
• System performance benchmarking against design parameters

Practical application: A controller logs compressor starts. If cycle rate consistently exceeds 6 starts per hour, the commissioning engineer widens the dead band or investigates heat pump oversizing.

Anti-Short-Cycle Protection

Definition: Anti-short-cycle protection is an electronic or firmware-based safeguard that enforces minimum off time regardless of thermostat demand.

Purpose: This feature prevents rapid restart after shutdown. It overrides thermostat signals until the minimum off time has elapsed.

Benefits:

• Prevents liquid slugging in the compressor
• Allows refrigerant pressure equalisation between high and low sides
• Protects motor windings from repeated thermal stress
• Reduces trip events on motor protection relays

Practical application: A power interruption during operation causes an unexpected shutdown. Anti-short-cycle protection holds the restart signal for 5 minutes, allowing pressures to equalise before the next start.

Defrost Control Integration

Definition: In air-source heat pumps operating in heating mode, ice accumulates on the outdoor coil. Defrost control temporarily reverses the refrigerant cycle to melt ice accumulation.

Purpose: On/off control must coordinate with defrost events. The system suspends normal thermostat-driven operation during defrost and resumes on/off control after defrost completion.

Benefits:

• Maintains evaporator efficiency during low ambient operation
• Prevents coil blockage from ice buildup
• Integrates defrost as a managed event rather than an unplanned interruption

Practical application: The heat pump operates normally in on/off mode. At a preset interval or when coil sensor conditions indicate ice formation, the controller initiates a timed defrost cycle. On/off control resumes automatically after defrost termination.

Types and Models of On/Off Control

Type 1: Single-Stage On/Off Control

Description: One compressor, one stage of heating or cooling. The compressor operates at full rated capacity when active.

Application: Residential heat pumps up to 12 kW. Domestic hot water heat pumps. Single-zone light commercial systems.

Control signal: Single binary input/output (1 digital channel).

Limitation: Full capacity output regardless of actual load. This causes temperature overshoot when heat pump capacity exceeds building heat loss.

Type 2: Two-Stage On/Off Control

Description: Two compressors or one compressor with two capacity steps. Each stage activates independently through separate on/off signals.

Application: Larger residential systems. Dual-compressor commercial heat pumps. Systems requiring partial-load capability without variable-speed hardware.

Control signal: Two binary channels, sequenced by the controller.

Stage sequencing logic:

1. Stage 1 activates when demand is detected
2. Stage 2 activates if Stage 1 run time exceeds a preset limit without satisfying setpoint
3. Stage 2 deactivates first on setpoint satisfaction
4. Stage 1 deactivates when temperature stabilises

Benefit over single-stage: Closer capacity matching to part-load conditions. Reduced average cycle rate per compressor. Improved seasonal efficiency.

Type 3: Lead-Lag On/Off Control (Multi-Unit Systems)

Description: Multiple heat pump units operate in rotation. The lead unit runs first. The lag unit activates only when the lead unit cannot satisfy demand alone.

Application: Large commercial buildings with multiple heat pump units. Centralised plant rooms. District heating nodes.

Purpose: Lead-lag control distributes runtime hours equally across units. This extends service intervals and reduces the probability of simultaneous failure.

Lead-lag rotation methods:

• Fixed lead/lag assignment
• Time-based rotation (e.g., weekly changeover)
• Runtime-based rotation (changeover when lead unit reaches a threshold run-hour count)
• Load-optimised rotation (controller selects most efficient unit as lead)

Type 4: Buffer Tank-Assisted On/Off Control

Description: A hydraulic buffer tank is installed in the heat distribution circuit. The heat pump charges the buffer tank in on/off cycles. The heating distribution system draws heat from the buffer continuously.

Purpose: The buffer tank decouples heat pump cycling from distribution system demand. This allows the heat pump to run in longer, more efficient cycles.

Benefits:

• Reduces compressor starts per hour
• Improves seasonal COP by allowing full-load operation during each cycle
• Compatible with low-temperature underfloor heating systems
• Enables demand-response control without comfort penalties

Practical application: A 200-litre buffer tank stores heat from a 9 kW heat pump. The heat pump charges the tank from 40 °C to 50 °C during each cycle. The underfloor heating system draws heat continuously at a lower flow rate than the heat pump’s output rate. Compressor cycle rate drops from 8 starts/hour to 3 starts/hour.

Type 5: Room Thermostat On/Off Control

Description: A standalone room thermostat provides a direct on/off signal to the heat pump. No external controller or BMS is involved.

Application: Simple residential installations. Retrofit applications replacing gas boiler controls.

Wiring: Typically a dry contact output from the thermostat to the heat pump’s terminal block (commonly labelled T1/T2, C/W, or equivalent).

Limitation: No outdoor temperature compensation. No weather-responsive adaptation. Fixed flow temperature regardless of ambient conditions.

Type 6: Weather-Compensated On/Off Control

Description: On/off switching is combined with outdoor temperature compensation (OTC). The heat pump activates and deactivates based on room temperature. The heating water flow temperature adjusts according to outdoor temperature.

Purpose: OTC improves efficiency during mild weather by reducing flow temperature when less heating is required. On/off switching controls start and stop events.

Benefit: Retains the simplicity of on/off activation while achieving partial efficiency gains from temperature modulation.

Standard reference: EN 14825 testing accounts for seasonal operation including part-load conditions. Weather-compensated on/off systems achieve higher SCOP ratings than fixed flow temperature on/off systems.

Use Cases

Residential Space Heating

A single-family home requires 8 kW peak heating capacity. A single-stage on/off heat pump operates in response to a room thermostat set to 21 °C. The building thermal mass moderates temperature swings. The heat pump cycles 4–5 times per hour during the heating season shoulder period.

Control configuration:

• Thermostat dead band: ±0.75 K
• Minimum off time: 5 minutes
• Defrost: time/temperature initiated, automatic resume

Domestic Hot Water (DHW) Production

A heat pump water heater maintains stored water between 45 °C and 55 °C. The on/off controller activates the heat pump when tank temperature drops to 45 °C and deactivates it at 55 °C. The 10 K dead band limits cycle rate effectively.

Control configuration:

• Sensor: immersion NTC at mid-tank height
• Dead band: 10 K (45 °C on, 55 °C off)
• Legionella protection: periodic thermal disinfection cycle (≥ 60 °C) overrides normal on/off logic

Regulatory note: Legionella thermal disinfection at ≥ 60 °C is required by L8 COSHH ACoP (UK) and VDI 6023 (Germany) for stored domestic hot water systems. The on/off controller must support this override function.

Commercial Retail Cooling

A retail unit requires comfort cooling during trading hours. A packaged air-source heat pump (cooling mode) provides 14 kW cooling capacity. The building management system (BMS) provides an on/off demand signal based on zone temperature. After-hours, the BMS withdraws the demand signal and the heat pump shuts down.

Control configuration:

• BMS output: volt-free contact to heat pump
• Scheduling: BMS occupancy programme
• Anti-short-cycle: 3-minute minimum off time enforced by heat pump controller

Industrial Process Temperature Maintenance

A manufacturing process requires coolant fluid maintained at 18 °C ± 1 K. A water-cooled heat pump activates when coolant temperature rises to 19 °C and deactivates at 17 °C. The 2 K dead band balances control precision against cycle rate.

Control configuration:

• Sensor: PT100 immersion probe in coolant circuit return line
• Dead band: 2 K
• Alarm: Cycle rate exceeding 8 starts/hour triggers maintenance alert

Heat Pump Pool Heating

A commercial swimming pool uses an air-source heat pump to maintain water temperature at 28 °C. The heat pump operates on an on/off thermostat with a 2 K dead band. Pool thermal mass is high, so cycle rate is inherently low (typically 1–2 starts per hour).

Control consideration: A flow switch interlock is required. The heat pump must not activate unless the pool pump is confirmed running. The on/off controller incorporates a flow-proving input before permitting compressor start.

Benefits of On/Off Control

System-Level Benefits

1. Low capital cost Binary switching requires minimal control hardware. A thermostat and a relay constitute the entire control system in basic configurations. This reduces installation cost significantly compared to modulating or inverter-drive systems.

2. High reliability Fewer electronic components mean fewer failure points. On/off control systems have demonstrated decades of reliable service in residential and commercial applications. The technology is mature and well-understood by installation and maintenance technicians.

3. Simple commissioning On/off systems require minimal parameter configuration. Dead band, minimum timers, and setpoint settings are typically the only commissioning steps. This reduces installation time and the risk of configuration errors.

4. Wide technician familiarity On/off control logic is universally understood across the HVAC&R trade. Diagnosis, fault-finding, and repair require no specialist training in inverter drives or modulating control algorithms.

5. BMS and legacy system compatibility On/off control signals are universally compatible with building management systems. Volt-free contacts, 24 V AC signals, and 0–10 V demand signals all translate into on/off switching without protocol conversion.

Efficiency and Performance Benefits

1. Full-load efficiency at every cycle When a heat pump runs in on/off mode, it always operates at its rated full-load condition during each cycle. The compressor is never throttled to a low-efficiency part-load operating point. This is advantageous in climates where the heating design temperature occurs frequently.

2. Predictable energy consumption On/off systems consume energy in discrete, measurable blocks. Energy metering is straightforward. Maintenance and performance monitoring based on run-hour counters is accurate and reliable.

3. Effective buffer tank synergy Combining on/off control with a buffer tank achieves near-modulating efficiency without inverter hardware. Long, efficient heat pump cycles charge the buffer tank. Short-cycling is eliminated. This approach is cost-effective in retrofit and new-build residential applications.

Selection Criteria for On/Off Control

When to Select On/Off Control

Select on/off control when the following conditions apply:

1. System capacity closely matches peak load. An oversized heat pump in on/off mode causes excessive cycling. Match installed capacity to calculated peak heat loss (per EN 12831 in Europe or ACCA Manual J in North America).
2. Thermal mass is adequate. Buildings with significant thermal mass (concrete construction, underfloor heating) buffer temperature swings. On/off cycling causes less comfort disruption in high-mass buildings.
3. A buffer tank can be included. Buffer tanks decouple heat pump cycling from distribution system demand. On/off control with a buffer tank achieves low cycle rates even in low-mass buildings.
4. Budget constrains control system cost. On/off controls cost significantly less than inverter-drive modulating systems. Where lifecycle efficiency gains from modulating control do not justify the capital premium, on/off control is the appropriate choice.
5. System replacement or retrofit of existing on/off infrastructure. Replacing a gas boiler with a heat pump in an existing on/off wired installation minimises rewiring and commissioning effort.

When Not to Select On/Off Control

Avoid on/off control when:

• Building heat load is highly variable. Highly glazed, low-mass commercial buildings experience rapid load changes. On/off control cannot match output to load quickly enough, causing comfort complaints or excessive cycling.
• Cycling rate will exceed 6 starts per hour. This indicates oversizing or inadequate thermal buffering. Modulating or variable-speed control is more appropriate.
• Low-temperature heating systems require precise flow temperature. Underfloor heating systems below 35 °C benefit from continuous modulating output, not cycling.
• Noise or vibration is a critical concern. Compressor start transients generate noise and vibration. In acoustically sensitive applications, modulating systems with soft-start or variable-speed compressors are preferred.
• Energy regulation mandates higher seasonal efficiency. Some national building codes require minimum SCOP levels only achievable with modulating or inverter-driven systems. Verify compliance before specifying on/off control.

Sizing Guidance

Correct sizing is critical for on/off control performance. Use the following hierarchy:

1. Calculate peak heat loss using a certified method (EN 12831, ACCA Manual J, or national equivalent)
2. Select heat pump rated capacity within 10–20% of calculated peak load
3. If oversizing by more than 20% is unavoidable, include a buffer tank
4. Verify minimum cycle rate at design conditions does not exceed manufacturer limits

Rule of thumb: For every 1 kW of oversizing beyond peak load, add approximately 15–20 litres of buffer tank volume.

Efficiency Gap: On/Off vs. Inverter in Practice

An inverter-driven heat pump typically achieves SCOP 4.0–5.5 in residential heating applications. A correctly specified and installed on/off system with buffer tank achieves SCOP 3.2–4.2 for the same application. The gap narrows in cold climates where full-load operation is frequent and widens in mild climates where part-load operation dominates the annual run profile.

Financial implication: In a typical Western European residential installation (8,000 kWh annual heat demand), the difference between SCOP 3.5 and SCOP 4.5 represents approximately 500–600 kWh per year of additional electricity consumption with on/off control. At an electricity price of €0.30/kWh, this equals approximately €150–180 per year. Against a capital premium of €1,500–3,000 for inverter control, the simple payback period is 10–20 years. This analysis guides the selection decision.

Integration with Other Systems

Integration with Building Management Systems (BMS)

On/off heat pump controllers communicate with building management systems through standardised interfaces:

• Volt-free contact (dry contact): Universal compatibility. BMS relay output connects to heat pump demand input.
• 24 V AC/DC signal: Standard in commercial HVAC. Compatible with most BMS digital output modules.
• Modbus RTU/TCP: Some on/off controllers include Modbus registers for enable/disable, alarm status, and run-hour data.
• BACnet IP: Available in controllers designed for commercial BMS integration. Enables full operational visibility.

Integration function: The BMS sends a demand enable signal to the heat pump. The heat pump’s internal on/off logic (thermostat, dead band, timers) operates independently within the BMS permission window.

Integration with Smart Thermostats and Demand Response

Smart thermostats communicate with on/off heat pump systems through standard thermostat wiring (typically 24 V C/W terminals) or wireless protocols. This enables:

• Time scheduling: Pre-programmed heating and cooling profiles activate and deactivate the heat pump according to occupancy patterns.
• Geofencing: The thermostat triggers heat pump activation based on occupant proximity to the building.
• Demand response: Energy suppliers communicate grid stress signals to the smart thermostat. The thermostat delays heat pump activation during peak demand periods. Building thermal mass provides comfort buffering during the demand response event.
• Data logging: Smart thermostats log cycle counts, run hours, and temperature profiles. This data supports maintenance planning and performance optimisation.

Relevant programme: OpenADR 2.0 defines the communication standard for automated demand response. Smart thermostats and on/off heat pump controllers participating in demand response programmes use OpenADR signals to manage activation timing.

Integration with Solar Photovoltaic (PV) Systems

On/off heat pump control integrates with solar PV output monitoring to enable self-consumption optimisation.

Operating principle: A PV monitoring device signals the heat pump controller when surplus solar generation is available. The controller activates the heat pump to consume excess solar energy before it is exported to the grid. The heat pump charges the buffer tank or raises DHW temperature during solar surplus periods.

Control logic:

1. PV monitoring detects generation exceeding household base load
2. Controller receives surplus signal (digital contact or energy meter pulse)
3. Heat pump activates in on/off mode, consuming surplus solar energy
4. Heat is stored in buffer tank or DHW cylinder
5. On-grid heating demand during non-solar periods is reduced

Benefit: This reduces annual electricity purchase cost and improves PV self-consumption fraction without battery storage. This aligns with the European Renewable Energy Directive (RED II) self-consumption provisions.

Integration with Hydronic Distribution Systems

On/off heat pump control must coordinate with hydronic circuit components.

Circulation pump control: The circulation pump typically activates a fixed delay before the heat pump and deactivates a fixed delay after shutdown. This ensures flow is established before heat production begins and dissipates residual heat after shutdown.

Zone valve coordination: Multi-zone hydronic systems use motorised zone valves. The on/off controller activates the heat pump only when at least one zone valve is confirmed open. This prevents heat pump operation against a closed circuit.

Buffer tank hydraulic separation: In on/off systems with buffer tanks, a hydraulic separator or low-loss header decouples the heat pump circuit from the distribution circuit. This prevents variable distribution flow from interfering with heat pump operation.

Flow temperature limiting: The on/off controller incorporates a high-limit thermostat on the heat pump flow line. If flow temperature exceeds the set limit (typically 55–65 °C for standard heat pump systems), the controller deactivates the compressor. This prevents condenser overpressure conditions.

Integration with DHW Priority Control

In combined space heating and DHW systems, on/off control must manage priority between functions.

DHW priority: When DHW demand is detected (tank temperature below setpoint), the controller diverts heat pump output to DHW production. Space heating is interrupted. DHW is satisfied first. Space heating resumes after DHW setpoint is achieved.

Parallel operation: Some systems operate space heating and DHW simultaneously using separate heat exchangers. The on/off controller manages two independent demand signals.

DHW priority timing: To prevent extended space heating interruption, DHW priority is typically time-limited. If DHW production exceeds a set duration (e.g., 60 minutes) without satisfying setpoint, the controller generates a fault alarm.