I think the Arotherm plus' have an option to set up an adaptive curve using a specific controller and sensors, (not available to me, of course 😏 as I have been told that's an unrealistic expectatation of mine)

Off the top of my head I can think of a few reasons

1. Heat pumps are not as powerful as the boilers they replace so simply can't just slam on at the huge output a boiler can to catch up when the thermostat goes below a temperature
2. It is much more efficient to aim for the lowest flow temperature possible, saving the user money
3. It can honestly just be more comfortable than a thermostat since a thermostat will always allow the temperature to drop (probably by 0.5C to 1C) then ramp heating up until it overshoots and the thermostat turns off, but allowing the radiators to carry on chucking out heat. This gives you a constant roller coaster whereas a good weather comp curve will feel more steady and comfortable

I doubt many people run in pure weather compensation mode. Most have a combination with thermostats, the best setup if which would be a modulating control that can help smooth out solar gains or windy nights by changing the weather compensation mode based on the interior temperature. 

I think it's simply because the outdoor unit already has a thermometer on it, whereas they can charge you extra for an indoor "smart thermostat" that's priced competitively with a Hive or Nest etc. If they bundled that into the price of the heat pump I don't think it would be enough of a differentiator for installers/consumers to choose one over the other, whereas they can capture a bit of consumer surplus (and save some costs) by supplying it as an addon.

Posted by: @oswiu

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Heat pumps are not as powerful as the boilers they replace so simply can't just slam on at the huge output a boiler can to catch up when the thermostat goes below a temperature

Very true, but I am not thinking of huge output, just a resetting of the output to match 'as if' it was a bit colder outside. For example, this morning my controls saw an outside temp of 3 degrees, and the LWT was about where it should be on the curve, at 46 degrees, but the house really needs say 50 degrees to warm up a bit, ie the LWT you would get if it was say zero outside. The bottom line is the house feels a bit chilly this morning.

Posted by: @oswiu

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It is much more efficient to aim for the lowest flow temperature possible, saving the user money

Very true, but there is a cost in comfort, as I say, the house feels chilly.

Posted by: @oswiu

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It can honestly just be more comfortable than a thermostat since a thermostat will always allow the temperature to drop (probably by 0.5C to 1C)

Not sure I agree! It currently 3 degrees below design temps in my downstairs rooms, at or just below upstairs! At 3 degrees below design, it is not more comfortable. It will also take ages for the house to warm up (could be days) unless i manually intervene, and boost the LWT for a few hours (and burn a few more deep sea divers while I'm at it).

Posted by: @oswiu

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the best setup if which would be a modulating control that can help smooth out solar gains or windy nights by changing the weather compensation mode based on the interior temperature. 

This is something I have suggested before. Weather comp curves assume everything else remains constant, only the outside temp changes, but of course this is a long way from reality. But why not use just indoor temps? They factor in things like solar gain and wind chill and leaving the back door open by mistake. I'm not suggesting control by indoor temps works as in a fossil fuel system ie boiler on/off, but as in a weather comp curve, ie the heat pump output increased and decreases in proportion to how distant the indoor temp is from the target temp. But I can see the problem, when the indoor temps reach target, the heat pump will shut off, because it doesn't know it needs to output xkWh to maintain that target temp, which brings us back to your best setup, an adaptive control that boosts the weather comp curve LWT when the interior temp is below that expected, and vice versa. Apropos of other posts elsewhere, you don't need AI or machine learning to do this, just simple sensors and I imagine (because I am not an electrical engineer) very simple circuits, with a logic that says something like if the interior temp is 2 degrees below expected, raise the LWT by 2 degrees etc etc. 

Posted by: @scrchngwsl

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whereas they can charge you extra

Good point. Add-ons are the marketeers dreams come true. 

Posted by: @saz

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I think the Arotherm plus' have an option to set up an adaptive curve using a specific controller and sensors, (not available to me, of course 😏 as I have been told that's an unrealistic expectatation of mine)

I think that many of the modern controllers already have an inbuilt temperature sensor, but unfortunately many of the installers locate the controller in the airing cupboard, loft or garage, which of course is not measuring the temperature in the living area. A further problem would appear to be that the response of the controller cannot be adjusted to match the characteristics of each home, which can be vastly different.

@cathoderay I did say a good weather comp curve can be more comfortable. If your house is cold then the curve isn't tuned properly. Either that or you're turning it off overnight, which is fine, but then you might need to programme a "set forward" on the curve to boost it a bit in the morning. I suspect not all controls allow this, so you might need to get the heating to come on earlier.

@cathoderay

I suspect in your situation, the indoor temperature fell because your heat pump is not capable of providing the required amount of heat energy at lower outdoor air temperatures.

Posted by: @derek-m

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I think that many of the modern controllers already have an inbuilt temperature sensor, but unfortunately many of the installers locate the controller in the airing cupboard, loft or garage, which of course is not measuring the temperature in the living area. A further problem would appear to be that the response of the controller cannot be adjusted to match the characteristics of each home, which can be vastly different.

I'm pretty sure the Midea controller can have an external room temp sensor fitted, there is a line in the 'Operational Parameters' for it, Ta Room Temp, but it doesn't get fitted, display just shows --, and I don't know whether it just controls the heat pump via on/off, or whether it can be set up to adapt the curve, something I need to look into. 

Posted by: @oswiu

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I did say a good weather comp curve can be more comfortable. If your house is cold then the curve isn't tuned properly. Either that or you're turning it off overnight, which is fine, but then you might need to programme a "set forward" on the curve to boost it a bit in the morning. I suspect not all controls allow this, so you might need to get the heating to come on earlier.

Posted by: @derek-m

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I suspect in your situation, the indoor temperature fell because your heat pump is not capable of providing the required amount of heat energy at lower outdoor air temperatures.

I think @derek-m is on the money here. The system is on all the time (room stat set to 26 degrees, which is never reached), no night time setback, the curve at the lower ambient end is as as high as I want the LWT to be (so as to avoid atrocious COPs), at 55@-4 degrees. Downstairs has been cold over the last few days because it has been so cold outside, and my marginal Midea pump with notably poor cold weather performance simply couldn't keep up, nor can it supply enough heat to warm the house up a bit, unless I manually tweak the curve (ie do what I have suggested should happen automatically). But I shouldn't have to tweak the curve everything the weather changes. The problem at the moment is that the weather comp curve is correctly set up providing the house is in a steady state, ie all rooms at design temp, it is it putting out the heat the house needs at ambients of around 6 degrees (which is what it currently is, but it is not enough, because the house is colder than it should be, and needs extra heat.

The other fundamental problem, as noted before, is that the single weather com setup assumes the only thing that varies is the outside temp, when of course that is far from true.

Rather annoying, my attempts over recent days to divert some of the heat from upstairs to downstairs by fine tuning the lockshield valve adjustments has only achieved lower temps upstairs (all down by about 2-3 degrees, they were just over design before I did this), but no corresponding increase in warmth downstairs. I guess the heat pump reads slightly warmer RWT, and as a consequence drops the output a bit, so all that has been achieved is a cooling down upstairs. 

@derek-m Yes, I have a portable thermostat though for my UFH (which generally stays in the lounge with me). Vaillant have a specific weather compensating controller the VRC 700. I don't have this though, I have the Sensocomfort. I'm following some of the Arotherm plus installs online with these controls, they seem to be achieving increadible COPs.

Posted by: @saz

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@derek-m Yes, I have a portable thermostat though for my UFH (which generally stays in the lounge with me). Vaillant have a specific weather compensating controller the VRC 700. I don't have this though, I have the Sensocomfort. I'm following some of the Arotherm plus installs online with these controls, they seem to be achieving increadible COPs.

SensoComfort is fully capable of performing weather compensation control if it is set to do so. Correct me if I am wrong, but I seem to remember that your controller is located in the loft.