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.

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. 🤓

Bob

from looking at your graphs of one day: you’ve got loads of scope to get more energy into  your house, if you want/need to,  by changing your control strategy. 

he issue is that the heat pump cannot get to the set flow temperature during the coldest periods

Hello, I confused about why this is a problem for you.  Is your house not getting warm enough?

Thanks Bob

I'm saying this from my experience, not as a heating expert:

Your compressor is turned off for too long periods. You may think the UFH is at temp, but low down its cooling. and when its OAT is cold, needs constant flow going through it. I would turn down the flow temp and run it constantly.  DT 10 the unit straining like mad to heat the screed. I too though my unit was undersized, but after I turned to flow down and ran it constant, I could keep the house topped up with heat for 8kw an hour produced at freezing (heat loss 11) and the pump had lots more in the tank. 

My graphs looked just lie yours when was aiming for a temp that the screed could not get to. 

In mild weather, I tend to batch heat a bit more until I get a weather comp that I'm happy with

at DT10, your little unit is putting in over 10-11kw of energy, then is idle or just ticking over for large parts in the day. You need to even this out more

Can I just say that this is what I think too, but I was waiting for @gary to answer the questions before being certain about the conclusion. 

Heat pumps need to be on 24*7 during the height of the heating season, assuming that they have been correctly sized.  The FT needs to be turned down as low as possible (to be done with thermostats, TRVs etc set well above desired temperature).  In the shoulder season, or with a well oversized heat pump, batch heating may be possible and may work out cheaper, although there is no guarantee. 

Operated this way its reasonable to expect the running cost to be similar to gas or oil, or a little cheaper.  If you qualify for OvOs 15p tariff then definitely much cheaper, particularly if you have UFH and so can operate at a max FT of 35C or below. 

As a concrete example I am operating with radiators and a max temp of 42C, the heat pump is working out ~12% cheaper than gas.  Im buying electricity for it at an average of 19p by using EON Next Drive which gives 7hrs cheap rate at night in return for a 10% daytime price hike.  I operate 24*7 without setback or set forward, so I'm not exploiting the overnight cheap rate to any great extent.