Posted by: @cathoderay

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Thanks for doing the modelling.

Posted by: @derek-m

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I think that I have discovered the primary reason why your heat pump cycles at temperatures above 4C, according to the OAT sensor, the root cause is the Plate Heat Exchanger, which I believe that I am correct in saying is still installed within your system.

It is still installed. Could you please elaborate on how why the PHE is the root cause? I don't have an absolute objection to removing it, but there are also some good reasons for keeping it. I also want to be sure about the relative importance of various factors. For example, I have today put forward a suggestion that maybe almost all heat pumps cycle at most temperatures to varying degrees, we just didn't know before because we didn't have good enough data. Certainly @harriup's right hand chart shows clear cycling in an Ecodan that that has LWT/RWT that looks remarkably similar to mine posted earlier, occurring over a range of temperatures. If cycling is inherent to heat pumps, then that might be the root cause, with the PHE perhaps aggravating it. I can't ignore the observation that all three (OK n=3 is small, but it is larger than zero) heat pumps we have data for show cycling of various forms across a range of OATS. Someone needs to pull the black swan out of the bag!

I will be happy to explain the reason why I think the above is the case.

The Modeling Tool was developed based upon a basic system, no buffer tank, no PHE, no room thermostats, no zoning, just a heat pump supplying heat emitters. The heat emitters should therefore receive water at virtually the same temperature as the heat pumps LWT, which therefore limits the maximum thermal energy output from all the heat emitters combined. 

Taking the calculated heat loss value it is possible to estimate the heat loss and hence energy demand for any given IAT and OAT combination, the IAT, you are measuring with a degree of accuracy, but the OAT is from the heat pump sensor which I do not believe is accurate and could be out by 4C or 5C at low temperatures.

Based upon the calculated heat loss of 12400W @ -2C, at an IAT of 21C? (please clarify if different), the heat loss at an OAT of 4C would be in the region of 9165W, but the actual IAT in this case is 18.8C, so the heat loss would be more likely in the region of 8087W.

The heat pump thermal energy output calculated from the LWT - RWT DT, and flowrate given in the raw data at an OAT of 4C, gives a value of 6749W.

There is obviously quite a difference between the two figures, so how can we account for this difference. Looking at the detailed data in the Modeling Tool for your system, a heat loss of 6749W would require an OAT of approximately 6.5C at an IAT of 19C.

So if the correct heat loss is 6749W, the water temperature at the heat emitters would need to be 43C, but the heat pump LWT was an average of 47.4C over this 1 hour period. The difference, I believe, is caused by the PHE.

The higher required LWT means that the heat pump has to work harder to produce the hotter water. If the air going into the heat pump is at 4C, and the LWT is 47C to 49C, the minimum thermal energy output shown in the Midea data tables is over 7600W, but if instead the LWT was 43C, the minimum thermal output would be more likely 7000W. Much closer to the 'not cycling' point.

I do agree with your finding that the turn down on the 14kW Midea heat pump does leave a great deal to be desired.

The problems created by the PHE, and the cold intake air, will further exacerbate your heat pumps inability to keep your home warm in colder weather conditions.

Posted by: @cathoderay

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Posted by: @jamespa

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I was faintly conscious that you had come to the conclusion that it was in fact undersized, but not sure on what basis.  I wouldn't trust the Freedom Heat Pumps model to discriminate reliably at this level (nor any other fabric model - there are too many fabric uncertainties).  Id be surprised based on your previous comments if you would trust a fabric model either.  But if you have actually measured it then that's a different matter!

Of course I am sceptical of heat loss calculators - after all, they are classic whatiffery models! What if the walls are made of this and are that thick, what if there are two air changes an hour, what if the roof space is only partly insulated, the whole thing is pure whatiffery. But we also have to have something to to help us choose the right size heat pump, and I took a pragmatic view that a spreadsheet based heat loss calculation would have to be 'good enough', otherwise i might as well roll the dice twice and add to results together to get my heat loss in kW.

As you say heat loss calculators are a tool we have and when you can't make a measurement you have no choice but to resort to a model.

Posted by: @cathoderay

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Real heat loss measurements are of course not easy to do. I seem to recall there was a discussion some time ago on whether you could use the energy input to the building (ie energy out from, not in to, the heat pump) when both IAT and OAT were stable as a guide to if not measurement of the heat loss in those prevailing conditions, and it might just be possible to extrapolate based on an assumption of a linear relationship between heat loss and OAT given a stable IAT. Interestingly, my IAT has been steady all day at about 18.5 to 18.8, the current Midea OAT is 6, and the predicted loss (from the Freedom calculator) at that OAT is 7.5kW, and the actual current 'ability' value from the wired controller over modbus is 7.44kW.       

I think you can do this but it needs to be done over several days at least to take out the noise, and we dont get many runs of days when oat is stable day and night. 

With a fossil fuel system, measuring the energy in over a season (and if you wish using degree days to adjust for annual variations) is going to give a pretty good answer, almost certainly I would say better than any model.  With half hourly readings of actual gas consumption and IAT in a house heated by gas, I would be pretty certain that a suitably intelligent system could work out everything you need to know including a very good estimate indeed of the load and the house thermal mass.  I would be very surprised indeed if octopus (who have the data from the smart meters, albeit not iat data, unless they have already fitted a sensor) aren't already working on this as its going to give a much better whole house answer than any model, gives them a competitive advantage and plays to their tendency to favour lower capacity pumps.   I'm not even certain you need an IAT sensor as you can quite probably infer IAT by correlating variations in OAT and the demand (and mathematical derivatives thereof), if you have sufficient data.

I closely watched my consumption last season and downloaded the half hourly readings, which is why I am >95% confident that my house needs 7.5kW, 8.5kW max, not 10.5-16kW that the spreadsheet merchants tell me.  I can get to 7.5kW on a fabric model by a) putting in the right fabric details as best I know them and b) assuming 0.5 ach.  Neither of the individuals that did the full monty survey (costing me £300 in one case, foc in the other) did either, and both came out with 16kW which is pure fantasy.

As my district council has blocked my heat pump installation for another season, I am monitoring again this year and have also capped my boiler output at 8,5kW (the lowest it will go).  I think that will either result in me being 100% confident that 7.5-8.5kW suffices, or tell me I need a little more.  So far it's looking like the former.

Posted by: @cathoderay

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@harriup - thanks, especially for posting the charts. We now have data from three installations, yours mine and @kev-m's, and they all show some form of cycling, even when the heat pump 'shouldn't' be cycling, ie lower but not defrost range OATs. Your right hand chart is really rather similar to mine posted earlier, slow cycling most of the time at the rate of about once an hour, all happening at OATs that shouldn't trigger cycling.  

Posted by: @harriup

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this golden standard of continuous, even running seems to be held up as an ideal that users can expect any heap pump to achieve if only we tickle the right couple of parameters in the right way.

Indeed, and as I said right at the end of my last post in different words, we need to ask how and why has it become the gold standard. If most heat pumps generally cycle most of the time, but no end users noticed because they weren't doing adequate monitoring, then perhaps it doesn't matter, especially if it is low frequency.

 

I think you are right that cycling happens a lot but nobody noticed before. It really doesn't matter though, as long as it's not excessive.  However while yours isn't excessively frequent, I do think it's a problem for you. 

But lots of heat pumps don't cycle a lot of the time.  Here's mine from earlier in the year.  The only interruptions to its continuous running were DHW and my (at the time) night time switch off.  It's modulating down to just over 4kW, which is close to its minimum. It does this when it's between about 4 and 10 deg OAT. Any colder and it may defrost, any warmer and it may cycle.  

Posted by: @cathoderay

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Posted by: @kev-m

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I think your 'cycles' after 8pm or so are defrosts.

I'm not sure they all are. The hallmark, at least on my system, of a defrost cycle is the LWT falls below the RWT (if you think about it, it does make sense). On @harriup's right hand chart this only happens during the 23:30 cycle, and there is another clue, the OAT jumps up, because of the warmer exhaust air. That in turn leads me to suggest that this means the heat pump is drawing in perhaps quite a lot of exhaust air - how otherwise would we get that OAT spike?

  

You're right, only the last of these is a defrost, the bigger drop in LWT is the sign of the heat extraction. I have only ever looked out of the window at the top of the unit when it is defrosting but I am pretty sure the fan does not operate during this operation – given that the unit is pumping hotter fluid round the coils to heat them, the last thing it would need is to draw cold air through at the same time. If there is enough heat to drive off the ice as water vapour there is enough in the air to raise the temperature a few degrees right behind the unit for the duration.

BTW the timeframe stops at 1am as that is when a setback stops the pump.

Posted by: @harriup

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Posted by: @cathoderay

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Posted by: @kev-m

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I think your 'cycles' after 8pm or so are defrosts.

I'm not sure they all are. The hallmark, at least on my system, of a defrost cycle is the LWT falls below the RWT (if you think about it, it does make sense). On @harriup's right hand chart this only happens during the 23:30 cycle, and there is another clue, the OAT jumps up, because of the warmer exhaust air. That in turn leads me to suggest that this means the heat pump is drawing in perhaps quite a lot of exhaust air - how otherwise would we get that OAT spike?

  

You're right, only the last of these is a defrost, the bigger drop in LWT is the sign of the heat extraction. I have only ever looked out of the window at the top of the unit when it is defrosting but I am pretty sure the fan does not operate during this operation – given that the unit is pumping hotter fluid round the coils to heat them, the last thing it would need is to draw cold air through at the same time. If there is enough heat to drive off the ice as water vapour there is enough in the air to raise the temperature a few degrees right behind the unit for the duration.

BTW the timeframe stops at 1am as that is when a setback stops the pump.

 

I'm happy to stand corrected, I often am!

Posted by: @jamespa

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Posted by: @newhouse87

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@jamespa I take it you mean my house will be yoyoing in a sense with regard IAT so by having setback. My counter point is it doesn't affect our current lifestyle as we are warm when we are there. I get what you mean theoretically but i suppose in my case its theoretical as its not having any real world impact on our comfort. The lost energy is generally going to be replaced when OAT is higher so heatpump cop will be better. Im just trying to articulate that running 24/7 may not be a universal fit.

I completely agree with the last sentence, and have said so on this thread, but... (Almost) without doubt there are circumstances where setback saves energy, but there are also circumstances where, at least in principle, it doesn't.  That's essentially what I said in my initial response - be a bit careful interpreting the results and look out for contraindications (eg turning up the WC curve because you feel cold).  

In your particular case, where you don't attempt to recover for breakfast, It's much more likely to save money than a case where you do attempt to recover for breakfast.  Equally its much more likely to save money when the daytime temperature is significantly higher than the night time temperature.  

Until we explore this a bit more we cant currently put definite boundaries on it.  @derek-m has created a model which enables some theoretical exploration, @cathoderay is trying to do some experiments, and we are collectively realising that its more difficult, particularly to do the experiments, than we might first have thought!

  

Thinking about this a bit more, for a house like yours (which, based on your figures, has a high thermal mass and is thus essentially an energy integrator), we can put some figures on it with a simple model.  I think its instructive to do so as it will aid understanding, much more than guesses or excel spreadsheets will do (for the avoidance of doubt I'm absolutely not saying excel spreadsheets aren’t valuable, they definitely are, but they are even more valuable if supported by a bit of an understanding of the drivers.)

Lets do a few worked examples.  We are going to use a very simple model of the OAT, its one value during setback (night) and another higher value when the heating is on (daytime).  This is crude of course, but much less crude than constant OAT, and sufficient to give us a very good idea whats going on and very likely good enough to draw some tentative conclusions.  Certainly it will point the direction to look.

We are going to work out the ‘breakeven daytime temperature’, ie the daytime temperature where the effect of setback is neither to reduce nor to increase the required energy in.  If the daytime temperature is above this, then setback will result in a saving in overall energy consumption (compared to 24*7 operation).  If its below this then setback will result in a higher energy consumption overall.   

Below I reproduce the COP curve for a 11.2kW Ecodan (chosen because I hapen to have it plotted).  We will come back to that later

Lets suppose that nighttime is 0C, I have radiators, and at 0C the FT according to the WC curve is 40C.  Lets also suppose that the heating is on for 12hrs and off for 12 hrs, and that during the off period the IAT drops by 2C (so an average drop of 1C). 

Here our ‘reference’ situation is heating 24*7.  The reduction in energy lost from the house (relative to the reference) by setting back is 1/20*12/24 = 2.5%. 

Put another way, relative to the reference situation our setback situation requires 97.5% as much energy in total to be delivered into the house, to match the energy lost.  But since I am heating only for 12hrs I will need to operate at just less than twice the power output (to be precise 195%) as I would in the reference situation, to deliver this amount of energy.  To do this my flow temperature will need to be 53.5C instead of 40C (radiators emit power proportional to dt^1.3 where dt is flow temp - room temp). 

Using the curves below and performing eyeball interpolation we can see that the COP at 40/0 is about the same as the COP at 53/4.  So a first estimate is that the daytime OAT must be at least 4 to ‘break even’.   But this isn’t quite good enough because, if the daytime OAT is 4C and the night time temp 0C, then my COP in the reference situation (where I am operating at constant FT=40) is on average about 3.2 so actually I need a daytime OAT of more like 6C to get the same COP on average (its actually a bit higher still because we need to iterate the last step to get a final answer, but its good enough for now). 

So if the daytime OAT is 6 we will just break even, and only if its more than 6 will we be in profit.  On the relatively few days in spring/autumn (in the South East) when the night time temp falls to zero this is actually quite likely, much less so in winter where the diurnal range is smaller.  Today for example (where I live) would be a day when setback results in a higher energy consumption based on this model.

If the nighttime temperature is a bit higher eg 4C the picture is quite different.  In this case the daytime temperature will need to be something like 14C for the COP on average to be the same at FT53 as it would have been at FT40.  There are many dull autumn, spring and particularly winter days when this will not be the case, perhaps the majority.  At this daytime temperature setback just breaks even, more and Im in profit, less I make a loss.

We can repeat the exercise for UFH.  Lest suppose at 0C the UFH FT is 30.  UFH (I am told) emits power proportional to dt, so it will need to operate at 39.5 to give 195% of the output.  If the night time temperature is 0 then the break even daytime temperature is a little over 5C.  With a night time temperature of 4C, the break even daytime temperature is about 14C.  In late spring and early autumn this isn’t infrequent, but elsewhere in these seasons I think it’s fairly rare (obviously this depends on your location).

Assuming I haven’t made a mistake in the logic (entirely possible) I think this illustrates the uncertainty around whether setback actually saves energy, especially in a house with high thermal mass.  Obviously, to get a fully accurate picture, one would have to run the calculations for the specific heat pump and also model the thermal variation over a year (or a representative selection of days) and do a weighted average of the results.  if I have made a mistake I hope someone will point out the error, in which case I apologise (but suspect that the ensuing discussion will further aid our understanding!).

Posted by: @derek-m

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Based upon the calculated heat loss of 12400W @ -2C, at an IAT of 21C? (please clarify if different), the heat loss at an OAT of 4C would be in the region of 9165W, but the actual IAT in this case is 18.8C, so the heat loss would be more likely in the region of 8087W.

The heat pump thermal energy output calculated from the LWT - RWT DT, and flowrate given in the raw data at an OAT of 4C, gives a value of 6749W.

There is obviously quite a difference between the two figures, so how can we account for this difference. Looking at the detailed data in the Modeling Tool for your system, a heat loss of 6749W would require an OAT of approximately 6.5C at an IAT of 19C.

Thanks for the explanation. My desired IAT is in fact 19 not 21 degrees, so the latter figures apply, and the OAT was at times close to 6 degrees, but my estimate of the building heat loss at 6 degrees (obtained from the original heat loss calculation) is around 7400W. Nonetheless, I get the idea that the PHE restricts (this is not an efficiency question, energy is not lost, because it can't be, it is I suggest more of a throttling question) energy transfer (eg only 95% of the available energy delivered to the inlet side heat exchanger ends up at the outlet side of the PHE, the other 5% gets returned to the heat pump, and it is this inability to transfer all the available heat that leads to cycling. The fundamental problem, though, the one that actually causes cycling is the heat pump's inability to modulate it's output down low enough to avoid cycling.

One way to think about this is to consider a train analogy again. Imagine a train that travels round a circular track, collecting goods on one side of the circuit, and offloading them on the opposite side. Unfortunately, on the delivery side, there is a blockage, and not all of the goods can be offloaded, and some end up going back to the collection point. This carries on for a while until the train is so full that the collection side has no where to put goods on the train, and so it stops all loading for a while, until enough goods have been offloaded at the delivery end to allow the supply end to start adding goods to the train again.

BTB, the energy out calculation needs to take into account the fact that I have glycol in my primary circuit. In my calculations, I use a specific heat capacity value of 4.05 for the circulating fluid.

Posted by: @jamespa

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I think you can do this but it needs to be done over several days at least to take out the noise, and we dont get many runs of days when oat is stable day and night. 

I agree, and I think that was our conclusion last time the idea was discussed. I mentioned the idea because it remains nonetheless one way of getting a measured heat loss, and it is at least useful as a sanity check.

Posted by: @jamespa

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Neither of the individuals that did the full monty survey (costing me £300 in one case, foc in the other) did either, and both came out with 16kW which is pure fantasy.

Sadly this was my experience too, though I did at least make sure I didn't pay for some Smart Alec to do some whatiffery on my behalf. I also had one installer come up with the magical 16kW loss, and I think they do this because most of the time fitting a 16 kW heat pump will cover the building' demand - ie they are just applying the old fossil fuel boiler sizing approach to heat pumps. Another installer came up with a figure of 9kW, and when pressed, couldn't explain how he got to that figure. Another huge problem is that the vast majority of people in the market for a heat pump will not have done O level maths, and the maths and physics of it all will be pure mumbo jumbo to them. This creates a severe imbalance of power (knowledge), much as indeed all too often happens in medicine, such that the patient has to trust the doctor to have the patient's best interests at heart. Whether Joe Public can trust heat pump installers to have Joe Public's best interests at heart is another matter. To be fair, in some cases they will (my installer did, even though we ended up with a slightly under-sized pump), but others? Adam Smith springs to mind.   

Another real world problem with spreadsheet based heat loss calculators is that they are very tedious to fill in, meaning corners get cut (that wall looks like it is 16 inches thick rather than actually measuring it), and typos are an ever present risk. Nonetheless, with all these caveats in mind, we use them because they are shall we say the least bad way of getting an estimate of heat loss.

Posted by: @jamespa

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As my district council has blocked my heat pump installation for another season

Again, something similar very nearly happened to me, though in my case I knew exactly what 'the problem' was: bats. I had done my homework, and satisfied myself that no bats were going to be harmed by my installation, which did involve a very small amount of demolition to make space for the heat pump. The council claimed otherwise, citing an internal document not made available to the public that showed I had bats, and therefore I needed the whole bats rigmarole, which costs thousands and can all too easily end up in failure. I finally got the council to show me their internal document, and it was immediately apparent it was based on modelling: no one had actually done any fieldwork. I applied my standard model demolition procedure on the model, and sure enough it fell apart, and the council had to concede there was no evidence at all for bats on my property. My point here is the average applicant for planning permission and listed building is not going to know that models are whatiffery, let alone how to demolish them (in this case I used the fact that the model showed that one end of the small lean to that would be demolished had bats, the other end didn't, and pointed out that such an implausible situation could only exists in the realms of an inaccurate model). It's all a game of fools and knaves...      

Posted by: @kev-m

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Here's mine from earlier in the year.  The only interruptions to its continuous running were DHW and my (at the time) night time switch off.  It's modulating down to just over 4kW, which is close to its minimum.

Yes, that does look much better. It might even be a few tail feathers from the black swan I mentioned earlier. It does seem, not unsurprisingly, that not all heat pumps are created equal. In similar conditions, my heat pump would only be able to lower output to around 7kW, making cycling inevitable. But in practice it does at least seem to be low frequency cycling, though I have yet to find a way to determine in the real world how harmful (or not) such low frequency cycling is.

Posted by: @harriup

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I have only ever looked out of the window at the top of the unit when it is defrosting but I am pretty sure the fan does not operate during this operation – given that the unit is pumping hotter fluid round the coils to heat them, the last thing it would need is to draw cold air through at the same time. If there is enough heat to drive off the ice as water vapour there is enough in the air to raise the temperature a few degrees right behind the unit for the duration.

This is an interesting question, whether the fan runs during a defrost. I very rarely see my heat pump defrost, as it usually happens at night, and despite having a bad dose of heat pump deficit hyperactivity disorder, I have not yet been forced to camp out in the garden on winter nights watching for defrosts. However the few that I have seen do suggest the fan is running, as the water vapour appears to be ejected from the pump, and moves several feet away from it. Although I agree one one level it makes good sense not to draw in cold air while attempting to defrost, on another level, it does make sense to draw in air, to displace the vapour as it forms.

Looking at my modbus register address tables, I see that it includes an address for fan speed. I will add this to the data I collect, and it should give a definitive answer as to whether my fan runs during defrosts. Do you have any data you could collect to do the same thing? I think it would be useful to know, as it would aid our interpretation of that rise in OAT seen during the defrost, and perhaps also go some way towards answering the question of how much intake air comes from exhaust air when the fan is running.   

Here is my last 24 hours worth of data:

Three clear defrost cycles, each with a marked uptick in OAT. There is also a clear three hour period between 0300 and 0600 when the unit achieved steady running, without cycling. This is interrupted by the third defrost, and then it again continues steady running until 0755, when for some reason the LWT starts to rise, and we get a cycle. Is this triggered by the OAT falling from 3 to 2 degrees? Possibly, the Set LWT can be seen to rise as the OAT fell (as indeed it should). The heat pump then over-shoots, and that triggers the cycle.   

Posted by: @jamespa

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Posted by: @jamespa

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Posted by: @newhouse87

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@jamespa I take it you mean my house will be yoyoing in a sense with regard IAT so by having setback. My counter point is it doesn't affect our current lifestyle as we are warm when we are there. I get what you mean theoretically but i suppose in my case its theoretical as its not having any real world impact on our comfort. The lost energy is generally going to be replaced when OAT is higher so heatpump cop will be better. Im just trying to articulate that running 24/7 may not be a universal fit.

I completely agree with the last sentence, and have said so on this thread, but... (Almost) without doubt there are circumstances where setback saves energy, but there are also circumstances where, at least in principle, it doesn't.  That's essentially what I said in my initial response - be a bit careful interpreting the results and look out for contraindications (eg turning up the WC curve because you feel cold).  

In your particular case, where you don't attempt to recover for breakfast, It's much more likely to save money than a case where you do attempt to recover for breakfast.  Equally its much more likely to save money when the daytime temperature is significantly higher than the night time temperature.  

Until we explore this a bit more we cant currently put definite boundaries on it.  @derek-m has created a model which enables some theoretical exploration, @cathoderay is trying to do some experiments, and we are collectively realising that its more difficult, particularly to do the experiments, than we might first have thought!

  

 

Thinking about this a bit more, for a house like yours (which, based on your figures, has a high thermal mass and is thus essentially an energy integrator), we can put some figures on it with a simple model.  I think its instructive to do so as it will aid understanding, much more than guesses or excel spreadsheets will do (for the avoidance of doubt I'm absolutely not saying excel spreadsheets aren’t valuable, they definitely are, but they are even more valuable if supported by a bit of an understanding of the drivers.)

Lets do a few worked examples.  We are going to use a very simple model of the OAT, its one value during setback (night) and another higher value when the heating is on (daytime).  This is crude of course, but much less crude than constant OAT, and sufficient to give us a very good idea whats going on and very likely good enough to draw some tentative conclusions.  Certainly it will point the direction to look.

We are going to work out the ‘breakeven daytime temperature’, ie the daytime temperature where the effect of setback is neither to reduce nor to increase the required energy in.  If the daytime temperature is above this, then setback will result in a saving in overall energy consumption (compared to 24*7 operation).  If its below this then setback will result in a higher energy consumption overall.   

Below I reproduce the COP curve for a 11.2kW Ecodan (chosen because I hapen to have it plotted).  We will come back to that later

Lets suppose that nighttime is 0C, I have radiators, and at 0C the FT according to the WC curve is 40C.  Lets also suppose that the heating is on for 12hrs and off for 12 hrs, and that during the off period the IAT drops by 2C (so an average drop of 1C). 

Here our ‘reference’ situation is heating 24*7.  The reduction in energy lost from the house (relative to the reference) by setting back is 1/20*12/24 = 2.5%. 

Put another way, relative to the reference situation our setback situation requires 97.5% as much energy in total to be delivered into the house, to match the energy lost.  But since I am heating only for 12hrs I will need to operate at just less than twice the power output (to be precise 195%) as I would in the reference situation, to deliver this amount of energy.  To do this my flow temperature will need to be 53.5C instead of 40C (radiators emit power proportional to dt^1.3 where dt is flow temp - room temp). 

Using the curves below and performing eyeball interpolation we can see that the COP at 40/0 is about the same as the COP at 53/4.  So a first estimate is that the daytime OAT must be at least 4 to ‘break even’.   But this isn’t quite good enough because, if the daytime OAT is 4C and the night time temp 0C, then my COP in the reference situation (where I am operating at constant FT=40) is on average about 3.2 so actually I need a daytime OAT of more like 6C to get the same COP on average (its actually a bit higher still because we need to iterate the last step to get a final answer, but its good enough for now). 

So if the daytime OAT is 6 we will just break even, and only if its more than 6 will we be in profit.  On the relatively few days in spring/autumn (in the South East) when the night time temp falls to zero this is actually quite likely, much less so in winter where the diurnal range is smaller.  Today for example (where I live) would be a day when setback results in a higher energy consumption based on this model.

If the nighttime temperature is a bit higher eg 4C the picture is quite different.  In this case the daytime temperature will need to be something like 14C for the COP on average to be the same at FT53 as it would have been at FT40.  There are many dull autumn, spring and particularly winter days when this will not be the case, perhaps the majority.  At this daytime temperature setback just breaks even, more and Im in profit, less I make a loss.

We can repeat the exercise for UFH.  Lest suppose at 0C the UFH FT is 30.  UFH (I am told) emits power proportional to dt, so it will need to operate at 39.5 to give 195% of the output.  If the night time temperature is 0 then the break even daytime temperature is a little over 5C.  With a night time temperature of 4C, the break even daytime temperature is about 14C.  In late spring and early autumn this isn’t infrequent, but elsewhere in these seasons I think it’s fairly rare (obviously this depends on your location).

Assuming I haven’t made a mistake in the logic (entirely possible) I think this illustrates the uncertainty around whether setback actually saves energy, especially in a house with high thermal mass.  Obviously, to get a fully accurate picture, one would have to run the calculations for the specific heat pump and also model the thermal variation over a year (or a representative selection of days) and do a weighted average of the results.  if I have made a mistake I hope someone will point out the error, in which case I apologise (but suspect that the ensuing discussion will further aid our understanding!).

 

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Posted by: @cathoderay

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model showed that one end of the small lean to that would be demolished had bats, the other end didn't, and pointed out that such an implausible situation could only exists in the realms of an inaccurate model). It's all a game of fools and knaves...   

Well actually... it is perfectly possible to have a bat roost on one wall of a building and not on the opposite, depending on the building location, orientation and time of year. Most UK bats that use buildings will nursery roost in the summer on the back of a warm wall and hibernate in the winter on the back of a cooler wall. However I am glad that no bats were harmed in the making of your heat pump installation!

Posted by: @harriup

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Posted by: @cathoderay

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Posted by: @kev-m

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I think your 'cycles' after 8pm or so are defrosts.

I'm not sure they all are. The hallmark, at least on my system, of a defrost cycle is the LWT falls below the RWT (if you think about it, it does make sense). On @harriup's right hand chart this only happens during the 23:30 cycle, and there is another clue, the OAT jumps up, because of the warmer exhaust air. That in turn leads me to suggest that this means the heat pump is drawing in perhaps quite a lot of exhaust air - how otherwise would we get that OAT spike?

  

You're right, only the last of these is a defrost, the bigger drop in LWT is the sign of the heat extraction. I have only ever looked out of the window at the top of the unit when it is defrosting but I am pretty sure the fan does not operate during this operation – given that the unit is pumping hotter fluid round the coils to heat them, the last thing it would need is to draw cold air through at the same time. If there is enough heat to drive off the ice as water vapour there is enough in the air to raise the temperature a few degrees right behind the unit for the duration.

BTW the timeframe stops at 1am as that is when a setback stops the pump.

 

this mornings HP activity here concurs with harryup and cathoderay Regarding defrosting.

the fan definitely stops so there is just the circulating pump moving the warmed water through the HP heat exchanger And presumably the HP gas is also being circulated to defrost the fins.

And there is a distinct signature on the MELCloud chart where the flow temp drops below the return temp - which would follow since the flow thermistor would be the first to receive the cooled water from the defrost cycle. The chart above shows  no other cycling and as you can see it is happily modulating up to DT7 and back down to DT3 as it gets close to set temp. I think there is some anxiety  about achieving rigid DTs when a balanced system can vary it’s output for many undisclosed reasons. Most of which we seem to only guess at.

Cathoderay do you know the lowest power output of your HeatPump? On the Ecodan registration plate it states it’s design output but also shows it’s minimum output and maximum output. Eg our system is a Ecodan 8.5 but has a range of outputs from 3.4 to 9.7. So I know that my 21kw rated radiator capacity will cycle if I don’t  maintain a full heat exchange of all the radiators. My 21kw rated rad installation will only accept around 3kw at 30c flow temp. Then again other elements such as pipe sizes and flow resistance can also have a say in cycling.

@abernyte - ah, but you are making batty assumptions!! Note I said it was a lean to. It was an old privy about 6ft by 4ft that had been 'converted' into a shelter for the oil tank. It had a single sloping roof of the same footprint, tiles completely exposed on both upper and lower surfaces, and you could simply see there was no evidence of bat activity there.

It wasn't an eyesore, but it wasn't pretty either, being built of a rather harsh urban style of brick (the house is mellow sandstone), and it was definitely a later addition, and not an essential part of the character of the original building, and arguably the visual impact of the heat pump, selected because it had a low profile - that's why I have a Midea heat pump - is actually less than that of the privy it replaced. This, plus the benefits of going green (without disturbing non-existent bats), and the fact it is in a not very visible part of the garden, meant I could put together a convincing case that permission and consent should be granted, which they were.