Posted by: @johnmo

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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. 🤓

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.