Integral vs Separate Heat Pump Water Heaters

Heat pump water heaters are often described as integral, separate, all-in-one, split, or stand-alone. These terms matter because the physical arrangement affects testing, documentation, AS/NZS 4234 modelling, and scheme applications.

Manufacturers typically develop a range of HPWH units across different styles to serve different use cases and market segments. This article explains the terminology and technical differences. It is not a recommendation for one design over another; both architectures are valid production choices with different trade-offs.

The key differences between integral and separate systems affect:

Installation complexity: Integral systems are simpler to install; separate systems require additional plumbing and pump integration.
System efficiency: Each architecture has different COP (coefficient of performance) characteristics and part-load performance.
Thermal stratification: Tank temperature distribution differs significantly between the two approaches.
Modelling methodology: AS/NZS 4234 treats integral and separate systems differently in annual simulations.
Performance testing: The DTHX (thermal difference) parameter in integral systems requires specific calibration.
Cost: Integral systems are often more affordable upfront; separate systems offer greater installation flexibility.

Integral HPWHs

An integral HPWH has the condenser heat exchanger integrated with the storage tank. This design is also known as an “all-in-one” or “monoblock” system.

Common Integral Arrangements

Common arrangements include:

Wrap-around condenser coil. A copper or aluminum coil wrapped around the tank exterior.
Microchannel heat exchanger. A thin aluminium jacket that effectively transfers heat directly to the tank wall. This is the most common modern approach.
Immersed condenser coil. A coil inside the tank itself.
Refrigerant-cycling split integral. The compressor and evaporator are in a separate outdoor unit, with refrigerant circulating to a wrap-around condenser on the tank.

How Heat Transfer Works

In an integral system, the refrigerant transfers heat directly to the tank through the integrated condenser. There is usually no pumped water loop between a separate heat pump unit and the tank. The condenser materials (copper or aluminum) are chosen for their effective heat transfer properties, contributing to overall system efficiency.

Notably, integral HPWHs don’t always require the entire heat pump unit to be ‘integral’ to the tank. Refrigerant-cycling split integral systems feature a separated HP unit with refrigerant circulating between the unit and the tank via the wrap-around condenser, offering a balance between compactness and flexibility.

Advantages and Considerations

Integral HPWHs offer a compact and efficient solution with benefits including:

Simpler installation: No external pump or water piping required.
Improved efficiency: Direct heat transfer to the tank in some designs.
Space efficiency: All-in-one footprint in many models.
Reduced complexity: Fewer moving parts and connections compared to separate systems.

However, thermal stratification can be affected by the design of the heat exchanger and its position on the tank.

Separate HPWHs

A separate HPWH (also known as a “stand-alone” unit) has the heat pump unit separate from the storage tank. The refrigerant transfers heat to water through a heat exchanger inside the heat pump unit, typically a plate heat exchanger. A pump circulates water between the tank and the heat pump unit.

The Refrigerant Loop

The refrigerant loop follows this cycle:

Evaporator. A fan draws in ambient air over the air heat exchanger, causing the refrigerant to become a warm, low-pressure gas.
Compressor. The warm gas passes through the compressor, transforming the refrigerant into a hot, high-pressure gas.
Condenser (heat exchanger). The hot gas moves through the water heat exchanger, transferring heat to the water loop. The refrigerant transitions into a cool gas/liquid.
Expansion valve. The refrigerant passes through the expansion valve, becoming a cold, low-pressure liquid and repeating the cycle.

The Water Loop

The water loop complements the refrigerant loop, ensuring effective heating of the water:

Cold water extraction. Cold water is drawn from the lower part of the tank.
Circulation pump. The cold water is pumped through a circulation pump.
Heat exchanger. The cold water is heated in the water heat exchanger (inside the HP unit) by the hot refrigerant.
Hot water return. The now-hot water enters the upper part of the tank.
Pipework. Flow and return pipework connects the heat pump to the tank.

Pump Flow Rate Control

Pumps in separate HPWHs operate in one of two modes:

Fixed Flow Rate:

The pump operates at a single speed setting, delivering a constant flow rate
This ensures consistent water circulation regardless of ambient or tank conditions
Simpler control but less efficient at part-load conditions

Variable Flow Rate:

The pump varies flow rate using a variable speed drive (VSD) motor
Achieves a near-constant heat pump outlet temperature (typically 60-90°C)
Flow rate adjusts based on inlet water temperature and HP operating capacity
The VSD control algorithm optimizes outlet temperature efficiency
More efficient across varied operating conditions, especially at lower heating loads

Advantages and Considerations

Separate HPWHs offer several benefits:

Flexible installation. HP unit can be located away from the tank (up to 10–15 metres with longer piping).
Better thermal stratification. Natural convection in water circulation can create better temperature layering.
Modular design. Easier to replace or upgrade components independently.
Higher outlet temperatures. Some designs can achieve 80–90°C outlet temperatures for specific applications.
Scalability. Easier to adapt for different tank sizes and system demands.

However, they require:

Additional plumbing. Flow and return connections between tank and HP unit.
Circulation pump. Adds complexity and power consumption.
More space. Separate units require more installation footprint.
Potential heat loss. Pipework between tank and HP can lose some heat.

Comparison Summary

Aspect	Integral	Separate
Condenser location	On/in the tank	In the HP unit
Water loop	No external loop	Pump circulates water between tank and HP
Installation complexity	Simpler, fewer connections	More complex, requires plumbing and pump
Thermal stratification	Can be poorer (whole tank heated at once)	Better (natural convection in circulation)
Flexibility	Fixed tank-HP integration	HP unit can be located away from tank
Outlet temperature	Typically ≤ 70°C	Can achieve 80–90°C with VSD pump
Modelling complexity	Requires DTHX calibration parameter	Standard model for stand-alone systems
Upfront cost	Generally lower	Generally higher
Space requirement	Compact	Requires additional space for HP unit
Efficiency at part-load	Can be lower (fixed operation)	Better with variable-speed pump

Why This Matters for Scheme Applications

The integral vs. separate distinction affects several compliance and regulatory pathways:

AS/NZS 4234 modelling. Integral systems require specific parameter calibration (DTHX) that separate systems do not.
Performance testing. Testing protocols may differ slightly between the two approaches.
Documentation requirements. System schematics and technical details must clearly show the architecture.
Certificate calculations. Modelled annual energy savings may differ due to thermal stratification differences.
Installation manuals. Integral and separate systems require different commissioning and control procedures.

Understanding your system’s architecture is essential when preparing any HPWH for testing, modelling, or scheme registration.