Not sure this is precisely what happens or how the control loop works.  What I observe on plots I have seen (including mine) is that the flow temperature rises above the set point (because the heat pump is putting in more heat than the radiators are emitting). 

Here might be a case in point James.

 Here are 2 hourly charts showing flow and return. These are supposed to be operating at fixed flow 34C and 35C respectively. They both have a little burst of temperature before closing down for around 4 minutes. I’ve always assumed this was a deliberate duty cycle function to bridge the shutdown temp drop… but it’s just a guess. There is no thermostat controlling, it’s just fixed flow. The room temp is close to 21 with an ambient of around4C last night and 9C for this afternoons chart

This 24 hour indoor/outdoor temp chart covers both hourly graphs above.

When this happens, the heat pump can cycle off to allow the water to cool and maintain a more optimal dT (somewhere in the controller logic the heat pump decides when dT drop below a certain value, the heat pump should cycle off).

Not sure this is precisely what happens or how the control loop works.  What I observe on plots I have seen (including mine) is that the flow temperature rises above the set point (because the heat pump is putting in more heat than the radiators are emitting).  Eventually the flow temperature exceeds the target + allowable hysteresis, at which point the heat pump will shut down.  You could in fact restore deltaT by simply slowing down the water pump, but the flow temperature will still rise above the target value because the heat pump is putting in more heat than the radiators are emitting. 

Having a volumiser is not going to change the above fundamentals, as unlike a radiator it cannot dissipate heat (it's well insulated so it is reasonable to assume near zero heat loss relative to a radiator or other emitter). Consider it as simply a well lagged fat pipe, so there is more volume of water in the system. It may take longer to reach the initial point of cycling as there is initially more water to heat, so at best it may extent the periods between cycling  (so short cycling now becomes longer cycling)

Agreed, particularly with the sentence in brackets.  A volumiser changes the period of the cycling but not the on/off ratio.

I wondered if short cycling would be reduced if it was caused by mild outdoor ambient circa 10c and a corresponding low flow temperatures and possibly insufficient radiator capacity at low flow temperatures…. Can you use a volumiser as a replacement for increased radiator capacity to reduce cycling? Eg. Flow temps down to 30c and rad capacity sized for 50c at -3c might not be large enough… as an example. 

or is this just a red herring?

As you say, short cycling can occur during milder ambient conditions where the flow temps are reduced and the radiators/emitters then cannot dissipate the heat that is being produced. This causes the return temperature to rise (as the heat cannot be effectively dissipated into the property) thus narrowing the dT between flow and return. When this happens, the heat pump can cycle off to allow the water to cool and maintain a more optimal dT (somewhere in the controller logic the heat pump decides when dT drop below a certain value, the heat pump should cycle off).

Having a volumiser is not going to change the above fundamentals, as unlike a radiator it cannot dissipate heat (it's well insulated so it is reasonable to assume near zero heat loss relative to a radiator or other emitter). Consider it as simply a well lagged fat pipe, so there is more volume of water in the system. It may take longer to reach the initial point of cycling as there is initially more water to heat, so at best it may extent the periods between cycling  (so short cycling now becomes longer cycling). Extending this premise, if the system volume were infinitely large, the return temperature would never rise to the point where the dT narrows to cause cycling, so there is a point of diminishing returns and as long as the system volume is large enough, then short cycling shouldn't be an issue. I do not know what this point is, but I have heard recommendations of having a system volume that is 3-4 times the recommended minimum system volume.

I wondered if short cycling would be reduced if it was caused by mild outdoor ambient circa 10c and a corresponding low flow temperatures and possibly insufficient radiator capacity at low flow temperatures…. Can you use a volumiser as a replacement for increased radiator capacity to reduce cycling? Eg. Flow temps down to 30c and rad capacity sized for 50c at -3c might not be large enough… as an example. 

or is this just a red herring?

Here is a chart showing how our defrosts occur. There is a 2 or 3 minute defrost followed by a slow recovery to the mean water temperature. It’s not a noticeable issue but I guess we haven’t been looking for it. If we were in a colder climate it might be an issue with repeat defrosts.

Posted by: @sunandair

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Firstly is it true that the circulation pump continues to push the flow and return water around the heating circuit during the defrost? Secondly is it true that there is no heating of any water as it passes through the heat pump

 

Great question… I hope that @heacol can pick this one up.

I didn’t see your earlier reply to @old_scientist which goes a lot further in detail. Which is pretty convincing.

ive copied it here for a reminder.

@old_scientist, you’re right that the volumiser stores heat energy, and a larger volume does reduce the overall temperature drop during defrost. However, the key issue is where this stored energy is most effectively utilised. On the flow side, the volumiser contains warmer water from prior heating cycles. During defrost, this stored warmth moderates the cold influx, reducing the temperature gradient between the incoming cold water and the water entering the radiators. This minimises discomfort and stabilises the system. By moderating the cold influx, the flow-side volumiser reduces the energy required to reheat the system after defrost, aligning with the First Law of Thermodynamics (conservation of energy).

On the return side, while the volumiser does store energy, it’s less effective during defrost because the return water is already at a lower temperature. Cold water from defrost mixes with this cooler water, exacerbating the temperature drop and forcing the heat pump to work harder to reheat the system. This increases energy consumption and reduces efficiency, negating the benefits of the stored energy.

You’re also correct that at high flow rates (e.g., 25L/min), a 50L volumiser will empty in about 2 minutes. However, the critical factor is what happens during those 2 minutes. On the flow side, the volumiser’s stored warmth moderates the cold influx, preventing cold shocks to the radiators. This is crucial for maintaining comfort and system stability. Even after the volumiser empties, the moderated water temperature reduces the overall temperature drop, making it easier for the heat pump to recover.

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, potentially causing discomfort and increasing the energy required to reheat the system.

While the volumiser on the return side does provide a larger system volume and some energy storage, its role is passive and less effective compared to flow-side placement. The flow-side volumiser actively moderates temperature fluctuations, reduces energy consumption and improves comfort – benefits that the return-side volumiser cannot match.

So by moderating cold influxes during defrost, the flow-side volumiser aligns with thermodynamic principles to improve efficiency, reduce energy consumption and maintain comfort.

This reply makes a lot of sense since it seems to confirm that the volumiser is doing more than just adding volume. It is clearly adding surplus volume but it’s also adding a huge reservoir of FLOW TEMPERATURE FLUID. 
furthermore by placing it at the LWT end of the installation this means it has the greatest opportunity of absorbing and mixing with the cold defrost slug of water which might be as much as 50 or 60 litres in volume.

so in my own clumsy way here is a few possible scenarios of the mixing journey of the defrost water…. Which may or may not be true…

So if the cold water dilution and blending process is going to work, the system needs to mix all of its surplus Flow Temperature volume held in the volumiser as the cold slug leaves the HP. but THIS MIGHT ONLY PARTIALLY BLEND as it flows through the volumiser itself. If the defrost recovery is going to be shortened or lessened the cold slug needs to be fully mixed with the 200 litre circuit volume and preferably in one cycle of the heating pipework.

My interpretation is that the rest of the mixing then continues as the water enters each radiator. But  it’s also important to see that each radiator has a different pipe length meaning that the temperature gradients are further blended as the return flows enter the CH return pipework at the different mixing points along the lengths of the flow and return pipework. 

So does this mean that by placing the volumiser at the LWT position in the system, (Position A or C on the sketch) the extra volume is capable of cancelling out the chill down and slow recovery of a typical defrost.

@editor  does this scenario work for you and perhaps of equal importance which position are you saying the volumiser should be placed on the sketch?

1. The heat pump can't defrost and produce heat at the same time. It's the process of extracting heat from the air which causes the frosting (which depends on air temperature and humidity - damp British air causes more frosting than drier air in central Europe) so the heat pump has to temporarily stop generating heat while clearing the ice.

2. The heat pump needs a source of heat for the defrosting and therefore needs to use some warm water, hence the temporary drop in the flow and return temperatures as shown on my graphs above. However, while radiators will temporarily feel cooler, it's unlikely that they will go cold. My detailed graph shows the flow temperature dropping for 6 minutes before starting to rise again. That's for water which started at less than 35C. I would expect a higher temperature system to defrost even faster.

3. My buffer tank contains a pocket for an immersion heater element (none fitted). I understand that this is a frost protection option by providing a source of warm water to circulate through the heat pump should the compressor fail but the circulating pump is still working. It wouldn't give protection against an electricity failure. It would be technicially possible to turn this immersion heater on during the defrost cycle instead of cooling the CH circuit (a 4-port buffer system is needed to semi-isolate the primary circuit through the heat pump from the secondary CH circuit). However, an immersion heater is a much less energy-efficient source of heat than the heat pump itself.