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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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JamesPa

(@jamespa)

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17/03/2025 7:39 pm

Posted by: @abernyte

↑

This whole "hot water from the primary is used in the defrost" is confusing me. Unless it is not general across all makes the Ecodan reverses the flow of hot refrigerant gas to defrost the evaporator after stopping the fan and primary flow pump.

Are you sure. Where does the defrost energy come from on that case?

Most stop the fan but leave the primary pump going thus extracting the defrost energy from the house. There are pumps that can start an inline electric heater and I think someone posted about one that has a built in water tank (not sure exactly how it's used).

The energy must come from somewhere.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Abernyte

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17/03/2025 7:59 pm

Well according to Mitsubishi's technician in the video it is the hot refrigerant gas being reversed via a reversing valve and expansion chamber. No need to steal heat from the primary circuit or any volumiser. There may well be brands that do but that seems a very low tech solution compared to using the hot refrigerant gas in the system.

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JamesPa

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17/03/2025 8:51 pm

Sofaik all others do the same, ie they operate the refrigerant circuit in cooling mode to effect defrost. They thus cool the house and warm the heat exchanger using heat extracted from the house.

I think it's highly likely mitsubishi don't switch off the water pump because the water is the source of the energy.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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davidnolan22

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17/03/2025 9:04 pm

@jamespa

My mitshubishi does not turn off any pumps flows continue and takes some heat from the house. I’m fairly sure this is normal behaviour

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Abernyte

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18/03/2025 4:22 am

"I think it's highly likely mitsubishi don't switch off the water pump because the water is the source of the energy." So the official Mitsubishi video is wrong? He clearly says that they use the already compressed hot refrigerant to defrost along with a tiny bit of heat from the HE at the end of the process, not the water from the primary as the heat source. Surely that is where the energy comes from.

On my Ecodan 14kW the primary pump stops for the 2/3 minutes it takes to clear the ice from the evaporator. The entire process from start to finish is over so quickly, and infrequently, that if it wasn't for the vapour cloud and water running off you can easily miss it.

Now I am really confused!

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davidnolan22

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18/03/2025 7:19 am

@abernyte

Post a picture of your flow and return, you will see they swap over for a short period during the defrost.

the video does say in takes a tiny bit of heat from the water.

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davidnolan22

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18/03/2025 7:58 am

the picture shows it stole a tiny bit of heat from the house to sort itself out.

👍

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Abernyte

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18/03/2025 9:46 am

Is that not just indicating that the primary pump has stopped momentarily while the unit defrosts using the hot refrigerant gas?

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JamesPa

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18/03/2025 9:52 am

Posted by: @abernyte

↑

Is that not just indicating that the primary pump has stopped momentarily while the unit defrosts using the hot refrigerant gas?

No, that wouldn't account for the flow temp being lower than the return temp. This shows that the water pump continues to operate and as a result the water output from the heat pump is chilled by the fact that it has given up energy to the refrigerant and ultimately to the fins. Its quite clear from this plot that the Mitsubishi does defrost in exactly the same way as almost all other heat pumps.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Gary

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18/03/2025 10:01 am

My Mitsubishi does exactly the same as David's above, there wouldn't be enough volume of water/heat in the primary circuit alone to defrost the heat pump my run is very short.

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davidnolan22

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18/03/2025 10:04 am

@abernyte no, it doesn’t turn the pumps off on mine.

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Abernyte

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18/03/2025 11:46 am

Thank you gentlemen/people (can't be too careful now) perfectly reasoned arguments that fit the observed evidence. The only fly in this ointment is why the manufacturer does not mention this in it's otherwise clear and technical explanation of defrost. Taking a "tiny bit of heat from the HE at the end of the process" is the only mention. One would think if they were setting out to make such a video then the simplest route would be to say "we are briefly going to steal some heat from the house or DHW to defrost the HP" But they don't.

Sigh....split jury, should never have studied law!

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