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Volumisers in Heat Pump Systems: Does Placement Matter?

Mars · 2025-03-10T08:19:29Z

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: During defrost, the heat pump introduces colder water into the flow pipe, which then circulates through the emitters. If a volumiser is installed on the return, it offers no thermal buffering… by the time the water reaches it, the cold slug has already passed through the radiators. This can cause a noticeable drop in room comfort. 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: 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. Focus on Defrost Performance: A well-placed volumiser can improve comfort during defrost cycles by preventing cold shocks to the radiators. Consider System Efficiency: Flow-side placement can reduce energy consumption and improve overall system efficiency, saving you money in the long run.

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Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

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SUNandAIR

(@sunandair)

Honorable Member Member

Joined: 3 years ago

Posts: 569

06/05/2025 6:51 pm

Posted by: @Anonymous

↑

Not sure if you have the defrost cycle and what happens correct. The heat pump reverses the refrigeration cycle not the heating water. The circulation pump runs as normal. Hot and cold refrigerator gases swop roles at the condenser only. So instead of being in heat mode it moves to cooling mode.

So cold water from defrost does not enter the return it enters the flow side. But a big volume of water on the flow side will allow some mixing and less very cold water will hit the radiator. Just slightly cooled water.

@johnmo are you commenting on @editor s opening post? If it is then I guess you’re commenting on this part of it;

”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.”

The first bullet point is incorrect as you say @johnmo.

interpretation: What actually happens in the context of this article is the defrost water leaves the heat pump in a chilled state and continues to flow along The Primary Flow Pipe straight into the emitter circuit (ie the secondary flow pipe). This means it passes through all the emitters absorbing heat from the emitters instead of adding heat. The defrost water then enters the volumiser on the return side. Mixing with the existing warmer return water. So this is described as not good… because it has passed through all the emitters in its chilled state. It only mixes with the volume of return water in the volumiser after it has robbed the emitters of some of their heat.

By placing the volumiser on the Flow side the chilled water first mixes with the higher flow temperature store in the volumiser. Before it enters the emitter circuit.

it might be that @editor has already edited this part of the article but I haven’t seen the final copy.

Of course, this is just one view of the intended document. 🤓

This post was modified 11 months ago by SUNandAIR

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4475

Topic starter 06/05/2025 10:42 pm

You’re absolutely right that during defrost the refrigeration cycle reverses, not the flow and return pipes. And yes, the cold water produced by defrost enters the flow side of the system… that’s exactly the point I was building toward.

The key issue here is how that cold water interacts with the heating circuit. If there’s no thermal buffer (in this case, a volumiser) on the flow, then that cold slug heads straight into the emitters. Because flow temps are typically what the radiators “see,” this can cause an immediate and noticeable drop in radiator surface temperatures, reducing comfort for the occupants, even if only briefly.

When a volumiser is installed on the return, it does nothing to moderate this cold influx. It simply adds volume at the cooler end of the system, where flow and return temperatures are already much closer together. In contrast, when the volumiser is installed on the flow, it stores water that is several degrees warmer than the return (typically at the current flow temperature) and this warmer mass helps temper the defrost water before it reaches the emitters.

This placement mitigates the thermal shock and helps maintain comfort during and after defrost cycles. Temperature is a valuable commodity in low-temperature systems like heat pumps, and conserving that energy (especially during transitional events) is key to performance and efficiency.

So yes, volumisers can technically go on the flow or return, but in practice, only the flow-side position delivers that buffering effect where it matters, before the emitters. That’s the core of my argument.

I’ve reread my post and my wording isn’t clear so I’ll update it now.

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