Hysteresis Band (Dead Band)<\/h3>\n
Definition:<\/strong> 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.<\/p>\nPurpose:<\/strong> 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.<\/p>\nBenefits:<\/strong><\/p>\n\n- Reduces compressor short-cycling<\/li>\n
- Extends compressor service life<\/li>\n
- Reduces electrical inrush stress on contactors and windings<\/li>\n
- Lowers mechanical wear on reversing valves in heat pump configurations<\/li>\n<\/ul>\n
Practical application:<\/strong> A system set to 21 \u00b0C with a \u00b10.5 \u00b0C dead band activates at 20.5 \u00b0C and deactivates at 21.5 \u00b0C. This creates a 1 K hysteresis band.<\/p>\n<\/div>\n\n
Typical dead band values:<\/strong><\/p>\n\n\n\n| Application<\/th>\n | Typical Dead Band<\/th>\n<\/tr>\n<\/thead>\n |
\n\n| Residential comfort heating<\/td>\n | 0.5 \u2013 1.5 K<\/td>\n<\/tr>\n |
\n| Commercial HVAC<\/td>\n | 1.0 \u2013 2.0 K<\/td>\n<\/tr>\n |
\n| Process cooling<\/td>\n | 0.2 \u2013 0.5 K<\/td>\n<\/tr>\n |
\n| Domestic hot water (DHW)<\/td>\n | 3 \u2013 8 K<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\nThermostat Integration<\/h3>\nDefinition:<\/strong> A thermostat is the sensing and switching device that provides the on\/off signal to the heat pump controller.<\/p>\nPurpose:<\/strong> The thermostat converts ambient temperature into a control signal. It acts as the primary feedback element in on\/off control loops.<\/p>\nBenefits:<\/strong><\/p>\n\n- Widely available and standardised<\/li>\n
- Low installation cost<\/li>\n
- Compatible with building management systems (BMS) via dry contacts<\/li>\n
- Simple commissioning and replacement<\/li>\n<\/ul>\n
Practical application:<\/strong> A bimetal room thermostat closes a circuit at 19 \u00b0C in heating mode. This energises the heat pump controller input, starting the compressor.<\/p>\nThermostat types used in on\/off heat pump control:<\/strong><\/p>\n\n- Bimetal mechanical thermostat (legacy installations)<\/li>\n
- Electronic NTC-based thermostat (residential and light commercial)<\/li>\n
- Programmable room thermostat (scheduled control)<\/li>\n
- Smart thermostat with connectivity (demand-response integration)<\/li>\n<\/ul>\n<\/div>
Minimum Run Time and Minimum Off Time<\/h3>\nDefinition:<\/strong> 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.<\/p>\nPurpose:<\/strong> These timers protect the compressor from thermal and electrical stress caused by frequent short cycles.<\/p>\nBenefits:<\/strong><\/p>\n\n- Prevents compressor overheating from insufficient oil circulation<\/li>\n
- Protects against high-pressure surges during rapid restarts<\/li>\n
- Reduces electrical demand spikes from repeated motor starts<\/li>\n
- Extends refrigerant circuit component life<\/li>\n<\/ul>\n
Practical application:<\/strong> 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.<\/p>\n<\/div>\n\n Industry-standard timer settings:<\/strong><\/p>\n\n\n\n| Parameter<\/th>\n | Typical Setting<\/th>\n<\/tr>\n<\/thead>\n | \n\n| Minimum run time<\/td>\n | 2 \u2013 5 minutes<\/td>\n<\/tr>\n | \n| Minimum off time<\/td>\n | 3 \u2013 10 minutes<\/td>\n<\/tr>\n | \n| Restart delay after power loss<\/td>\n | 3 \u2013 5 minutes<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\nCycle Rate (Starts Per Hour)<\/h3>\nDefinition:<\/strong> Cycle rate is the number of compressor start-stop cycles per hour. It is a direct measure of on\/off control activity intensity.<\/p>\nPurpose:<\/strong> Monitoring cycle rate identifies whether the system is correctly sized and configured. Excessive cycling indicates oversizing or an incorrectly set dead band.<\/p>\nBenefits of controlled cycle rate:<\/strong><\/p>\n\n- Predictable compressor wear profiles<\/li>\n
- Accurate maintenance scheduling<\/li>\n
- System performance benchmarking against design parameters<\/li>\n<\/ul>\n
Practical application:<\/strong> 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.<\/p>\n<\/div>\n\n Acceptable cycle rate benchmarks:<\/strong><\/p>\n\n\n\n| System Type<\/th>\n | Maximum Recommended Starts\/Hour<\/th>\n<\/tr>\n<\/thead>\n | \n\n| Residential heat pump<\/td>\n | 4 \u2013 6<\/td>\n<\/tr>\n | \n| Commercial packaged unit<\/td>\n | 3 \u2013 5<\/td>\n<\/tr>\n | \n| Scroll compressor systems<\/td>\n | 4 \u2013 6<\/td>\n<\/tr>\n | \n| Reciprocating compressor systems<\/td>\n | 3 \u2013 4<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\nAnti-Short-Cycle Protection<\/h3>\nDefinition:<\/strong> Anti-short-cycle protection is an electronic or firmware-based safeguard that enforces minimum off time regardless of thermostat demand.<\/p>\nPurpose:<\/strong> This feature prevents rapid restart after shutdown. It overrides thermostat signals until the minimum off time has elapsed.<\/p>\nBenefits:<\/strong><\/p>\n\n- Prevents liquid slugging in the compressor<\/li>\n
- Allows refrigerant pressure equalisation between high and low sides<\/li>\n
- Protects motor windings from repeated thermal stress<\/li>\n
- Reduces trip events on motor protection relays<\/li>\n<\/ul>\n
Practical application:<\/strong> 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.<\/p>\n<\/div>Defrost Control Integration<\/h3>\nDefinition:<\/strong> 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.<\/p>\nPurpose:<\/strong> 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.<\/p>\nBenefits:<\/strong><\/p>\n\n- Maintains evaporator efficiency during low ambient operation<\/li>\n
- Prevents coil blockage from ice buildup<\/li>\n
- Integrates defrost as a managed event rather than an unplanned interruption<\/li>\n<\/ul>\n
Practical application:<\/strong> 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.<\/p>\n<\/div><\/span>Types and Models of On\/Off Control<\/span><\/h2>\nType 1: Single-Stage On\/Off Control<\/h3>\nDescription:<\/strong> One compressor, one stage of heating or cooling. The compressor operates at full rated capacity when active.<\/p>\nApplication:<\/strong> Residential heat pumps up to 12 kW. Domestic hot water heat pumps. Single-zone light commercial systems.<\/p>\nControl signal:<\/strong> Single binary input\/output (1 digital channel).<\/p>\nLimitation:<\/strong> Full capacity output regardless of actual load. This causes temperature overshoot when heat pump capacity exceeds building heat loss.<\/p>\nType 2: Two-Stage On\/Off Control<\/h3>\nDescription:<\/strong> Two compressors or one compressor with two capacity steps. Each stage activates independently through separate on\/off signals.<\/p>\nApplication:<\/strong> Larger residential systems. Dual-compressor commercial heat pumps. Systems requiring partial-load capability without variable-speed hardware.<\/p>\nControl signal:<\/strong> Two binary channels, sequenced by the controller.<\/p>\nStage sequencing logic:<\/strong><\/p>\n\n- Stage 1 activates when demand is detected<\/li>\n
- Stage 2 activates if Stage 1 run time exceeds a preset limit without satisfying setpoint<\/li>\n
- Stage 2 deactivates first on setpoint satisfaction<\/li>\n
- Stage 1 deactivates when temperature stabilises<\/li>\n<\/ol>\n
Benefit over single-stage:<\/strong> Closer capacity matching to part-load conditions. Reduced average cycle rate per compressor. Improved seasonal efficiency.<\/p>\nType 3: Lead-Lag On\/Off Control (Multi-Unit Systems)<\/h3>\nDescription:<\/strong> 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.<\/p>\nApplication:<\/strong> Large commercial buildings with multiple heat pump units. Centralised plant rooms. District heating nodes.<\/p>\nPurpose:<\/strong> Lead-lag control distributes runtime hours equally across units. This extends service intervals and reduces the probability of simultaneous failure.<\/p>\nLead-lag rotation methods:<\/strong><\/p>\n\n- Fixed lead\/lag assignment<\/li>\n
- Time-based rotation (e.g., weekly changeover)<\/li>\n
- Runtime-based rotation (changeover when lead unit reaches a threshold run-hour count)<\/li>\n
- Load-optimised rotation (controller selects most efficient unit as lead)<\/li>\n<\/ul>\n
Type 4: Buffer Tank-Assisted On\/Off Control<\/h3>\nDescription:<\/strong> 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.<\/p>\nPurpose:<\/strong> The buffer tank decouples heat pump cycling from distribution system demand. This allows the heat pump to run in longer, more efficient cycles.<\/p>\nBenefits:<\/strong><\/p>\n\n- Reduces compressor starts per hour<\/li>\n
- Improves seasonal COP by allowing full-load operation during each cycle<\/li>\n
- Compatible with low-temperature underfloor heating systems<\/li>\n
- Enables demand-response control without comfort penalties<\/li>\n<\/ul>\n
Practical application:<\/strong> A 200-litre buffer tank stores heat from a 9 kW heat pump. The heat pump charges the tank from 40 \u00b0C to 50 \u00b0C 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.<\/p>\nType 5: Room Thermostat On\/Off Control<\/h3>\nDescription:<\/strong> A standalone room thermostat provides a direct on\/off signal to the heat pump. No external controller or BMS is involved.<\/p>\nApplication:<\/strong> Simple residential installations. Retrofit applications replacing gas boiler controls.<\/p>\nWiring:<\/strong> Typically a dry contact output from the thermostat to the heat pump’s terminal block (commonly labelled T1\/T2, C\/W, or equivalent).<\/p>\nLimitation:<\/strong> No outdoor temperature compensation. No weather-responsive adaptation. Fixed flow temperature regardless of ambient conditions.<\/p>\nType 6: Weather-Compensated On\/Off Control<\/h3>\nDescription:<\/strong> 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.<\/p>\nPurpose:<\/strong> OTC improves efficiency during mild weather by reducing flow temperature when less heating is required. On\/off switching controls start and stop events.<\/p>\nBenefit:<\/strong> Retains the simplicity of on\/off activation while achieving partial efficiency gains from temperature modulation.<\/p>\n | | |