Great data.  I look forward to being able to generate some like it myself one day.

Posted by: @mike-patrick

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I've also shown the best fit line but in fact I think the distribution of results would be better explained by a curve.

I might be getting carried away in my excitement, but it looks from the plotted data like there's a non-zero lower asymptotic value (which makes sense as there will be a non-zero lower limit on the energy consumption), so an exponential with an offset (y=a+c*exp(b*x)) would probably give a good fit.  Polynomials aren't so useful for this type of response.

You can select an exponential fit in Excel after adding the trend line, which should increase the R-squared value a bit, but this doesn't include the offset, so it won't be a perfect fit.  Unless you have access to some more capable non-linear curve-fitting software, a quick fix would be to estimate the lower limit (say, from the mean of the values above 15C) and subtract this from all of the values before refitting the curve.

I'll be quiet now.

You are perfectly correct, there is a non-zero element which needs to be considered. It is due the the 'human infestation' along with their toys and gizmo's which often tend to produce thermal energy inside the thermal envelope of the home.

@mattc Here's the same chart with the Excel exponential line added.

I could subtract the background 8-10 kWh per day that is used when the ASHP is off and we are away but wouldn't that just drop the whole curve by that number of units. As we don't have separate metering of the pump, yes there is also some daily variation in total kWh due to daily activities, with cooking probably being the major one. Anecdotally I'd say that also varies seasonally - with more baking during the winter (hot meals) than the summer (mostly salads).

It's interesting how these exercises answer some questions but produce more as well. Having just acquired a weather station, with the ability to monitor the outside humidity, I wish I had been collecting that data too over the last 3 years. I've seen during the recent cold spells what a big difference it makes to the kWh. On low humidity cold days the ASHP is much less likely to go into a defrost cycle, with the resultant lower kWh - maybe as much as 20kWh compared with similar temperatures on a high humidity day.

Mike

This is precisely why it is not possible to predict Power Input or Power Output, with any degree of accuracy by merely using outside air temperature variations. There are many more variables that need to be taken into account.

Posted by: @mike-patrick

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I could subtract the background 8-10 kWh per day that is used when the ASHP is off and we are away but wouldn't that just drop the whole curve by that number of units

Yes.  That would just drop the curve on the graph, so you'd have to re-label the axis to compensate. The idea of subtracting a constant from the results is just a sneaky trick that I hoped would fool the software into producing a better-fitting curve.  However, after trying it out, it turns out that Excel won't fit the exponential curve in the presence of negative or zero values, so unfortunately it won't help us (I suspect it's just log-transforming the y variable and using linear regression, rather than using a proper non-linear algorithm).  The best option would be to fit the appropriate curve using some more capable software - I'd be happy to give it a go if you don't mind sending me the data - or I could point you to some software if you prefer to do it yourself.

To all Forum members and others.

Could I please ask any forum members or guest who have received advice to spend a little time and report back upon the quality and helpfulness of the advice received.

Many thanks in anticipation.

Returning to an old theme (it must be the onset of colder weather) I have done a new analysis of our kWh usage over a year. The chart is attached.

Firstly I've just noticed that our total kWh to date for 2024 (as at 8th November) is within 65 units of the figure for 2023 at the same date. That's a difference of only 0.78%, despite the significant variation in the weather when comparing individual months on a like for like basis. It seems that it "all comes out in the wash".

The data are again the daily records of Max and Min outside temperature and our kWh usage (all uses since we don't have a separate meter for the ASHP). Numbers are for 2023.

On the chart the bars show the number of days in the year that the Minimum outside temperature occurred. The line shows the cumulative kWh for the year at all temperatures up to each Minimum. For example there were 35 days when the temperature fell as low as 5C and that days when the minimum temperature was up to 5C accounted for about 52% of our total annual kWh.

I put this together because I was mostly interested in the very coldest days. These are when our ASHP uses astounding amounts of electricity. Thankfully there are very few of them in this part of the country (rural West Oxfordshire). There were 30 days when the minimum temperature was 0C or less. These account for 17.2% of our total annual kWh. There were only 14 days when the minimum was -1C or less. These 14 days (3.8% of the year) account for 9.15% of the total annual kWh. The single day when the temperature fell to -6C accounted for 0.89% of the total annual kWh.

My experience then is that it is simply not the case that ASHPs perform well at low outside air temperatures. This could be further confirmation that our installation/set-up was not done correctly and/or that this is just a feature of our particular ASHP (COP rating 2.66).

I would be really interested to know the experience of others with their ASHPs at sub-zero air temperatures.

Mike

@mike-patrick I don’t have any graphs for ‘historic’ data as Homely Dashboard only spans a few weeks then consigns it to ‘bin’ I suppose [I have suggested to Homely that it would be very useful if historic data could be fetched but I suspect the data storage is the problem here].

Anyway, I do remember that last November / December when we had -7 and -8 degrees C over a few days, our consumption jumped to some ~40 - 42 kWh per day. Normally, our consumption doesn’t exceed ~ 20 kWh in a day. Mercifully, those -7 or -8 degree days are rare but even then, our cost is ~ £5.20 a day at today’s tariff cheap rate time via the battery. I feel this is good value for the comfort level we experience.

I found the biggest disadvantage of heating with a heat pump is that having been cushioned and comfortable indoors with no apparent effort to keep warm - leaving the house to walk or catch a bus comes as a great shock!!! Regards, Toodles.

@mike-patrick  It is a fact that ASHPs are less efficient at low air temperatures, thats all documented in the specs and is a direct result of the thermodynamics.  Its also a fact that houses lose (and so need) more energy at low air temperatures.  Finally any ASHP should be set up with weather compensation, which means that the flow temperature will be higher at low OAT, and that also degrades COP.    

Put the three together and its entirely expected that the days when its very cold will result in the consumption of a disproportionate amount of electricity.   For example the Mitsubishi Ecodan R32 8.5kW has a COP of 4.2 at 40C FT/ 7C OAT, and a COP of 2.4 at 45C FT and -2C OAT.  A house which loses 8kW at -2 will lose only about 4kW at +7.  Put these together and you expect the ASHP to consume 3.5 times as much electricity at -2 as it does at +7.   Since +7 is the typical temperature in the heating season its absolutely no surprise that 3% of the days account for 9% of the energy use!  Bear in mind, however, even the house loss alone (nothing to do with the ASHP) accounts for a factor of 2 in this ratio which will be the same whatever the heating source.

That's why SCOP (seasonal coefficient of performance) was created, to give a balanced view of the average over a typical heating season.  

As to whether yours is particularly bad its difficult to tell from what you have posted.  If your 'cop rating of 2.6' is actually  SCOP (average COP over the season), then this isn't particularly good.  I note that this a Grant.  Grant has, certainly in the past, tended to advise that its ASHPs are set up to run like boilers, and provided hardware to make that easier.  This is a very bad idea if efficiency is the target.  The underlying hardware is reputedly fine, its the way grant tell you to install it that is the problem.

Do you know how yours is set up.  Is weather compensation enabled and properly adjusted, is there a buffer tank (or low loss header - even worse)?  If you post a bit more info (ideally a system diagram) several on this forum may be able to advise  

@mike-patrick - an interesting way of looking at this question. I don't plot my data the same way, though probably could, but from the time series plots I do do it is very clear my heat pump uses a lot more energy in colder weather - as of course it would, but the increase is eye-watering. The COP also plummets, again as expected. Mine gets into CQC (can't quite cope) territory when defrosts start at around 5 degrees OAT.

However - one thing I do notice about you most recent chart, with the histogram for days with each minimum OAT, is that they do not look normally distributed beyond a vague overall bell shape. It seems most odd that the min OAT with the most days (10 degrees 45 days) has two of the smaller counts on either side 9 degrees 9 days, 11 degrees 11 days. Of course that might happen by chance, and it may be that minimum OATs are not normally distributed, but the variance would certainly make me double check my sensor and data (and also the normality assumption).     

The heat pump is running on weather compensation. My data readings are unsophisticated. I take daily kWh readings from the smart meter to track day and night usage. The outside temperature I record manually with a MaxMin thermometer. The house is fully insulated having been totally refurbished in 2017 and has underfloor heating. It is 100% electric. I only note the temperature to the nearest degree. This avoids spurious precision. But I'm sure that if the figures were to the nearest 0.5 degree then much of the apparently odd distiribution would be greatly smoothed.

The ASHP is an older Grant model. I've looked all over for a data sheet with a SCOP value but can't find one. The COP of 2.66 is from the sticker on the back of the unit. Looking at more recent Grant models they appear to have improved dramatically since then. The latest R32 model with a similar output has a claimed COP of 5.28 and a SCOP of 5.2! If true it would be enough to offset the price penalty of electricity compared with gas. In my dreams.

I understand that the KWh will be greater when the OAT is below Zero but at what cost? In that data series the one day when the OAT hit a Minimum of -6C, was 21st January 2023, when we used 93 kWh. That compares with our average daily usage across the entire year of 29 kWh per day. (About 8 to 10 of that is for all other non-ASHP uses). Despite this the DHW still did not reach normal temperature and we struggled to get the room temperature to its usual 20C.

I have daily data from the start of 2021. But there was definitely a problem with the set up then (since rectified) as our annual kWh is now only 2/3rds of the level at that time. The annual figures since 2022 have been very consistent.

I'm less concerned these days with further efforts to tweak my system (I can accept a monthly energy bill, now,  of circa £200) but I'm still interested to see how my experience compares with others. It's one of the few benchmarks that enables you to judge whether the results are good, bad or typical.

Given my experience I'm still puzzled by claims that ASHPs work well even in sub-zero temperatures. How is this so in locations such as continental Europe and Scandinavia? Do they have much lower humidity than in the UK?

My ASHP goes into a defrost cycle at the drop of a hat as temperatures approach Zero. It's already happened once this season on 10th October when the Min OAT was 2C (humidity 97%) But in persistent cold conditions it can be stuck in frosting/defrosting for much of the day.

Mike

It might well be a quirk of the Chofu unit in that it is less able at lower temperatures. My 2019 Ecodan was sold to me on the grounds that it was designed for northern west European damp cold conditions and sized appropriately.  It has never failed to comfortably hit the target IAT no matter how cold outside,  although yes there is a hit economically when that happens.

This is a fairly large timber frame, leaky construction but last January when we had 15 days with sub zero temperatures it coped no problem although it chugged through 60kWh/day in doing so.  £15 per day on the coldest days I can live with, yes I am fortunate, perhaps other Grant users can comment on your units performance better.