Buffer Tank in Heat Pump Systems - Do You Really Need One?

We’ll explore different hydraulic setups in heat pump systems and the ongoing debate about whether a buffer tank is necessary. Understanding these configurations is crucial for optimizing a heat pump's efficiency and performance. We will break down three common setups and their implications for system design.

1. Open Loop (Direct)

Direct (open-loop) heat pump system supplying the heat emitters directly.

In a direct system setup, the heat pump’s internal circulation pump sends water directly through the heat emitters, such as underfloor heating or radiators. This is the simplest system configuration.

Advantages

• Most efficient setup, delivering the highest COP (Coefficient of Performance)

• No additional components, keeping the system simple and reducing installation costs
  

When to Use

• Best suited to open-loop systems where heating zones stay open

• Ideal for underfloor heating and radiators or systems with low pressure drop
  

Key Considerations

• Closing zones (e.g. via thermostatic radiator valves) can cause flow rate issues

• System pressure drop must remain within the capacity of the heat pump’s internal pump

In practice, the decision to add a buffer tank often depends on the heat pump’s operating range and system curve. This is explored in more detail in the video Buffer Tank or Not? Heating Pump Curve Explained.

2. Buffer as a Volumiser (Two-Pipe Buffer on Return Circuit) 

Buffer tank used as a system volumiser.

In this configuration, a buffer tank is placed strategically (often in the return circuit) to increase the system's total water volume. 

Advantages

• Ensures a minimum water volume

• Flow rate is available for the heat pump (preventing short-cycling)

• Helps with defrost cycles
  

When to Use

• Systems with many thermostatic valves or motorized circuits

• Installations with low system water volume

• Cooling applications, as heat pumps require higher water volume at lower temperatures. 

You can explore this topic further in the video Can a Heat Pump Run on Microbore Pipes Without a Buffer?

3. Buffer as a Hydraulic Separation (Four-Pipe Buffer)

Four-pipe buffer tank used as a hydraulic separator between the heat pump and heating circuits.

This setup introduces a buffer tank that acts as a hydraulic switch, creating a primary and secondary circuit. Usually, this is not efficient way to manage system and in most cases is not needed:

System Layout

• Primary circuit: heat pump with its internal circulation pump

• Secondary circuit: one or more secondary pumps supplying the heating zones
  

Advantages

• Helps manage multiple temperature zones simultaneously (e.g., fan coils and underfloor heating).

• Provides stable operation when zones open and close independently
  

When to Use

• Systems with multiple zones requiring different flow rates and temperatures

• Installations where the heat pump’s internal circulator pump does not have enough available pressure drop for the entire system

Choosing the Right Setup

The best approach depends entirely on the specific system design and control requirements:

System Type	Recommended Setup	Primary Goal
Simple, Open-Loop Systems	Direct System Setup	Maximum Efficiency (highest COP)
Multi-Zone Systems with Different Temperatures/Flows	Buffer as a Hydraulic Switch	System Decoupling and Flow Stability
Systems with Frequent Zone Closures	Buffer as a Volumizer	Minimum Volume Assurance and Defrost Support

 Want to dive deeper into buffer tanks and hydraulic configurations?

Watch the video