Posted by: @sunandair

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@heacol I’ve also herd that a volumiser can help with short cycling.

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.

Posted by: @old_scientist

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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. 

Posted by: @old_scientist

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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.

Posted by: @jamespa

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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.