You've raised some interesting points about living with an oversized heat pump, and it's great to see you making the most out of your situation. While a properly sized heat pump is indeed more efficient, your approach of leveraging Quiet Mode and time-of-use tariffs like Octopus Agile to optimise performance is smart. The ability to reheat your DHW cylinder quickly and take advantage of cheap electricity slots are definite perks.

Another potential advantage might be in resilience; an oversized system could have more headroom to handle particularly cold spells or increased demand in your home, ensuring comfort even in extreme conditions.

Mine is also pretty oversized our -9 heat loss is about 3.5kW, but we have a 6kW heat pump, so in percentage term huge. 

You can batch charge thick screed UFH in the E7 down to sub zero, but the coldest days need to run for quite a few extra hrs. 

With some tuning you get the cycles under control, mine is currently once per hour. When running within a couple of minutes it seems to run close to manufacturer CoP.

We now heat and cool a summer house, which adds another 1kW+ to heat demand, without having to add additional heat source.

Downside 

Because you cycle, you have to slightly compensate with slightly warmer flow temps, so take a hit CoP wise.

Cycling in general takes a CoP hit.

You really HAVE to run single zone to keep things happy.

Posted by: @old_scientist

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compressor is operating in the 30-70 percent range.

Are you sure about that? Do you have a reference?

In general the longer the run time the more efficient the heat pump becomes, because the compressor gets to a steady state temp throughout, the refrigerator gases and an the throttle valves are all settled to their correct running position. Plus the water temp is in a steady state.

@editor Yes, I have no doubt that we will not be cold in winter. It's generally mild where we are, and our design temp was -1.9C, but I'm sure we are good to go even if we were to see temps much lower. Of course the main "issue" is too much heat output in the milder months, but rather than try to run low and slow, we may just control when the heat pump runs and try to schedule bursts during cheaper Agile slots. Having yet to experience a winter, I'm guessing we'd need temps of below 10C (maybe as low as 5C) to run continuously, even at minimum output.

But I'd much rather be in this boat than those who have cold houses and very high bills.

Posted by: @old_scientist

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I'm guessing we'd need temps of below 10C (maybe as low as 5C) to run continuously, even at minimum output.

It may not then either. You can't control your running hysterisis directly, but you can control in a hybrid mode of water law and the controller thermostat. So this mode I believe works by generally following weather compensation but allow some overshoot in room temp, to prolong compressor run, then some undershoot before restarting the compressor. This increases on time and off time. People have seen a best CoP of high 2s go to mid 4s by using this strategy. It's all built in to the controller as an option. Don't know the details of it directly, but there was a discussion on here where it was mentioned (short cycling) also saw it discussed on another forum.

I can change the stop and start hysterisis bands and have done so to get a reasonable run time even when I didn't have a large heat demand.

Posted by: @johnmo

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Mine is also pretty oversized our -9 heat loss is about 3.5kW, but we have a 6kW heat pump, so in percentage term huge. 

You can batch charge thick screed UFH in the E7 down to sub zero, but the coldest days need to run for quite a few extra hrs. 

With some tuning you get the cycles under control, mine is currently once per hour. When running within a couple of minutes it seems to run close to manufacturer CoP.

We now heat and cool a summer house, which adds another 1kW+ to heat demand, without having to add additional heat source.

Downside 

Because you cycle, you have to slightly compensate with slightly warmer flow temps, so take a hit CoP wise.

Cycling in general takes a CoP hit.

You really HAVE to run single zone to keep things happy.

Posted by: @old_scientist

↑

compressor is operating in the 30-70 percent range.

Are you sure about that? Do you have a reference?

In general the longer the run time the more efficient the heat pump becomes, because the compressor gets to a steady state temp throughout, the refrigerator gases and an the throttle valves are all settled to their correct running position. Plus the water temp is in a steady state.

@johnmo, I'd taken it from John Cantor's video here, see the slide from ecoforest towards the end of the video at 20:08min

which shows efficiency falling off a cliff below ~30%, which is probably why equipment manufacturers do not allow their ASHPs to modulate below 25% output as COPs would be disastrous so they are restricted to operate in the range in which they are most efficient. It shows a less pronounced roll off at higher compressor speeds which is also somewhat temperature dependant.

I believe this is why I can measure less required electrical input with my heat pump in Quiet Mode (restricted to ~70% of output) when reheating my DHW cylinder to it's set point from the same starting temps and ambient temps. When quiet mode is off, the heat pump ramps up to full output rather than it's output being limited, which is less efficient. Maybe a short DHW run is not a good example as the system never achieves a steady state of operation as LWT is continuously rising until the DHW set point is reached. Maybe I am observing some other effect.

Posted by: @johnmo

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Posted by: @old_scientist

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I'm guessing we'd need temps of below 10C (maybe as low as 5C) to run continuously, even at minimum output.

It may not then either. You can't control your running hysterisis directly, but you can control in a hybrid mode of water law and the controller thermostat. So this mode I believe works by generally following weather compensation but allow some overshoot in room temp, to prolong compressor run, then some undershoot before restarting the compressor. This increases on time and off time. People have seen a best CoP of high 2s go to mid 4s by using this strategy. It's all built in to the controller as an option. Don't know the details of it directly, but there was a discussion on here where it was mentioned (short cycling) also saw it discussed on another forum.

I can change the stop and start hysterisis bands and have done so to get a reasonable run time even when I didn't have a large heat demand.

Yes, I've not had a chance to really play with settings yet as we've not experienced a winter, but because I assume I'd be cycling (of some description), I've initially decided to control that cycling myself rather than allow the heat pump controller to do it based on weather compensation and LWTs.

So I've disabled that setting in the Samsung controller, so my heat pump basically runs continuously and is controlled by my 3rd party add on room thermostat's call for heat (on/off). My room stat has a hysteresis setting of 0.2C to 1C, and we can further modulate that by turning the set temp up and down extending any run/off periods. I plan to run like that a while to get a baseline for performance, and then we can reverse the settings and let the Samsung controller have a go at things based on it's WC curve, and see how that compares COP wise.

I think the hysteresis settings you refer to may only be applicable if using the Samsung controller as the room stat - I'm not using it as my room stat (it's bolted to the front of my DHW tank) as we have a separate add on room stat to control room temp / call for heat, but I can achieve the same hysteresis effect with my room stat controller.

Posted by: @old_scientist

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think the hysteresis settings you refer to may only be applicable

The hysterisis referred to is the difference between target flow temperature and at what temp the compressor starts and stops.

So mine at a target flow temperature of 25, the compressor starts at return temp of 20.1 and runs until water leaving temperature hits 27.8. I can move these numbers about and they are a long way off what was set in the factory.