Understanding Compressors for Heat Pump Water Heaters

The compressor is one of the main components that determines heat pump water heater performance. For product design and scheme modelling, one of the important choices is whether the unit uses a fixed speed compressor or an inverter-driven variable speed compressor.

Both approaches can be suitable. The right choice depends on target cost, refrigerant, control strategy, test performance, noise requirements, and the markets where the product will be sold.

Fixed speed compressors

Fixed speed compressors, also known as single-speed compressors, operate at a constant speed. When the heat pump is running, the compressor generally delivers a consistent heating capacity rather than modulating to match the tank condition.

This simplicity can be an advantage. Fixed speed compressors are usually easier to design around, easier to control, and often cheaper upfront than variable speed compressors.

Fixed speed compressors are also a proven technology. They have been used across heating, cooling, and water heating systems for many years, which can make reliability and servicing more straightforward.

The main disadvantage is part-load performance. A fixed speed compressor continues operating at full speed near the end of the heating cycle, when water temperature is higher, COP is lower, and power consumption can be less favourable.

Variable speed compressors

Variable speed compressors, also known as inverter-driven compressors, can adjust operating frequency in response to demand and operating conditions. This gives the controller more flexibility as air temperature, water temperature, and tank conditions change.

When designed well, variable speed operation can improve COP by matching compressor speed to the useful part of the heating cycle. The unit may run at higher speed when water and air temperatures are favourable, then reduce speed as water temperature rises.

Variable speed compressors can also reduce noise during lower-load operation. This can matter for residential products installed near living areas or boundaries.

Frequency flexibility can also help manufacturers adapt one platform for different electrical markets. For example, a compressor platform may be configured differently for 230 V/50 Hz and 120 or 240 V/60 Hz markets.

The main trade-off is cost and complexity. Variable speed products usually have a higher upfront cost, and the control strategy needs to be properly documented for testing and scheme modelling.

What this means for registration

For incentive scheme registration, the compressor choice matters because it affects AS/NZS 5125.1 test results and AS/NZS 4234 modelling outcomes.

EnergyAE needs to understand whether the compressor runs at fixed speed, variable speed, or a fixed frequency setting selected for the tested model. The control description should match the tested configuration and the product supplied to the market.