@jamespa Thanks for your thoughts James.  The idea of monitoring with Home Assist is certainly attractive and new to me.  However I know my IT skills (and patience) are very limited so the idea will sit on the back burner for now.

A further factor is that the HP never brings our temperature up to the set one of 19C.  It hovers about 18.6 but never reaches 19 unless there is a marked increase in ambient temperature (middle of the day) or obvious solar gain.  I believe Vailant have built in a way to stop an overshoot of the temperature and wonder if this is detrimental to the way my system is running.

Posted by: @davidb

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 I believe Vailant have built in a way to stop an overshoot of the temperature and wonder if this is detrimental to the way my system is running.

Yes it does, but you can disable it.  Sensocomfort-Settings-Installer Level-Installation Configuration-Circuit 1-Room temp mod.  Set to inactive to run on pure WC.  I run on inactive for most of the season, switching to 'expanded' only very late season when solar gain becomes significant.

Posted by: @davidb

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Another factor which might be working against us is the heat demand of our house is low.  It was subject to a deep renovation with lots of insulation. I am wondering if, in the range 5 to 10C, the unit is struggling to dial back the heat produced to match the building heat loss is cycling instead??  Does this make any sense??

It makes perfect sense and is where monitoring comes in at least to understand whats going on.  The best you can expect is that the system cycles above about 10-12 C.  An oversized system will cycle at a lower temp.  Vaillant appears wuite good at maintaining COP through cycling, but if your system volume is small this could well compromise performance.

https://energy-stats.uk/vaillant-arotherm-basic-settings/

Mick Wall has written up a good article about the modes, better than I can. Please ask more questions if you need to. There’s info on the app too.

@judith Thanks for the article.  So far a rapid skim read suggests this may well be helpful.  Not least to explain some of the capabilities of the software.  I will let you know how I get on and what changes seem to make sense.  Thanks again.

@davidb you have said that your house is very well insulated and therefore your heating will be lower compared to your DHW usage.

Michael de Posesta has a good example of his extremely well insulated house here. 
https://protonsforbreakfast.wordpress.com/2023/11/01/heat-pumps-cop-envy-is-pointless/  
your house may be similar and in which case your COP is probably a correct reading

if you download his spreadsheet and put in appropriate values for your house you’ll find out.

Alternatively your COP value may be somewhat under reading due to one or more of the following factors 

*the temperature monitoring thermistors (measuring water flow out and return) are not very well matched 

*the temperature monitoring is not tightly attached to the pipework in the ashp

These two above are under reading yet the COP would be fine, and the following is an additional heat loss and will affect your consumption but not the COP, as far as I understand it.

*your observations of not very good external insulation and current damage 

My inclination is that your very well insulated house means your DHW dominates. Have you split each part out in turn and noted the COP of each? Our heating was 4.87 and hot water was 3.57, and heating is 20x the power usage of hot water. If your heating goes off after you light the fire there is even less usage. The more you DHW dominates the closer you will be to that value 

@johnnybudith Thanks for all the information.  It’s really helpful to know it’s out there.  Up to now I have been trying to get help from Vaillant customer service - which has been pretty variable!  The graph was eye opening!  I say that as our usage of heat is about 9kWhrs per day in January.  By the look of things that is low by comparison to the graph data so I assume many households.  We used 280kWhrs in January with hot water of 40kWhrs so roughly 15%. Screen shot below of the data.  

The ashp measures exit and return flow temperatures which are usually designed for up to 5C. Thermistors have a measurement uncertainty of about 0.5C, depending on type and measurement circuit.

Re matching: If both sensors over read by 0.5C then the measurement difference is still 5C, but if one under-reads and the other over-reads then the temperature difference may be 4C or 5C, 5C +/-10% depending on on which way around they are fitted. The heat generated is calculated from this temperature difference and your 3kW may be within the range 2.7 to  3.3. The value of accuracy for the sensors follow a normal distribution so mostly they tend to the middle. But if the sensors were matched they are in the middle, and the heat output and hence the COP is accurate.

I suspect ours reads ~5% high and yours could be up to 10% lower than reality.

MdP’s is arithmetic manipulation based on the performance of his 5kW Valliant in a 3kW highly insulated house. His proportion of DHW is much higher I think, than either of us.

Our energy consumption is over 3 times yours for January it was particularly cold here at the beginning of the month. But the warmer later period lifted the COP average.

There are many installations where the design and implementation is poor and the owners would love your value of 3.?

An engineering training is valuable in optimising and you can never have too much understanding (imho).

I’ve got a similar system, but it’s a 5kW unit, I understand that is the same hardware as the 3.5. I get a mildly disappointing 3.8 cop. This seems to be caused by the system being oversized, so it always cycles. 

in your case I think adding the wood burner does make things worse as far as cop is concerned, as by adding heat to the house you in effect reduce the heat demand of the house as far as the heat pump is concerned and hence increase cycling. 
secondly your fireplace is probably on when the outside temperature is highest and hence would get the best cop. 

I wasn’t sure if you’re already doing it, but if not I suspect actively switching off the heatpump when the fireplace is on. 
With home assistant you could automate that, for example by putting a thermostat near the fire and making an action to switch the heatpump off when the temperature rises above e.g. 22C and switching it back on when it falls below 20C.

Also scheduling the DHW cycle for the afternoon when the outside temperature is highest might help somewhat. 

I agree with the comments made by @adrian both about the fire and the DHW.  You should also bear in mind that, with a low loss house, COP isnt so important.  Its a bit ironic that very low loss houses can be some of the more difficult ones when it comes to efficiency, systems generally aren't designed to go that low and of course introducing supplementary heating just makes the matter worse.  Your 3kW Vaillant is actually a 5kW machine derated, as are almost all others out there with a couple of niche exceptions that are probably not yet ready for mainstream.  If you tried to find a boiler matched to the job, it would be even less possible!  Resistance electric is the only mainstream option I can think of where you might have a perfect match, but then you will be getting a COP of 1!

Some people with very low loss houses and UFH in a concrete slab (do you have UFH in a concrete slab) find that heating part time works well, effectively treating the slab as a storage heater.  Obviously this can be particularly relevant if you can take advantage of ToU tarifs.  The challenge is of course that UFH, unlike storage heaters, doesn't have vents that you can close to reduce the output, so you are reliant entirely completely on getting the WC curve right so that the charge is at the right level.  Someone else on this forum has been struggling with exactly that, albeit with a different (and well oversized) heat pump.  Its possible of course, but takes some tweaking, and certainly wont deal with a log fire which may or may not be on.

@jamespa thanks to both you and Adrian for the comments and suggestions.  In one regard I am confused about your comments on our wood burner and impact on COP.  Once the stove is generating heat, the thermostat cuts out the HP - so probably 8 to 12 hrs a day.  During this period the heating is off so why would that have an impact on COP?  Obviously I am missing something.  Can either of you explain?

incidentally James, you mentioned the capacity of water in the system.  It’s about 300 litres in 7 radiators plus some underfloor pipes.  I had to import mains water to stay within the Vaillant spec.  We don’t have mains water and get our supply from a spring which can be acidic.  The easiest was to fill an IBC and the installation team simply pumped that into the house - hence I know the rough capacity.  It’s a small house and system but is that volume likely to have an impact?

Hi David, as I understand it running the system in active mode means that it is not running in pure weather compensation mode and is hence a little less optimal wrt cop. Also the fire goes out there is a period were the effective “heat loss” of the house is low enough to cause cycling. 

As James says being well insulated, you are not going to get a perfect COP, as a house that has a higher demand would have. (Which why the question of whether you can install a heatpump in a badly insulated house is mostly nonsensical).

one thing you could try is to not have active mode, but have it set to none, set a schedule that sets the temp to let’s say 5C from 0:00 -02:00 and when you start your fire set the temp manually down, that way it would go back to normal at 02:00 and you might get something closer to pure weather compensation cop. I wouldn’t want to guarantee that it makes a big difference, but might be interesting. 

@davidb 

Once again I agree with Adrian.  Houses have memory and different bits cool at different rates.  So for example (this is just one mechanism) if your fire heats the house above the 'normal' temperature both the walls and the air will heat up.  When the fire is switched off the air will likely cool quicker so the heat pump will switch on whilst the walls are still warmer than normal. They will thus lose energy to the room (whereas normally they would be taking energy from the room) thus reducing the load on the heat pump thus reducing its efficiency.  Depending on how long the fire was on for it could take several hours for 'normal' conditions to be restored.

Another thing to bear in mind is that measurements of COP are dependent on two sensors being matched.  Sometimes they arent as well matched as we might like and, when load is low, that begins to affect the measured value significantly.  So the value you read might not be that reliable.

It might help understand it better if you gave us the following information all of which is available in the App

• For the month of January:  What was the electricity consumption and environmental yield for space heating and DHW respectively
• What is the total running time and the number of on/off cycles (this is in status, aroTHERM plus)
• Roughly where are you

With that information we can estimate your actual (as opposed to surveyed) house loss, the likely cycling, and a few other things that may clarify further,.