@johnmo Ah! You didn’t go to the right Duvet Counter - it’s no good getting the TOG values sold on the ordinary counter, you have to look for the ‘Rich-Customer-who-can-afford-a-heat-pump’ counter. This is where you will find the Tog 5 down to Tog 0.005 duvets! (The lowest values of course are the most expensive ‘cos they are for those customers who can afford to heat their houses the most) 🤪 (Sorry) Toodles.

We have been here before, on the question of continuous vs intermittent heat pump running, and the effects on costs and comfort. My conclusions to date (it is a far from settled matter) are that while the physicists and engineers can bludgeon us with the laws of thermodynamics (which no sensible person denies, though few of us like being bludgeoned with them) their models (reams of spreadsheet whatiffery as in what it the OAT is X and the IAT is Y etc etc to the end of time) are not perfect, not by a long shot. There are a lot of variables involved, and no one, so far as I know, has managed to develop a model that incorporates all those variables correctly. A further problem is model complexity: the models get so complicated that no one but the person who created them can understand them. That's bad science, because it can't be readily and independently verified, and we end up with a sort of pseudo priesthood who pass down the laws from on high to us mere mortals who must believe what we are told or else face the music.

In place of high priests, I prefer experiments, and analogies that the person on the Clapham omnibus can understand. I think I can say without being shouted at that everyone agrees that long setbacks (a less than ideal term, because it can mean either active heating but to a lower, setback temperature or no heating ie on/off/on running, however that is achieved, either by use of a timer or turning the thermostat down) save money. If you go on holiday for two weeks and turn the heating off (or more likely leave it at a frost protection level) you will save money, and won't suffer discomfort, because you are not there.

The problems start with shorter periods of setback, for example, overnight, or during the day when no one is at home. On the face of it, surely a setback must save money, but as is so often the case, it is not that simple. My own preferred analogy for what happens is this. Imagine the house is a leaky old bath, with the water (indoor air temperature, IAT) at a constant level, maintained at that level by a tap that lets water in at exactly the same rate as the old bath loses water from its leaks. That is the continuous running heat pump setup. Let us now put the tap on a timer, so it goes off overnight. The bath still leaks during the night, and the water level (IAT) goes down. Come the morning, when we want a full bath again (IAT back to what it should be), we have to add extra water via the tap at a faster rate than needed during a period of steady state running. This extra effort (over and above what would have been used with continuous running) then starts to cancel out the overnight savings. If, for example, you saved ten units during a setback, only to have to put in an extra ten units during the catch up period, they cancel out, and you save nothing.

With a long setback (while you are on holiday) the savings are massive, yet there is only one catch up period, and you clearly save money. The problem is what happens with shorter (say overnight) setbacks? The setback is short, the savings modest, and for each setback there is yet another period of catch up. The models can't tell us, and even if they could, can we trust them? Do we really understand how the model gets to its conclusions? If we can't understand what happens in the model's black box, how can we know whether we can trust the conclusions?

Which is why I prefer experimental evidence, but even that is not perfect. Nonetheless, if models are black boxes, then at least experiments are transparent (though perhaps not as transparent as @transparent). Here is a period of running from my heat pump when I had an overnight setback in place. I also have an auto-adapt mechanism (a python script) that adjusts the the LWT (leaving water temperature, in effect the amount of heat delivered) depending on how far the actual IAT is from the desired IAT. If the actual IAT is too low, then the LWT goes up, and this causes a hump (increase) in energy use during the catch up period. These humps can be clearly seen in this plot from last April (trailing sixty minute values, green bars = energy in, red bars = energy out, blue line = COP, middle of the day spikes are domestic hot water heating): 

Furthermore, not only does the energy in go up during the catch up period, the COP generally goes down. I was using more energy, less efficiently, because the heat pump was working harder during the catch up period.

That much I know, I can see the evidence. I can also tot up the energy use per 24 hours, and know my daily energy consumption. The problem is, how much energy would I have used had I not had the setback? I would have used more energy overnight, but less during the setback, but how do I quantify this, so I can compare 24 hour energy use with and without a setback? This is after all a natural experiment (an arrangement whereby you observe things as they actually happen in the real world), and natural experiments have an infuriating habit of failing to supply what happens in the control conditions, in this case, what would have happened without a setback, while everything else stayed the same.

There are two solutions. The first is to do a bit of whatiffery on my own account, ie what if I hadn't had a setback. I can 'model' what happens when I don't have a setback, and then apply the pattern that I find then to the conditions present when I did have a setback. To cut a long story short (see posts passim ad nauseam), it looks as though I do save a bit, but not a lot, indeed even it approaches the 'is it really worth all this faffing about' threshold, and furthermore, given the conclusion is based on whatiffery, it is heir to all the failings inherent in whatiffery. The second solution is much much better, but it takes a long time: I wait until I have collected enough data from both continuous running and setback running to have periods for both in which all the other relevant factors remained constant. This situation then does approximate pretty well to a proper experiment: two groups of data, with only one difference between them, in this case setback or no setback. I am hopeful that come the end of this heating heating season, I may just have enough data to do this analysis.

There endeth that lesson. A couple of other relevant and I hope not too contentious points:

(1) despite the above, which is only about heat pumps, if you switch from timed fossil fuel heating eg two fast and furious (which fossil fuel systems can do) heating periods per 24 hours to continuous heat pump heating, you will use more energy, and your bills will increase. If the cost per kWh from fossil fuels is less than the cost per kWh for electricity, you will suffer a double whammy, more kWh used plus higher cost per kWh, and your heating bills will go up, possibly by a painful amount. I know, because that is exactly what happened to me.            

(2) One less talked about benefit of continuous heat pump running is that it can seriously restrict or even eliminate condensation. With a fossil fuel (oil) central heating system run on a timer, I was very comfortable, but at this time of year and onwards, I had terrible window condensation in the mornings. With the heat pump, especially with continuous running, I have very little or no condensation. I also sense (do humans have such a sensor?) that overall the house is less damp, and that feels more comfortable.          

Posted by: @cathoderay

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We have been here before, on the question of continuous vs intermittent heat pump running, and the effects on costs and comfort. My conclusions to date (it is a far from settled matter) are that while the physicists and engineers can bludgeon us with the laws of thermodynamics (which no sensible person denies, though few of us like being bludgeoned with them) their models (reams of spreadsheet whatiffery as in what it the OAT is X and the IAT is Y etc etc to the end of time) are not perfect, not by a long shot. T.....

Apart from the slur on engineers/physicists (I have a degree in physics and spent the first part of my working life in engineering), I agree with almost everything you say and reach similar conclusions.  (BTW I dont use complicated models which nobody can understand to explain this particular phenomenon, I use one simple principle, namely conservation of energy!)

@jamespa - I owe much of my meagre but at the same time much improved understanding of the thermodynamic principles involved to your many generous contributions to this forum, for which I am extremely grateful.

@cathoderay Which ever system and depth of complexity is employed to model all those factors mentioned above, as you say there are still the unavoidable [and possibly inexplicable] factors to be allowed for (unknown unknowns perhaps?). If this ‘factor’ is perhaps called ‘And Goodness Knows What Else’, perhaps the AGKWE factor might have a value of 10-15%. By applying this AGKWE factor to the bottom line of the most carefully constructed models would show that any and all models are showing results within a milli nadger of each other! Cynically , Toodles.

@jamespa Intuition and common sense are the enemy of reason, why do I keep forgetting that! 🙂

I look forward to running some more experiments when the weather gets a bit colder!

I for one have thoroughly enjoyed this exchange and just hope it has answered the original questions from LickOfPaint.

Thanks to all

Posted by: @jamespa

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...

Finally just to emphasise, the only debunking I wish to do is to debunk the idea that turning off heating (whether heat pump or boiler) for say 25% of the time saves 25% of the cost.  It doesn't, the saving is much less because the house continues to lose heat even though the heating is off.  With a heat pump any modest saving may be wiped out entirely because it has to work harder to recover, so care is needed.

Absolutely, and understood. Quite right too.

Posted by: @jamespa

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...

As to 'do I have any evidence that money rather than comfort is the primary driver' then I would say yes some, for some people.  The evidence (in as much as it exists) is that some people come onto this forum (and others) and make extravagant claims about how much money setback is saving, sometimes going so far as to say that they don't mind feeling a bit cold in the morning in order to make the saving.  This suggests that their primary motive is cost saving. 

...

And I've seen the same thing. My uncertainty is whether these people are just a vocal minority or in some way representative since, anecdotally, people wanting setbacks for comfort reasons are unlikely to recommend that setup to others since comfort is a very personal thing. As such, the situation is almost engineered to publicise only one side of the discussion. That's not, of course, to say that those after financial savings are necessarily in the minority; simply that I just don't know.

As for your subsequent guidance points, I think they are potentially very helpful and worth making more visible. I doubt there are many people who will entirely disregard their comfort to realise savings any more than there are many who will pursue comfort with money being no object. For most I imagine it's a matter of choosing the right compromise to fit our circumstances and as such guidance that pulls both strands together is likely to be well appreciated. Thank you.

Posted by: @mike-h

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Sorry, but it is true!

@jamespa I do apologise for a really crass error. You are absolutely right that 24kWh of energy produced at a COP of 4 would require 6 kWh of consumption. I do understand the difference between power and energy, but I must admit to doing the calculation in a muddled way and still don't understand how I managed to do the equivalent of adding 2 + 2 and making 5!

@majordennisbloodnok @davidalgarve @bontwoody @cathoderay @toodles @mikeh (and anyone else I have missed out) thanks for the good natured and productive discussion over this which I hope shows the benefit of sharing ideas.  My thinking on the matter, which of course must encompass things beyond the thermodynamics, has evolved considerably over the past months and evolved further and to and crystallised even more as a result of this exchange.

Heating, which we take largely for granted, is actually quite complex!  The physics may be simple but the real world scenarios much less so.

Posted by: @majordennisbloodnok

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I do get concerned that the constant drip-feed message on this forum is "setbacks are bad because they are inefficient" if (and here I simply don't know) most people's priority is comfort and they're being put off achieving that. Do you have any evidence or insights that might help identify the split between savings-seekers and comfort-seekers?

I fear that this evaluation is about to become a lot more messy.

In the DESNZ Consulation Delivering a Smart and Secure Electricity System (ended 21st June) Paper-1 proposed 3rd-party remote control of appliances in the home. DESNZ asserted without evidence that the UK needed to reduce domestic energy consumption during the early-evening peak. The evidence would've been useful because DUKES shows only 29% of electricity consumption is domestic.

DESNZ illustrated their proposal by referring to heat-pumps being switched off in return for financial compensation.

I wonder how much those at DESNZ understand about HP operation, efficiency, COPs etc.
The financial compensation would need to exceed the cost of bringing the home back up to temperature following a relatively short "set-back" of being switched off for a couple of hours.

As shown in those histograms from @cathoderay the COP is lower during recovery from such as set-back.
I doubt that most members of the public would factor that in if they opted-in the proposal from DESNZ.

Moreover, as @lickofpaint observed, condensation also becomes a problem.

I fear that it will be households who are already in energy poverty who are most likely to be attracted by having their electricity account credited by the HP being switched off. Indeed, those with a prepayment meter would see it as increased credit on their IHD, which "looks good" if you don't understand the physics.

If such households are tenants, then they may also assume that it's the landlord's responsibility to deal with the effects of increased condensation, such as black mould around window-frames.

There are ethical issues here too, of course.

Should the cost of energy be proposed as a motivation factor by DESNZ, whilst they remain ignorant of the long-term increases in respiratory problems from fungal spores in the home?

I opposed the DESNZ proposals in my responses to the 58 questions on Demand Side Response.
But the deep level of technical knowledge required to understand the questions means that it's unlikely there were many respondents outside of the Energy Sector itself.

The energy sector doesn't historically have a good reputation when a proposal needs evaluating on issues like ethics, poverty and threats to health. 🤔 

Posted by: @transparent

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I fear that this evaluation is about to become a lot more messy.

In the DESNZ Consulation Delivering a Smart and Secure Electricity System (ended 21st June) Paper-1 proposed 3rd-party remote control of appliances in the home. DESNZ asserted without evidence that the UK needed to reduce domestic energy consumption during the early-evening peak. The evidence would've been useful because DUKES shows only 29% of electricity consumption is domestic....

You are correct that ToU tarrifs, and more sophisticated tarrifs still, will further complicate the situation and exactly one of the things that Homely purports to deal with.

It is, in my view, inevitable that ToU/sophisticated tarriffs become either compulsory or very heavily incentivised and we should be embracing this transition not resisting it.  One of the major 'back-end' challenges of electrification of heating and cars is grid capacity.  The grid must be designed to meet the maximum demand and, if that increases dramatically as it will if our use is unmoderated, then the costs will soar.  Currently there is nothing to stop, or even dis-incentivise, the whole country from using a heat pump, charging an electric car and turning on the kettle at precisely the same time during the half time break in a cup final, leading to the potential for massive peaks.

The scale of the upgrade can be reduced substantially by taking advantage of the energy storage we have or will have, namely the fabric of our houses and our electric cars.   Moderating the demands of these down during peaks occurring for other reasons has the potential massively to change the grid balancing needs, and thus massively reduce infrastructure cost.  Done correctly there will be little or no amenity penalty, for a major infrastructure saving.

Homely is demonstrating that it can be done in practice for heating with no amenity or cost penalty, and the crude ToU tarrifs offered by the likes of Octopus are demonstrating that it can also be done for car charging, albeit with some loss of convenience.  Expect more of this and ever more increasing sophistication, with less need for user intervention and an impact on amenity which more or less vanishes.  Of course it needs to be done well, but it must be done otherwise we are all going to pay.

There is no question in my mind this is the way forward.  If you don't want it personally then please expect to pay handsomly.  Please don't, however, force the majority to pay for your luxury by objecting to the principle, which is sound and addresses a real problem which is otherwise going to be very expensive indeed to solve.