Posted by: @cathoderay

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@jamespa - I've been giving it some thought. As I said it was a long time ago especially the oil period. I probably put the basic timer settings in decades ago. I still have all my marbles, but remembering that sort of thing from that long ago is a long shot by any measure.

1. Best to ignore the standard mains electricity period. It was an infill period when the oil tank was not really usable (minor leaks and I mean minor,  although it did have a small amount of oil that I very occasionally used in very cold weather, but I didn't dare fill it) and getting the heat pump. As many others have found, that was a long and complicated process with both planning and listed building consent needed, cowboy installers, all the usual crap. The house using only standard mains electricity was very cold in winter (enough to make me think I'm doing myself in) because mains electricity is very expensive and I am a mean old bugger so I only used to heat one room, the room I was in. In cold spells, the kitchen would get down to 7 degrees in the morning. It was a highly atypical period, not at all comparable, and not much use for our purposes.
2. I live in a mid-southern county of England, in a small leaky old listed building. Heat loss is always going to be high, unless I do things that I am not going to do. What I have done is pumped up the loft insulation, and I am in the process of adding secondary glazing, which in turn needs window frame works to get a suitable level finish on ye olde frames, which in turns means I can reduce draughts, which in this house are a major cause of heat loss. The heat loss calcs done by my installer and submitted to MCS have a design OAT of -2, in earlier version it was -1.8. The -4 is not a design temp as such, it is my weather curve left hand end point OAT, I wanted the set LWT to carry on going up a bit after the design OAT (otherwise it flat lines).
3. I think my oil CH was probably on for less that 12 hours a day. For almost all of my working life, I worked away from home, ditto my partner, and I would not have had the heating on during the day (frost protection was by a frost stat). I have a very vague recollection the evening on period was 1830 to 2230, 4 hours. The boiler could heat up the house quickly, in 20 mins or so, and of course I was younger, and less likely to feel the cold, so the late start time was OK. I am now only guessing, but I think it is possible the morning on period was also 4 hours or shorter. If so, then the oil heating was only normally on for 8 or less hours, 1/3 of the heat pump time when running without a setback. I do also recall I did occasionally use the 'extra hour' on the time in cold weather, but I doubt that in the grand scheme of things that amounted to very much. I do have an open fire, but that tends only to be used on special occasions, it is not in regular use.

I think the daily hours of use is probably the main explanation. If I only ran the oil CH for 8 hours a day, 1/3 of the heat pump running hours, then that difference seems to me to a plausible explanation for the extra energy delivered by the heat pump. Another possibility is that, although I never felt cold with the oil CH, the house might in fact have been a degree or two less, that the current 19 degrees, but being younger I didn't feel colder. I don't have any IAT records from the oil CH time, just a recollection that I don't remember feeling cold.     

Thanks.  (I think you have probably provided the explanation for the apparently anomalous consumption (10MWh vs ~24MWh which would be expected for a 12kW house).  I jokingly mentioned earlier an uninsulated barn, it sounds like your house is fairly close.  It clearly has (or had) poor insulation and probably a fairly low thermal mass.  Thus when you turned off the oil heating the house would rapidly cool down perhaps not to OAT but quite close, rapidly reducing the heat loss.  Given that you ran the boiler for only 8 hours its quite plausible that the average temperature of the house was sufficiently low to account for a consumption about half to 40% of the expected value.

I also suspect you might have been rather cost conscious.  For most of the recent past Oil has been notoriously expensive compared to gas, and so there is additional pressure to reduce consumption.  perhaps, as you say, you were content with the house a degree or two lower than you are now, even when it was occupied.

Incidentally this sort of house is one where the general 'low and slow' rule for heat pumps probably does not apply.  Because it cools quickly and has low thermal mass, even with a heat pump there are savings to be made by timed operation (the 'limit case' is a house which cools instantly and heats up instantly, in which case just turn on the heat pump when you want it warm!).  However I suspect that this longer fits your lifestyle.

Earlier I said the following:

"The divide by 2000 approach (open university says 2200) comes from a simple conservation of energy calculation, based on a typical UK climate, and the assumption that, in any setback/timed heating the house cools by only a few degrees and not anywhere close to reaching OAT.  It also assumes that the heat pump is operated reasonably efficiently so it delivers it's rated output to the house  (or alternatively that you measure the output to the house not the input)and that the room temperature is around 20C. 

If those assumptions are true then the method should be pretty robust because the laws of thermodynamics are pretty robust, the average climate varies surprisingly little from year to year, and setbacks where the house temperature doesn't drop too much only reduces consumption by a modest amount even with fossil burners.  If any of the assumptions are not true (I may have missed one or two) then of course it may be misleading, but if this is the case then it may suggest an action that could be taken to improve matters."

I think your case fails the assumption that 'in any setback/timed heating the house cools by only a few degrees and not anywhere close to reaching OAT', which is why the rule of thumb doesn't work.  Although somewhat overlapping, perhaps it would be wise to add a further assumption that the house is heated for a substantial part of the day, not for only a short period.

I think the conundrum is, to all intents and purposes, solved, not in precise numbers but in general terms which, if we had more complete data, would have a good chance of stacking up.

@jamespa - something else to add to the mix. The fact the conundrum post plot used very early data, with energy out determined by multiplying the energy in by the COP, rather than calculating it independently, has been bugging me. I am not sure how reliable it is. I've just done an ultra quick and dirty overlay, adding (as the blue line) the weekly data from winter 23/24, a period when I had full and comprehensive monitoring running. There is still a conundrum, but much less of one, that I think can be more than explained by the ideas we have come up with:

Apologies again for the lack of visual appeal, I just wanted to get something posted. Also ignore the week 53 low value artefact.

@cathoderay 

Winter 23/24 was milder than 22/23.  Previously you multiplied input energy by SCOP, whereas in fact it should be multiplied by COP.  In general doing it correctly will depress the calculated output energy on cold days, and raise it on warm days.  The pattern change is not inconsistent with this and without doing a full analysis (which I do not intend to do) I cant see anything major awry.

This also affects your calculation of loss based on input energy vs OAT.  At FT55/-2 your machine will have a much lower COP than 3.2.  I dont know exactly what model you have but according to the databook for MHC-V14W/D2N8-B MHC-V14W/D2RN8-B the COP at -2/55 is about 2.3 and max output about 11.3kW (bit of estimated interpolation going on to reach these figures).  So your curve suggests a loss at -2 of more like 4.5*2.3=10.4kW.  Again without going into more detail than I wish, I cant explain the difference between this and your spreadsheet, except by the general statement that the spreadsheet method makes assumptions about the fabric which are often way out, the difference between 10.4 and 12 is well within this uncertainty and my calculation involved some interpolation by guesswork not by arithmetic.  Feel free to do the interpretation arithmetically to come up with a more accurate projection from your power vs OAT curve.

This does leave the final question of why there is not sufficient output at the lowest temperatures to keep the house quite up to the design temp.  This could of course be the emitters not the heat pump, although presumably your auto adapt script would take care of that by raising the FT.  Other possible explanations are loss in the PHE (which one would hope is small), the pipework (ditto) or something else.  I note from the databook that the max capacity drops off very sharply at FT>55, by about 2kW or so once FT reaches 60.  That might be the reason particularly if the drop off occurs early in the 55-60 range.  You may wish to take careful notes if there is a cold spell this winter, to see if anything can be done to improve the situation (if you care that is).  Removing the PHE would almost certainly fix it instantly, but I note you don't want to do this.

From my perspective I cant now see a discrepancy of sufficient magnitude to be worth particular comment, but I can see a line of investigation to get it working at the lowest temperatures..

Posted by: @jamespa

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Previously you multiplied input energy by SCOP, whereas in fact it should be multiplied by COP.

It was early days, I had little data, and only very basic (S)COP data. However, even allowing for that, the discrepancy was sufficient to create, at least for me, a conundrum, which I agree we have now explained, and I agree, no further analysis required on that one. That said, I don't think we should drop the message that for some people, swapping from tightly timed fossil fuel systems to heat pumps may not realise any savings, and may in fact cost more, because, in addition to all the other factors in play, the heat pump will oblige them to run their heating differently, always on or sometimes mostly on, and that will increase costs - and of course lead to a warmer for longer home with less condensation.

I haven't for ages thought my (S)COP is 3.2. Overall it is just under 3, but it varies greatly depending on the OAT, as can bee seen in the week's data I posted earlier. Earlier today I also started looking into but didn't post data on my heat pumps actual hourly output, where the headline is the highest it ever got to (at very low OATs) was 9.21kWh (can rephrase as an average of 9.21kW for an hour), which is a lot less than the 11 and bit kW suggested by the Midea engineering data (same figures as you quote above, 11.3kW) at these OATs (and my LWTs). The energy in at that time, by the way, was 5.13kWh, giving a COP for that hour of less than two.

Like you, I am left with a sense that things don't quite stack up. I wonder if COP volatility has something to do with it. I'm going to sleep on it, and if I have the enthusiasm (probably will have at some point, but don't bank on it!) do a plot of calculated from basics energy out (delivered) against OAT and then do a regression on that. Using a calculated energy out (from flow rate, LWT and RWT delta t, and SG) avoids using COP entirely, thus removing any pernicious effects it may have on the plots.          

Posted by: @cathoderay

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That said, I don't think we should drop the message that for some people, swapping from tightly timed fossil fuel systems to heat pumps may not realise any savings, and may in fact cost more, because, in addition to all the other factors in play, the heat pump will oblige them to run their heating differently, always on or sometimes mostly on, and that will increase costs - and of course lead to a warmer for longer home with less condensation.

Whilst this may be true I don't think we can be certain of that conclusion based on your one example. 

You are not comparing apples with apples.  Your house is fairly unusual in that it appears to cool very rapidly and (possibly) have a low thermal mass.  That's not the majority.  Your fossil fuel system controls were optimised for minimum cost in a situation where the house was vacant most of the time.  You haven't attempted to do this for the ASHP because your lifestyle has changed.  Because of the thermal characteristics of your house, its almost certain that, if you did, you would see your consumption drop quite significantly.  Slow and low is right for most houses, but not for houses that cool (and possibly heat up) rapidly and are rarely occupied! 

That said I dont personally think ASHPs should be marketed as cost saving other than to people who currently heat with resistance electric.  At current prices they are very roughly running cost neutral for gas.  The price of Oil is so volatile that I wouldn't want to comment.

I appreciate this thread may appear to be going off piste but I think if we take OP's original statement ('My approach is then, to simulate an ASHP with my current gas system boiler over the coming heating season.') and the thread title as the more general question of given various data what can we infer about our heating system, then I may be skiing on the edge of the piste, but haven't actually gone off it yet. I happen to have a heat pump as a source, but exactly the same thing can be done for a gas boiler, provided you have data for the same variables.

I have now plotted energy out vs OAT. I have done this for the month of Jan 2024, because it was a cold period, and I had the heat pump on all the time (no setbacks). The data is recorded every minute, and then summed (for energy out) or averaged (for OAT) for the hour. The energy out is calculated from the flow rate and LWT/RWT delta t as reported by the Midea controller using a specific heat of 4.05 (10% glycol). Because the energy out is calculated from fundamental data, no COP values have been used, and any pernicious effects caused by guesstimating COPs are entirely removed. I should also point out that the IAT was not constant (steady), because Jan 24 was a cold month, especially in the middle, and there were times when the actual IAT fell a degree or two below the desired IAT (of 19 degrees, but overall the average IAT (18.65) wasn't that far out). This failure to reach the desired IAT happens when my heat pump has reached the maximum it can put out, and can't go any further. During this cold period, the LWT approached 60 degrees, and at these high LWTs the heat pump starts performing very badly, with the COP falling below 2. Here is my standard plot of variables for the month. Note that there are a lot of defrost cycles (LWT goes below RWT) particularly during the cold spell, and that what might look like short cycling (saw tooth pattern) isn't, typically the heat pump cycles once or twice an hour, but as the chart covers a whole month, that's a lot of cycles, which appear densely packed. 

With the background out of the way, here is the plot of OAT against energy out ie the energy delivered to the house. As noted above, this does not use the COP to determine the energy out, instead, the energy out is calculated from the relevant variables. I tried both linear and polynomial regressions, the latter has a slightly better fit, no doubt because the relationship isn't linear, because the COP falls at lower OATs. Thus the effect of the falling COP can be seen in the plot, even though the COP was not used to determine the data used in the plot. 

Now, the first obvious question is this: why does my 14 kW heat pump, which Midea admits falls to 11 and a bit kW in low OATs with high LWTs, only putting out on average about 7kW (for those struggling with energy (kWh) and power (kW), the plot is energy (kWh), but 7kW for 1 hour is 7kWh) when the OAT is zero, with the peak, rarely reached, being 9kWh? Bear in mind the data behind the energy out data in this plot all comes live from my heat pump, as reported by whatever sensors Midea have in the pump, but at the same time I have some independent verification of the data (analogue flow rate meter and IR thermometer readings for the LWT/RWT) that suggest the Midea unit is reporting reasonably accurate data.

The second obvious question is what does this mean for my actual heat loss? The calculated loss at -2 is 12.3kW, but according to this chart, with the IAT not too far off steady at 18.65 degrees, the energy in needed to maintain this IAT is around 7kW, suggesting my actual heat loss is close to 7kW.

Go figure... 

Posted by: @cathoderay

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The second obvious question is what does this mean for my actual heat loss? The calculated loss at -2 is 12.3kW, but according to this chart, with the IAT not too far off steady at 18.65 degrees, the energy in needed to maintain this IAT is around 7kW, suggesting my actual heat loss is close to 7kW.

Three reasons I suspect:

1. I presume that the calculation assumes a room temp of 20/21 not 18.6.  That does make a difference

2. Calculated heat losses are a bit of a fiction unless the fabric construction is known (and properly used in the calculation).  The biggest uncertainty appears to be air change, the typical MCS assumption is 2-3ACH but many houses achieve much less than this (I have heard it said that <1ACH is quite common in reasonably, but not excessively, draught-proofed houses).  Thats why I have a serious beef about the insistence on using the GIGO spreadsheet!

3.  Plotting hourly energy vs temperature can be a bit misleading if low OATs only occur for a few hours (which is the normal situation in the south of the UK).  Because the house has 'thermal mass' (more accurately a heat capacity - ie it stores energy), it will 'smooth over' short term variations in OAT.  This will tends to depress the energy apparently used at low OAT unless there is a long period of such (several days!).  A long period of low OAT last occurred in Southern England in December 2022.  This may well account for the shape of your best fit line, which should definitely be straight unless you have phase change materials built into your fabric!.  You may be better off doing 6 hour averages or using degree-days.

Nevertheless you have presented other data suggesting your house loss is less than 12kW so I think this is very likely an overestimate.  You would need to refine both experimental determinations to home in on the actual figure

Posted by: @jamespa

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Whilst this may be true I don't think we can be certain of that conclusion based on your one example. 

You are not comparing apples with apples.  Your house is fairly unusual in that it appears to cool very rapidly and (possibly) have a low thermal mass. 

I think you are making assumptions here. Review the data for Jan 2024 above, and posts passim that include setback periods. The house has a relatively high thermal mass for its size, unsurprising because of a solid ground floor, and thick solid stone walls. they are both slow to heat up, and slow to cool down. I don't think it is that unusual, in fact I would go so far as to say it is very typical of most of the houses in my immediate vicinity (very similar sizes and construction), and furthermore a lot of the national housing stock in rural areas will be similar, average size (3 bed) small cottages with traditional construction.

Furthermore, although we all appreciate extrapolating from n=1 is potentially hazardous, the fact is it is only potentially hazardous. We have to apply common sense. Many properties like mine will be off mains gas, and they will do exactly as I did if/when they go over to a heat pump. They will replace a fossil fuel, most likely oil but sometimes LPG, on a strictly timed regime with a heat pump on a mostly or always on regime. The question is, why won't they have a similar experience to mine?

Despite that, I did hedge my statement somewhat: 'I don't think we should drop the message that for some people, swapping from tightly timed fossil fuel systems to heat pumps may not realise any savings, and may in fact cost more...'. Some, may - I'm not being didactic and saying everyone will, just some may. That's what happened to me, if you do the same thing as me, then you may experience the same outcomes.

Posted by: @jamespa

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You haven't attempted to do this for the ASHP because your lifestyle has changed.

NO! While it is true my lifestyle has changed (I've retired from normal work), the driver for how I run my heating is my heating system has changed. Having a heat pump is what causes me to have mostly always on heating. I even made determined attempts to add setbacks, in other words move back towards timed heating, only to be told repeatedly that I was wasting my time, although in due course they were some occasional reluctant admissions that in certain scenarios, it may not be a total waste of time.

I know it can be seen by many as heresy to say anything negative about heat pumps, but I am of the opinion that honesty is the best policy. Heat pumps do have many good things going for them - and heck, I even have one! - but for many people, saving money isn't one of them. Better to manage expectations than have irate post heat pump installation homeowners throwing bricks through windows!      

There is a factor which is glaring to me which is average heat loss with IAT of 18.5 (ish) versus std calculation temp of 21 for living areas. So 7kW must be scaled up by 21/18.5.

. There’s a huge scatter on your low temp results too, much more scatter that higher OAT (~10C) so perhaps some doors or window opening effects, meaning a true larger heat loss?

Posted by: @jamespa

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This may well account for the shape of your best fit line, which should definitely be straight unless you have phase change materials built into your fabric!. 

That's a good point, I forgot i am only dealing with a steady state linear relationship that doesn't involve a declining COP. Here's the linear regression version:

I agree about the looseness of the heat loss spreadsheets. Or as i would say, far too much whatiffery involved. Another cause of error is typos, easy to do because there is a a lot of tedious manual data entry to do. One of the installers - supposed to be OK - I had quote for my system produced a heat loss calculation based on more typos than you can shake a fist at, the clue being that a larger room with more outside walls and more windows had a lower heat loss than a smaller room with fewer outside walls and windows.

But, at the end of the day, we are still stuck with an empirical finding that suggests my actual heat loss may be getting on for half the heat loss based on whatiffery ie the standard MCS/Freedom/universal spreadsheet calculations (what if my walls a solid stone of such and such a size, what if the air changes are this, and so on). Could it be that your suspicions, and my empirical evidence, mean that the whole heat pump heating industry is using heat loss calculations that are way out of whack, perhaps getting on for twice what they should be? Even worse, that works, because heat pumps never actually put out what their manufacturers say they do? In other words, the industry matches incorrectly specced heat pumps incorrect heat loss calculations?  

I don't think we've got to the bottom of this one, not by a long way.       

Posted by: @judith

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There is a factor which is glaring to me which is average heat loss with IAT of 18.5 (ish) versus std calculation temp of 21 for living areas. So 7kW must be scaled up by 21/18.5.

There’s a huge scatter on your low temp results too, much more scatter that higher OAT (~10C) so perhaps some doors or window opening effects, meaning a true larger heat loss?

The design/heat loss calculations IATs varied between rooms, range 19 to 22 degrees, to satisfy MCS requirements. The average was 20.4, say 2 degrees above my actual IAT in Jan 2024 of 18.5 (ish). Applying the scaling, 20.5/18.5 = 1.11, 7 x 1.11 = 7.77kW is still way below the 12.3kW calculated loss.

There is a lot of scatter, but these are real world data, and real world data are often messy, sometimes very messy. There's not much I can do about that, except collect lots of data, and hope to benefit from some regression to the mean (repeated sampling gets you closer to the mean). The residuals are also not too bad: 

It is plausible the actual linear regression line isn't that far from the truth...       

Posted by: @cathoderay

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But, at the end of the day, we are still stuck with an empirical finding that suggests my actual heat loss may be getting on for half the heat loss based on whatiffery ie the standard MCS/Freedom/universal spreadsheet calculations (what if my walls a solid stone of such and such a size, what if the air changes are this, and so on). Could it be that your suspicions, and my empirical evidence, mean that the whole heat pump heating industry is using heat loss calculations that are way out of whack, perhaps getting on for twice what they should be? Even worse, that works, because heat pumps never actually put out what their manufacturers say they do? In other words, the industry matches incorrectly specced heat pumps incorrect heat loss calculations?  

Yes, Yes, Yes.

MCS admit that the current method is not entirely satisfactory, I have it in an email.  But they say they 'dont have an alternative'.

If you listen to the video about sizing on this forum you will hear what Graham Hendra (who supplies heat pumps amongst other renewables and is well respected, I believe, in the industry) has to say about the spreadsheet method; his comments start 3 minutes in.  You will also hear about my personal journey with heat loss (so far).

@johnmo is (almost certainly) right to say that wrong-sizing doesn't make as much difference to efficiency as some people might make out,  but that's not the point!  Going from 8 to 12KW, or even 16kW, has so many knock on effects on radiators, pipework, planning permission, physical appearance and (calculated) noise.  If your actual loss is 3kW then none of this matters because everything is anyway small. if your actual loss is 8kW (a more typical house) then it matters big time.