Er, nobody out there to support?

I'm quite sure that my wild guesses won't help, but possible will provoke some reaction from people more skilled than I:

From my point of view, this looks like a very normal reply on a heat demand from a hall thermostat, assuming that 'Circuit 1 thermostat temperature' is measured by such a device.

Like this:
At some point, as hall temperature falls below par (difficult to see in the graph since it may be only 0.1°C) the heat pump gets started.
HP strives hard to asap achieve nominal FT ('outlet tempeature').
When nominal FT is reached an equilibrium applies, and the HP happily hums along.
As a very first sign of saturation, the RT ('inlet temperature') starts rising and the HP answers with a little slowdown of rpm.
Then a while everything is stable again, until rpm starts rising. I understand that the HP sees a good time coming to its end and its reaction is panic (of course I'm joking .…)
While the HP starts cycling, a close look reveals that indeed the hall temperature, sorry, circuit 1 temperature, has risen, and this being the reason why TRVs start closing and choking flow in a way that prevent the HP from releasing its heat, thus cycling.
At some point hall thermostat sees set point reached, shuts down HP.
For a while now, residual heat of the radiators plus hall thermostat hysteresis keep the system from restarting, until .... the next cycle starts.
The second cycle shows a lesson learned: there's no panic attack in front of cycling.
The second cycle gets interrupted by the beginning of daytime mode.

Unfortunately, there are no units given in the graphs, but I understand that temperatures are in °C, compressor frequency in 1/s and that one block of the grid is one hour. Right? In the overview it's easier, there's one cycle per day.

While researching 'Decreasing Fixed Water Temp' I found a similar and contemporary request in another forum, this being you as well, I suppose?

The question now would be, why does the controller ignore your setback and 'Decreasing Fixed Water Temp' demands? Let me add that, sorry, I have no idea!

And now a second case of answering a question nobody raised:
Night setback on a heat pump is not a successful business case. As you can see, indoor temperature lowers from 22°C to 19°C only, being, during night shift, in average 1.5 K below setting. At an average outdoor temperature of 6°C this is a 1.5/(20.5-6) equalling to 10.3 % saving. On a 24-hour-cycle it's again 9/24*0.103 being shy of four percent.
On the other hand, without setback you could have lowered the outlet temperature from average 34°C during daytime mode to supposedly 30°C @ 24/7 which would gain ten percent (2.5 % per centigrade) efficiency!
Maybe the controller does wrong, but is right anyway?

@unsure what's the time scale on the horizontal axis?

Posted by: @jamespa

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@unsure what's the time scale on the horizontal axis?

The set back periods are 8 1/2 hours (between 9:30pm - 6:00am). The chart showing the spikes is one set back period (8 1/2 hours). 

@unsure

Thanks

Have you got a buffer tank or Low Loss Header or is the Grant directly plumbed to the radiators.  Have you got any secondary controls? On what controller are you setting the 'set back'.

I note that the compressor cycling is happening at the two points point OAT reaches its minimum.  Is it possible this is the anti-freeze cycle kicking in.  Heat pumps, just like boilers, do that whatever the settings (other than power disconnected obviously) to ensure that they dont freeze up.

I have the Grant Volumiser. There are no secondary controls, just the Smart Controller (EcoNET). Each time I approach Grant about this issue, I’m told the compressor reacts to the flow temp when pump blockade is off not the room stat. So, there must be a relationship between the Set Back and Flow temperature (Room Temp and Water Temp) otherwise what is the point of adjusting the Set Back via the room stat?

If I drop the room stat Set Point on the controller to its lowest setting (10 Degrees). I assumed that would also drop the point at which the compressor activates in relation to the flow temp.

Looking at it in more simple terms:

If the conditions (Set-back, Internal temp, External temp, Flow temp Out and In, WC), and every other setting available are ALL the same across a range of dates, then why does the compressor activate on some nights and not others.

The graph below offers a basic example of this, although I can find others with a far better match. The bottom graph (last night) shows the compressor activating at 2:15am when the conditions are the same for other nights when the comp does not activate (above graph).

Posted by: @unsure

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So, there must be a relationship between the Set Back and Flow temperature (Room Temp and Water Temp) otherwise what is the point of adjusting the Set Back via the room stat?

Not necessarily, it depends on what WC mode you are operating in (please clarify). 

If you have it in pure WC mode then it will probably ignore the set room temperature altogether, depending on how exactly it works.  With most heat pumps this is in fact the case.  There are a few, like my Vaillant, where the effect of adjusting the target room temperature is actually to shift the WC curve.  Thus even in pure WC mode changing the set room temperature alters what it does.  Its possible that Grant have implemented this algorithm in their smart controller.  It is actually very sensible and appears to have been developed initially for boilers in countries where WC has been compulsory for boilers for a couple of decades.  That said Grant have a history of being muddle-headed about their controls, trying for many years to make a heat pump look like a boiler, so I would believe almost anything!

What WC mode is yours operating in?  

Also you speak of 'fixed water temperature'.  What do you mean by this, are you NOT operating in weather compensation mode (the Grant Smart controller speaks of 'compensated' and 'uncompensated' circuits)

Do I deduce from the above that pump blockade is set to 'off'?

Has the smart controller been set to use: no thermostat, its own sensor, an external thermostat?

Also is the grant volumiser plumbed as a volumiser or a buffer.  You can tell most easily by whether there is a secondary water pump or not.

Finally is any of this actually causing a problem which you are trying to fix, or are you just trying to understand the logic?

Posted by: @unsure

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If the conditions (Set-back, Internal temp, External temp, Flow temp Out and In, WC), and every other setting available are ALL the same across a range of dates, then why does the compressor activate on some nights and not others.

The graph below offers a basic example of this, although I can find others with a far better match. The bottom graph (last night) shows the compressor activating at 2:15am when the conditions are the same for other nights when the comp does not activate (above graph).

Judging by the fact that the flow temperature stays up after the compressor activates, this looks like a long cycle to me, ie it switched on because it decided that the FT was just below some threshold, heated up, then the FT stayed at about 30 with the compressor running at minimum modulation because the demand was low.  Perhaps on the first night shown it didnt quite reach the switch on threshold until 6am when it looks like it switches on again.  

I’m using WC with a Stat attached. This is the standard setup with the Smart Controller. The manual points to a relationship between the Air and Flow temperature controlled by the WC. (The air temp relates to the decreasing water temp, see page 33 in the Smart Controller manual 2024). Even taking all that into account, when the conditions are the same, the HP should behave the same. E.G. what changed to activate the heat pump between 1pm-2pm on the bottom graph.

Pump blockade is off. I don’t have the issue when it is on (see my other thread regarding the problem with frost protection and pump blockade on).

The Volumiser is plumbed as a Volumiser.

I now see this as a fault (possible faulty flow temp sensor in the heat pump).

Finally, I've never seen any other owners have the same issue, thus it appears out of normal operation.

Posted by: @unsure

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I’m using WC with a Stat attached. This is the standard setup with the Smart Controller. The manual points to a relationship between the Air and Flow temperature controlled by the WC. (The air temp relates to the decreasing water temp, see page 33 in the Smart Controller manual 2024). Even taking all that into account, when the conditions are the same, the HP should behave the same. E.G. what changed to activate the heat pump between 1pm-2pm on the bottom graph.

Pump blockade is off. I don’t have the issue when it is on (see my other thread regarding the problem with frost protection and pump blockade on).

The Volumiser is plumbed as a Volumiser.

I now see this as a fault (possible faulty flow temp sensor in the heat pump).

 

Finally, I've never seen any other owners have the same issue, thus it appears out of normal operation.

OK this bit of the manual

So this says that, when blockade is disabled, the target flow temperature will be reduced when the IAT reaches the target IAT.  Its a sort of adaptive WC mode, ie its basically WC but 'modified' in that the FT is reduced when the room temperature goes over target.  Note that the amount by which the FT is reduced does not depend on the set temperature, its reduced by the value of DECEASE WATER TEMPERATURE.  Not also that this means there is always a target flow temperature even when the house temperature exceeds the set value, so its to be expected that the heat pump will come on from time to time.  Also note that this is not the case when blockade mode is set, which is what you are seeing.

In your two plots most recently posted what were the values of DECREASE WATER TEMPERATURE and Target Indoor Air Temperature.  Also what are the values of 'minimum flow temperature' if there is such a limit somewhere, and flow temperature hysteresis.

I will be frank and say I'm not at all convinced its a fault, it could just be the heat pump doing what it says it will do (which is admittedly quite difficult to unravel).  Is it actually causing a problem with the air temperature of the house?  Both heat pumps and modern boilers measure and react to many things so the behaviour is not always as simple as we are used to with older equipment, usually for very good reason if we choose to understand it.

Posted by: @jamespa

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what were the values of DECREASE WATER TEMPERATURE

Indeed, what were they! Everything relates to air temp in the manual.

The target indoor temp was 10 Degrees; the WC was set very low as an experiment to see the effect). In the two graphs the flow and return temperatures were 21 degrees.

I have graphs showing the flow and return temperatures at 17 degrees and the comp remains off

I’m comparing like with like. In fact, during some of the set back periods when the comp remains off, the flow and return temperatures was as low as 14 degrees. The house hardly loses any heat.

Like many technical issues, discussion’s evolve and before long the original problem grows exponentially. So, to reset. IF I could establish what the flow temp is when it activates the compressor at various Set Back and WC settings then I can see what is going on.

The 'min flow temp' setting will probably (I would say almost certainly) override the temp setting on the WC curve and possibly the reduction in target ft when iat goes below set value.  Thus target indoor temp and the WC curve may be irrelevant in the conditions for the graphs above.

I'm still can't see any behaviour which cannot be explained by the heat pump trying to maintain a given ft within certain hysteresis parameters.  It looks to me like this may be 30C with a downside hysteresis of 10C.  Do you see anything that cannot be explained by an assumption like that?  If not then the question becomes what parameter in the controller is determining that a ft of 30 should be targeted (hint, probably the min ft setting)

Sometimes it helps to think of the system as an dumb heat pump which tries only to maintain the flow temperature it's told to maintain, and a controller which works out what flow temperature to tell the heat pump to target based on all sorts of parameters that sometimes it can be difficult to unravel.  Mitsubishi make this explicit by calling their controller a 'flow temperature controller'

Trying to reverse engineer a modern heat pump or boiler takes quite some time.   If I take my vaiilant as an example there is a bit of a collaborative effort to do that on heatpumpmonitor and I still don't think they can explain everything Unless this is actually causing an actual problem I would personally note for future reference but not get concerned.

It's also worth adding that, unless your house is in a temperature controlled warehouse, the same conditions will never occur on separate days.

One final thing, the OAT looks a bit too steady in one of the plots, stuck at around 10.  Of course that does happen but would be worth keeping an eye on.

I’m going to take a look into what you’ve said over the next few days. I’ve also invited others from another forum who’ve been offering help who have not had access to the graphs.

Just for now, forget the technical discussion and look at if from a user’s point of view.

Mr Smith sets his WC and Stat on his Aerona 3 R32 / Smart Controller to achieve a house temperature of circa 20 Degrees. During the night, Mr Smith prefers it cooler, so the installer tells him to have a 3 degree Set Back (17 Degrees).  Mr Smiths house maintains a minimum of 19 Degrees throughout the night. However, under the settings he was told to use by the installer, his HP comes on during the night (Set Back) heating the house up to 21 Degrees. How should Mr Smith set up his HP?

IF, I never made any other settings, I get the same issue has Mr Smith.