I think that I might be able to push for a lower flow temperature once I get my extension built and have more emitters. 

It’s not really a problem as it only happens in warmish weather and the COP is fine running at over 5. 

On an academic level I wonder if it would be possible to change the hysteresis values via a modbus connection?

I believe that the Samsung compressor stops when LWT rises approximately 1degC above target LWT and restarts when LWT falls 2degC below target LWT. 

If system volume is 50L and the emitters are dissipating heat at 3kW, then the off time is around 3.5 minutes. This fits with your heatpumpmonitor graph more or less. 

I believe that the on time is longer because the power that heats the water up by the same 3degC is the difference between what the heat pump produces when it restarts and the 3kW that the emitters are emitting. 

For example, if the heat pump restarts at 4.5kW, which is 1.5 kW in excess of the emitters, then the on time should be approximately 7 minutes.  Your cycle time for the on   cycle plus the off cycle is about 8.5 minutes. When my 12kW Samsung short cycles, it also cycles on and off every 8.5 minutes and I have seen the same with other Samsungs. 

I don’t think that you can change the hysteresis on a Gen 6 Samsung. It appears to have one of the least sophisticated ways of avoiding short cycling of any common ASHP. Perhaps that is why a buffer is commonly installed as standard. 

so it can cycle quite a bit

What sort of run and off time do you get?

Can you change the setting for how many degrees of heat the boiler adds per minute. Mine had a setting hidden away, called gradient, the default setting was way to high, changed to 2 degs a min, made a huge difference

Hi can you explain this statement about the 40L

You just about explained it yourself.

Image a kettle, it has 3kW element, if you put a cup of water in it it boils very quickly, full it it takes a while to heat up. 

So a boiler or heat pump has a target flow temperature, the heater controller will use a hysterisis above and below this to manage heat input. If you say the hysterisis is around 5 to 8 degs, 40L and 5kW comes out about right and gives around 10 mins running, off time will depend on heat dispersion rate. More volume gives a longer run time, less volume, shorter runs and quicker cool down.

have a large amount of radiators (but cant remember the volume off the top of my head) but i can still cause my heat pump to cycle rapidly if I reduce the flow temperature sufficiently.

I had a similar issue and had to increase the target flow temp hysterisis which fixed the issue. See here for explanation 

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/hp-cycling-expected-or-undesirable/paged/3#post-32091

I can appreciate that having a larger body of water to heat will slow the onset of cycling, but I cant see how it would prevent it. If your heat loss is 1kW and your minimum output is 3kW then eventually you will reach a point where the heat in the water cannot escape quickly enough and the heat pump will stop heating.

I suppose that if the water pump keeps running then the stored heat in the water would eventually dissipate and this could be some time before it reaches the point when the heat pump will start again. Is this what you mean?

I have a large amount of radiator volume (but cant remember the exact value off the top of my head) but i can still cause my heat pump to cycle rapidly if I reduce the flow temperature sufficiently.

Thanks

Addendum: Just done a quick calculation and I have way more volume available than 40L (over 60L) and my heat pump is a 5kW output

min output wouldn’t be too far from 5kW, which is what my boiler’s minimum is. That boiler already cycles like crazy in mild weather. 

I would guess you have many thermostats or radiator valves, shutting off system volume, rather than an issue with minimum turn down of 5kW from your gas boiler. You only need about 40L of water engaged to stop short cycling from a 5kW input.

What makes up the 7/24/21 formula? I’ve not seen that one before

You have two modes for the heat pump, central heating and DHW. While you are doing DHW you are heating the house.

So heat demand from heat meter you said was 7kW, you multiply that by 24 (hours per day) to get kWh heat required per day.

You need to allocate time for heating DHW, so based on my experience we heat for about 2 to 3 hours per day. So I chose 3 hrs.

24 (hours per day) less 3 hours is 21.

Divide (7x24) by 21 to get the power output required from the heat pump, so it can heat the house and make DHW. Make sure your heat pump can deliver that energy on you coldest day. You won't go short of hot water or be cold or have to use a secondary heat source such as the in built immersion kicking on costing you loads.

Don't get hung up on a size, stated on the box, every manufacturer uses a different scale. Samsung is the max output, so output on a hot day, Vaillant is the output on the coldest. They are very similar in output not magnitudes different as the name would suggest.

I am not suggesting you go with a Samsung unit, I just used them as an example where model line made the choice for you. In you case look at a different brand