@cathoderay I've put a couple of charts showing our Gas Boiler usage in 2020/21 compared to ASHP usage in 2022/23. Both systems were/are doing Heating & DHW and there have been no changes to the fabric of the house so this should be a reasonable comparison. Weather over the periods is of course a factor and I am trying to find a decent set of historical temperature data so that I can do a comparison (any suggestions appreciated).

The data I have shows the ASHP (for November - April) using approximately a third of the gas equivalent period from 2020/21. And this is with a relatively low COP of 2 - 2.5, so I think I have plenty of room for improvement.

On the output side the ASHP has produced 86% of the gas heating output over the equivalent period. For info I used 88% as the efficiency of the gas boiler (it was 16 years old and was not a condensing boiler). The difference here could be caused by weather or how efficient the gas boiler actually was (something we will never know), but on the whole the output heating power seems to be similar to that of the gas boiler.

My ASHP data comes from the Midea App which is not the most accurate source in the world but is all I have at the moment, and for large scale comparisons such as this it is probably not too bad.

So at the moment I'm not seeing the paradox that you and others are (subject to all the usual caveats about the quality of the data).

@cathoderay feel free to think what you need.

You assume and estimate that the heat used is somehow similar, without measuring it. It's not only about the efficiencies of the two systems, but the amount of heat each put out, that you cannot measure by feel. I feel comfortable in a bracket of temperatures, running a house at 19 vs 21 and how constant or not, brings a huge difference of heat output.

Posted by: @alan-m

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I am trying to find a decent set of historical temperature data so that I can do a comparison (any suggestions appreciated).

Try the Met Office WOW website (<=link). You can search for a weather observation website near you and download the data (you have to register but its free). 

Others including obviously you but also @batalto and others have demonstrated around one third energy use when using a heat pump compared to using a fossil fuel. That's why I have a paradox. I am convinced I am missing something so obvious I just can't see it.

Posted by: @alan-m

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My ASHP data comes from the Midea App which is not the most accurate source in the world but is all I have at the moment

Indeed, but if it is any reassurance, when I compare the Midea app kWhs used with my external dedicated heat pump kWh meter, the two are reasonably close (within maybe 5% or less of each other) but the relationship isn't constant, sometimes over reads compared to the other, sometimes the other way round. 

Posted by: @alan-m

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I am trying to find a decent set of historical temperature data so that I can do a comparison (any suggestions appreciated).

I havent found a source of hourly data over an extended period of time which is available for free (you can get about a months worth at a time here: https://data.ceda.ac.uk/)

But degree days are available here https://www.degreedays.net/ and really tell you everything you need about the weather to do many calculations of interest.

I plotted the correlation between degree days and my gas consumption (I still have gas heating, no thanks to MCS) and its fairly linear as expected.  The correlation is better if I work on the previous days degree days, not entirely surprising my house appears to have a 12-24 hr time lag to respond to changes in outside temperature (I really need to determine this exactly some time).  Also note that, from degree days and your annual power consumption for heating you can, assuming that you keep the house reasonably warm most of the time, easily calculate the whole house load at any given design temperature.  This is a sense check on the MCS whole house calculations.  From the limited evidence I have seen it may well give a more accurate result. 

Posted by: @cathoderay

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I raised this paradox (the 'disappearing COP') in another thread a while back and didn't really get a satisfactory answer. The best I got was something along the lines of something else must have changed to explain the paradox, but no one really knew what that something was, apart from perhaps changing from timed heating using oil to 24/7 heating using the heat pump. I was left wondering, and still do wonder, whether we have got something fundamentally wrong with our assumptions about heat pumps. I would therefore very much welcome your thoughts.

It deserves an explanation if collectively we can find one.  You seem to have done many of the obvious things right, and it would be instructive to find one as its an interesting case study as well as an apparent anomoly relative to the many who report measured SCOPs of 3-4 as expected. 

You have told us about your set up, can you give some more info to get some context.

• Roughly where are you?
• What was your annual oil consumption?
• What is your annual electricity consumption for your ASHP and is this measured on your dedicated input meter?)
• What is your heating pattern (and rough temperature profile across the day/night) prior to the swap and now?
• Is your DHW heated by the ASHP also and have you got any idea what the heating pattern is for that?

You dont mention a buffer tank, can we presume that you don't have one?

You mention Freedom Heat pumps, what heat pump brand is it?

Posted by: @fazel

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You assume and estimate that the heat used is somehow similar, without measuring it.

The heat energy used (energy in) is measured, and accurately (meter on the oil delivery tanker, external dedicated kWh meter for the heat pump). The heat delivered (energy out) for the oil has to be estimated because the boiler no longer exists, and I didn't have monitoring at the time, but of one thing we can be sure, no oil boiler is 100% or more efficient, 80% is a fair enough figure, so we can make a reasonable stab at guessing how much heat energy was delivered to the house. The heat pump energy out is measured, both by the Midea app (but see notes above) and also by me over modbus (which is Midea data, but without any Midea app interference) but the numbers (flow rate and LWT/RWT delta t) make sense and are close to ad hoc physically measured values (I have a physical flow meter in the primary circuit, and have put various thermometers on the flow and return pipes. They may be a bit out, but they are not out by a factor of around three.    

Posted by: @fazel

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I feel comfortable in a bracket of temperatures, running a house at 19 vs 21 and how constant or not, brings a huge difference of heat output.

Maybe, but humans are generally considered quite sensitive to temperature. I can usually tell when the house is a degree or so below the design temp, and by the time it is three degrees below I start to feel chilly. To that extent, I am a thermometer, in that my senses can tell me what the temperature is. All I can say is that I do not find the house to be noticeably warmer or colder with the heat pump than it was with oil. I am of course a bit older, and cold tolerance may decrease as we get older, but we're not talking factors of three here.

The only big difference between the oil system and the heat pump system is timing. This may or may not be a red herring: maybe the oil boiler put out more heat per hour for shorter periods, while the heat pump puts out less heat per hour but over longer periods, but the 24 hours output is the same. 

And we are still stuck with the fact the agreed subjective but probably reasonably sensitive thermal comfort is about the same. If at the same time the building heat loss is reasonably constant, which it is (except for any secondary glazing improvements) then the heat delivered must be about the same. But it isn't (the second of my two earlier charts). And the difference is far from trivial. That's the paradox.

@jamespa

Using degree days for calculations with gas or oil boilers is not really a problem, but such does not work particularly well with ASHP's.

@derek-m Surely that depends on what you are trying to calculate.  Degree days will tell you about demand (load) profile whatever the heating technology.  If you already know your annual usage from fossil fuel measurements it should give you a good estimate of the load at design temp too (possibly better than the theoretical calculations)

I grant its not much good for simulations of COP, for that you ideally need hourly temp data which, so far, I cant find a free source of.  I suspect however you could get a pretty reasonable simulation of cop using a combo of degree days and max/min temps, making some assumptions about the rate of change of temp etc/times of min or max.

Posted by: @jamespa

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It deserves an explanation if collectively we can find one.

I certainly welcome any such attempt! I think it may be considered an interesting case, often it is the anomalies that lead to better understanding, so while I clearly stand to benefit, other may also benefit.

Firstly, my measured heat pump SCOP, measured/calculated both from the Midea app kWh in/out and from data collected over modbus is around 3 (over shorter time periods it varies of course). Secondly, the external heat pump kWh meter agrees reasonably well with the Midea data (see above for comments on discrepancies). The heart of the paradox is that it appears that the heat pump needs to deliver about three times more energy than the oil boiler did to achieve broadly the same heating levels.

To answer your questions:   

• Roughly where are you? Central southern England
• What was your annual oil consumption? 1000 to 1200L
• What is your annual electricity consumption for your ASHP and is this measured on your dedicated input meter?) 2/5/22 to 1/5/23 use = 8823kWh, readings taken from external dedicated heat pump meter
• What is your heating pattern (and rough temperature profile across the day/night) prior to the swap and now? Oil: on morning and evening, sometimes all day (but off overnight); heat pump on 24/7 (but see earlier comments: oil may have been on off but run hot, and heat pump slow and steady, but over 24 hours maybe much the same). I don't have room temp data for the oil period, but I do for the heat pump, as I have a data logger logging hourly temps in the kitchen, and most of the time (cold snaps and solar gain excepted), the room temp is within a degree or so of the design temp (19 degrees). All that said, as I said in a previous post, the timing is the obvious difference between the two systems (and it is why elsewhere I have discussed how to implement an overnight setback, as yet not practical as the system takes too long to recover, it needs an 'adaptive' weather comp curve implemented over modbus).   
• Is your DHW heated by the ASHP also and have you got any idea what the heating pattern is for that? Yes, and it is on a timer, comes 'on' between 1300 and 1400 each day, but only actually 'fires' once in every two to three days because I do not use a lot of hot water. Tank set temp is 50 degrees, delta t to trigger heating is 10 degrees, ie tank has to be below 40 degrees for DHW heating to come on. Oil use: can't remember if it was on a timer, but what I do know is that during the summer (ie when central heating was off) I used an immersion heater, but important to bear in mind the DHW component of my total energy use is relatively small.   

You dont mention a buffer tank, can we presume that you don't have one? No buffer tank (or volumiser), just a plate heat exchanger as mentioned before.

You mention Freedom Heat pumps, what heat pump brand is it? Midea M-Thermal Mono (block) 14kW

I am sure some of these things do make a difference, but a three fold difference? Or maybe a lot of small things add up to something quite big?

Posted by: @jamespa

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for that you ideally need hourly temp data which, so far, I cant find a free source of

The Met Office WOW website I mentioned has that. You need to register to download the data, but it is free to do so (csv format). I do it once a month for a local weather station. The data I get is for every few seconds but pivot table > then group by hours and days (average) gets hourly data. 

@cathoderay Thanks

The first observation I would make is this: 1100l oil per year is 11.4MWh.  Even if your oil is 100% efficient that would suggest that either

your whole house demand at say -2 (assumed - what is your actual design temp) is very roughly about 6kW.  If so 14kW may be well oversized and I am told (unverified) that Mideas dont modulate down much.

your house was previously not only not heated, but also well below the design temp for much of the time

Any ideas which?

Posted by: @jamespa

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The first observation I would make is this: 1100l oil per year is 11.4MWh.  Even if your oil is 100% efficient that would suggest that either your whole house demand at say -2 (assumed - what is your actual design temp) is very roughly about 6kW.  If so 14kW may be well oversized and I am told (unverified) that Mideas dont modulate down much.

Interesting, I stupidly didn't do that calculation. My calculated heat loss, done for the heat pump installation, is around 12.3kW, but that is also at minus 2, and most of the time the ambient temp is not minus 2, so the actual total kWh heat loss (and so demand) over a heating season will be much smaller. But we do still have the heat pump actual data - 8823kWh (8.823MWh) May to May. At a SCOP of 3 that becomes into 26.5MWh...

I think my Midea can get down to about 25% of it's nominal output, then it starts cycling, need to check.

I, and I am sure others, would be interested to know how you get from annual MWh use to a (kW) demand (the reverse is also potentially of interest, given a kW heat loss figure, how do you derive an annual MWh use)? Is this where degree days come in? It might also throw some light (good or bad) on EPC annual use figures.   

Posted by: @jamespa

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you didn't in fact heat the house for much of the time

True, though it was on a fair amount of the time. I do wonder if thermal mass may have a part to play here. I don't know, but is it possible the oil boiler overheated the house a bit when it was on, and so banked some heat in the fabric for when it was off? I'm thinking something along the lines of (for example, numbers chosen for easy calculation) two daily six hour heating periods putting out a total of say 48 kWh (4kWh x 6 hours x 2) amounts to the same 24 hour output as a heat pump putting out 2kWh per hour for 24 hours, and the thermal mass takes care of managing the warmth?