The debate over buffer tanks in heat pump systems has been a hot topic for years, but there’s another component that’s equally important yet often overlooked: the volumiser. While buffer tanks are designed to store and release heat energy, volumisers serve a different purpose: they store water volume to stabilise flow rates and prevent short-cycling. However, just like buffer tanks, it appears the placement of volumisers is a point of contention among installers and heat engineers.

Recently, I posed a question to my network of heat pump installers, engineers and experts: “Where do you place the volumiser: on the flow side return side?” The results were striking: 81% said the return side, 8% said the flow side, and 11% offered other suggestions, often with comments like “it’s needed at all if the system is correctly designed”.

While the majority opinion is clear, it raises an important question: Is this placement based on sound technical reasoning, or is it simply a case of following tradition?

What Is a Volumiser & What Does It Do?

A volumiser is a hydraulic component that stores water volume to stabilise flow rates in a heat pump system. Its primary role is to prevent short-cycling (frequent on/off cycling of the heat pump) and to ensure smooth operation during transient events, such as defrost cycles.

Unlike a buffer tank, which stores heat energy, a volumiser doesn’t store temperature. It stores volume. This distinction is crucial because it determines how the volumiser interacts with the system and where it should be placed for maximum effectiveness.

The Defrost Cycle: A Critical Consideration

To understand why volumiser placement matters, we need to look at what happens during a defrost cycle. When a heat pump defrosts, it temporarily reverses its operation to melt ice on the outdoor coil. This process introduces cold water into the heating circuit, which can disrupt comfort and system performance if not managed properly.

If the Volumiser is on the Return Side:

• Cold water from defrost flows directly into the return pipe, which is already at a lower temperature during heating mode.
• This cold water mixes with the cooler return water, further reducing the temperature entering the radiators.
• Result: Radiators can receive a sudden influx of colder water, causing potential cooling and discomfort.

If the Volumiser is on the Flow Side:

• Cold water from defrost enters the flow side, where it mixes with warmer water stored in the volumiser (heated by the heat pump during normal operation).
• The volumiser’s stored volume moderates the temperature of the cold influx before it reaches the radiators.
• Result: Radiators receive tempered water, minimising temperature shocks and maintaining comfort.

Thermodynamics and Energy Efficiency

The flow-side placement of the volumiser aligns with thermodynamic principles, particularly the First Law of Thermodynamics (conservation of energy). By moderating cold influxes during defrost, the flow-side volumiser reduces the energy required to reheat the system, improving overall efficiency.

On the return side, the volumiser’s stored energy does little to moderate the cold influx, as the return water is already cooler. This means the radiators receive colder water almost immediately, increasing the energy required to reheat the system and reducing efficiency.

Why the Majority Might Be Wrong

Given the technical advantages of flow-side placement, why do 81% of installers prefer the return side? The answer likely lies in tradition and training. Many installers are taught to place volumisers on the return side because it’s seen as the “standard” approach. But is that right?

What Homeowners Need to Know

As a homeowner, understanding the role and placement of volumisers can help you have more informed conversations with your installer. Here are a few key points to keep in mind:

1. Ask About Volumiser Placement: If your installer plans to include a volumiser, ask where they intend to place it. If they say the return side, ask why and discuss the benefits of flow-side placement.
2. Focus on Defrost Performance: A well-placed volumiser can improve comfort during defrost cycles by preventing cold shocks to the radiators.
3. Consider System Efficiency: Flow-side placement can reduce energy consumption and improve overall system efficiency, saving you money in the long run.