1. Buffer Tank Sizing – Ensuring Stable System Operation

A buffer tank plays a critical role in residential heat pump systems by increasing system water volume and stabilizing operation, especially in inverter-driven systems.

Why a buffer tank is needed

• Reduces frequent on/off cycling

• Improves temperature stability

• Supports defrost operation

• Protects the compressor under partial-load conditions

Typical sizing guideline

A commonly used rule of thumb for residential systems is:

Buffer tank volume (L) = 10–20 L per kW of heat pump capacity；

• Floor heating systems: lower end of the range

• Radiator systems: higher end of the range

• Systems with multiple zones: often require larger volumes

Proper buffer tank sizing ensures smooth operation without unnecessary energy loss or oversized system components.

2. Expansion Vessel – Installation Position and Volume Selection

The expansion vessel compensates for water volume changes caused by temperature fluctuations within the heating system.

Installation position

• Typically installed on the return water side

• Positioned close to the heat pump or hydraulic module

• Installed upstream of the circulation pump for stable pressure control

Volume selection

The expansion vessel size depends on:

• Total system water volume

• Operating temperature range

• System static pressure

For residential systems, expansion vessel volume is often selected as:

Expansion vessel volume ≈ 8–12% of total system water volume

Correct sizing and positioning help prevent pressure fluctuations, safety valve discharge, and long-term system stress.

3. Circulation Pump Selection – Type, Flow Rate, and Head

Residential heat pump systems often use multiple circulation pumps, each serving a different function within the system.

Common pump types

• Heat pump primary circulation pump

• Heating distribution pump (floor heating / radiators)

• Domestic hot water circulation pump

Key selection parameters

• Flow rate: matched to heat pump capacity and temperature difference

• Head (lifting pressure): determined by pipe length, fittings, and system resistance

• Pump type: high-efficiency ECM pumps are recommended for variable-load systems

A properly selected circulation pump ensures:

• Stable heat transfer

• Low electrical consumption

• Quiet operation

• Balanced flow distribution across heating zones

4. Auxiliary Electric Heater – Capacity Selection and Role

The auxiliary electric heater is designed as a supporting component, not the primary heat source.

Main functions

• Backup heating during extreme low outdoor temperatures

• Support during defrost cycles

• Supplementary heating during system startup

Typical sizing principle

For residential heat pump systems:

Auxiliary heater capacity = 20–40% of heat pump heating capacity

Oversized electric heaters increase operating cost, while undersized heaters may fail to provide sufficient backup under extreme conditions. Correct sizing ensures reliability without compromising system efficiency.

From Component Selection to System Performance

Each installation component plays a specific role, but true system performance is achieved only when all components are correctly matched.

Proper selection of buffer tanks, expansion vessels, circulation pumps, and auxiliary heaters leads to:

• Higher seasonal efficiency (SCOP)

• Reduced wear on key components

• Improved comfort stability

• Lower maintenance and service costs

In residential heat pump systems, the heat pump unit is only part of the solution. Thoughtful selection of installation components transforms individual devices into a well-balanced heating system, ensuring efficient, stable, and reliable operation throughout the entire lifecycle.