I’ve exchanged a few messages about this with Graham Hendra, and according to him this unit won't reduce its output below 40%, leading to cycling; it's simply how it operates. Since it can't modulate below this threshold, temperature control tends to be uneven and spikey, with noticeable fluctuations. This appears to be a common characteristic of newer models.

Posted by: @ecobaker

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I'd set the system to run based on the water outlet temp, which I had set at 30, which would have been about right for today.

But without the hive the compressor wont stay on. What happens is the compressor turns on for maybe 3 mintues, water outlet shoots up to 38 to 40ish, the compressor turns off. The water outlet temp very slowly drops until below 33, then fires up again for a few minutes. 

Any ideas on what is wrong?

As Mars says, the Samsung will cycle when the heat emitted by the radiators is lower than the minimum power that the heat pump can modulate down to - around 40% of its peak output. With a water outlet temperature of 30C, your radiators are unlikely to be emitting much heat especially if your room temperature is that high and it is mild outside. When the Samsung cycles, several users have commented on its tendency to cycle on and off every 8 minutes or so.

Posted by: @ecobaker

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However I wanted to try running the system without the Hive, using the standard Samsung wired controller, and setting up weather comp, so I disconnected the Hive.

If you change your settings to :

Indoor Zone Option --> Standard Temperature --> Indoor

and  FSV 2091 & 2092 --> 0 (Not use)

and FSV 2093 --> 1 (Thermostat only)

then the thermostat in the wired controller will switch the heat pump on and off at temperatures determined by your desired temperature settings. The outlet temperature will be determined by your weather compensation settings which are set using FSV 2011, 2012, 2021, 2022, 2031, 2032.

So you will still get cycling if you choose outlet temperatures that are too low.

Mine is set to 47.5C at -5C outside air temperature (OAT) and 33.5C at 10C OAT as my 12kW Gen 6 Samsung can only modulate down to 3.6kW and my radiators need  the outlet temperature to be above 33C to emit this minimum 3.6kW.

This just in from a Samsung Product Specialist: I think it might just overshoot target within the first 5 minutes of operation. That is often the case in systems with a too low active volume.

@ecobaker some more feedback on how to remedy your issue, also from the product specialist: "Calculate minimum required active volume for (extreme) part load operation of 12 minutes (as stated in Samsung's Installer Reference Guide). And check quantity in the system. If there is insufficient volume, increase it by adding a buffer vessel."

Hmmm.

Posted by: @editor

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@ecobaker some more feedback on how to remedy your issue: "Calculate minimum required active volume for (extreme) part load operation of 12 minutes (as stated in Samsung's Installer Reference Guide). And check quantity in the system. If there is insufficient volume, increase it by adding a buffer vessel."

Hmmm.

Add a Volumiser, NOT a Buffer Vessel.

@editor Thanks, sounds like you know all the right people to speak to. When I do have it set running with the hive I do the cycling. So I think my volume will be too low and below 40% at 30 degrees. I guess I should do the calculations like you say and see what my system requires. If I put more radiators in that would increase the volume right and maybe get it closer to the 40% right? I do have some larger rads arriving next week.

@mike-h Thanks thats really useful. 

So my system is set at 42C for -3C OAT and 30C for 16C OAT, but I actually think I can have a lower flow temp maybe down to 27 or 28c on warmer days. I have an 8kw pump, so I guess I need more volume in my system for it to get down to the lower flow temps and not cycle too much. 

I have got some new larger radiators coming next week, that would help wouldn't it?

@editor Any idea how to get my hands on an installer reference guide, my googling hasn't turned up much

Posted by: @editor

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@ecobaker some more feedback on how to remedy your issue: "Calculate minimum required active volume for (extreme) part load operation of 12 minutes (as stated in Samsung's Installer Reference Guide). And check quantity in the system. If there is insufficient volume, increase it by adding a buffer vessel."

Hmmm.

@editor This advice seems to fly in the face of everything I have learnt from the likes of @iancalderbank and @heacol . Perhaps that is why you have added a 'Hmmm', but I am not sure that it is very helpful to the OP. The minimum volume that Samsung recommends does not prevent short cycling and certainly in my case, a 50L buffer made short cycling worse due to the resulting 4-5C reduction in flow temperature to the radiators.

So an update to this issue, I reinstalled the Hive and went back to my old method, had the hive room temp set for 28, but I added water Law compensation. My flow temp has been around 29 all day, and my COP (taken from energy use/genertion on the unit controller, I don't have any monitoring) has been 4.5 today. Although that was without any DWP because I turned it off and forgot about it. 

I am short cycling, the compressor is coming off and on, but I don't know how often in an hour. At what point is too cycles in an hour?

Posted by: @mike-h

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Posted by: @editor

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@ecobaker some more feedback on how to remedy your issue: "Calculate minimum required active volume for (extreme) part load operation of 12 minutes (as stated in Samsung's Installer Reference Guide). And check quantity in the system. If there is insufficient volume, increase it by adding a buffer vessel."

Hmmm.

@editor This advice seems to fly in the face of everything I have learnt from the likes of @iancalderbank and @heacol . Perhaps that is why you have added a 'Hmmm', but I am not sure that it is very helpful to the OP. The minimum volume that Samsung recommends does not prevent short cycling and certainly in my case, a 50L buffer made short cycling worse due to the resulting 4-5C reduction in flow temperature to the radiators.

That is precisely why I wrote hmmm. That is advice straight from Samsung. I just don’t understand it, and I fully concur that this is in no way helpful to the OP. I find it frustrating how poor the advice is, but felt compelled to share because this is directly from Samsung.

I, however, certainly don’t recommend putting a buffer tank in.