Posted by: @sunandair

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So in terms of energy saving I believe, as a Retrofit installation the defrost temperature zone is a particularly inefficient and costly period which becomes only more costly the colder the temperature gets. (In other words the true defrost burden should really reflect a mix of colder temperatures) 

it’s worth emphasising and not to forget the energy saved in a setback is not just the defrost energy, there is also the actual running cost for the continually operated period at the colder temperature.

I am still not sure we have got our collective heads round what is going on. One way of looking at a defrost is that it is a setback, insofar as the heat pump stops putting energy into the building, albeit with a different trigger and mechanism, and will therefore save energy during the setback, and this is exactly what we see happening when we look at minute by minute data. But the heat pump then has to recover from that setback, and during the recovery period it uses more energy than it would have used had there not been a defrost. Very approximately, it does at least appear that the energy saved during the defrost is then subsequently added back as extra heat during the defrost recovery, making the entire defrost cycle in effect energy neutral, ie it neither increases nor decreases overall energy use itself, compared to running without defrosts at the same OAT (which of course it never does). If this is a correct assessment, then it follows that the reason for very high energy use and low efficiency (COP) during defrost periods isn't in fact the defrosts themselves, it is rather the lower OAT. There is some empirical evidence for this, in that we do not see a step increase in energy in when the OAT drops into the defrost zone:

The relationship is polynomial, rather than linear, as the efficiency drops at lower OATs. The variance appears greater at lower OATs, but as a proportion of the absolute OAT, it is about the same. There is no obvious step increase in energy use during the defrost range, the increase seen is simply a result of the lower OAT.

An alternative way of looking at defrosts is that they are inherently wasteful of energy because they pump heat away from the building into its surroundings, and must therefore use extra energy. Obvs, init? But I don't think the evidence bears that out.    

Posted by: @sunandair

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Posted by: @sunandair

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So in terms of energy saving I believe, as a Retrofit installation the defrost temperature zone is a particularly inefficient and costly period which becomes only more costly the colder the temperature gets. (In other words the true defrost burden should really reflect a mix of colder temperatures) 

it’s worth emphasising and not to forget the energy saved in a setback is not just the defrost energy, there is also the actual running cost for the continually operated period at the colder temperature.

 

 

I am still not sure we have got our collective heads round what is going on. One way of looking at a defrost is that it is a setback, insofar as the heat pump stops putting energy into the building, albeit with a different trigger and mechanism, and will therefore save energy during the setback

Nice re-focus but somehow Im not sure the defrost ‘setback’  saves energy it does after all put negative energy into the building for at least one whole cycle of the system volume.

and your graph does highlight the wonderfully wide range of high energy consumption a system can consume as it approaches the defrost zone

apologies for scrawling over the cold end of the graph but I thought I’d remind ourselves of the real high daily energy consumption of the defrost zone.

It may be my eyes but I do see a  widening of the energy spread within the defrost zone. Does this reflect the variability of the number of defrosts in a particular 24 hour period?

I also am thinking the HP is not in proper steady state (as @robs has implied) at the point just before it defrosts again. The system is still catching up from the influx of cold water so the system has to be operating at a compensatory higher output than if it was not in the defrost zone. This would be easily compensated by having the WCcurve set higher but it wouldn’t necessarily be appreciated that extra heat is being provided because of the real culprit - the cold slug of circulating water and the 11 minutes of down-time per hour for the defrost to take place.

Posted by: @sunandair

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It may be my eyes but I do see a  widening of the energy spread within the defrost zone. Does this reflect the variability of the number of defrosts in a particular 24 hour period?

There is certainly a visual widening, but as I said, as a proportion of the absolute OAT value, it is about the same across the range, perhaps in fact slightly less at lower OATs. In other words, for any given OAT, the spread is mean +/- x% of the absolute OAT, where x is fairly constant across the range.

The fact the curve fits very well, both visually and in the R squared value, suggests the change in energy in is very explained by the change in OAT, ie the defrosts are not having much, if any, of an effect.

Where I think the defrosts may have an effect is when they fall on an hour boundary. If the defrost happens in one hour block, and the recovery in the next hour block, then conceivably the first hour block will use less energy, and the second hour block will use more, but over time (and we are looking at a years worth of data) I would expect these effects to generally even out over time Or will they? Time's arrow only goes one way...

@cathoderay 

It occurs to me that there is one variable that 99.9% of heat pump installs cannot accurately check, it is the energy output. Most of us can check the energy used, either by smart meter, or inline energy meter, but very few have an accurate enough measure of the flow and temperature required to calculate the exact output energy. Correct me if i am wrong, but i believe the figure shown in the pumps data is one generated by a algorithm produced by the manufacture, using variables like the flow, outside temp,target temp, compressor speed, delta T and who know what else. If this is slightly out at some temperatures(ie in defrost or after setback) this could change the overall output enough to make the differences between the science and the real life observations. A defrost ejects heat to the environment,(to melt the ice) so this must be lost from the house.

Another consequence of the above, without a accurate energy out figure, when we are chasing a improved COP, what we are doing is trying to find the perfect match to the algorithm of that manufacture, if this formula isn't correct, we may not be changing settings to produce the most financially efficient system. As each manufacture will have a different algorithm, some may be more ambitious with there formula, therefore one pumps COP of 4.2 may be the same as another 3.5.

Posted by: @mikef

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Correct me if i am wrong, but i believe the figure shown in the pumps data is one generated by a algorithm produced by the manufacture, using variables like the flow, outside temp,target temp, compressor speed, delta T and who know what else.

I'm not sure, but I think those tables in the engineering data manuals are supposed to be direct measurements in lab conditions, perhaps not for all the values ie some interpolation, but there does have to be a person in a white coat with a clipboard involved at some point.

I don't know how Midea calculate their energy out (as seen in the wired controller), but I do know how I calculate energy out: kWh = flow rate x LWT-RWT delta t x specific heat capacity of the circulating fluid. The first two come from the Midea raw data, and I do have some independent checks: I have an independent analogue flow meter in the circuit, and I can directly measure the LWT and RWT, and generally there is adequate ball park agreement, while the specific heat capacity is a constant. All of which is to say I think the energy out calculation, which can be done per minute and upwards, is tolerably useful. I am actually more worried about my energy in value, since there is a discrepancy between the raw data calculated value (volts x amps) and the energy in shown on a kWh meter that only supplies the heat pump. I suspect the discrepancy is down to ancillaries, and I deal with it by applying a correction factor which brings the calculated value more into line with the metered value. But it is not ideal (AKA a bit of a bodge).   

@cathoderay 

Maybe i am a skeptic, men in white coats were involved with the Dieselgate emissions scandal, it didn't stop them, when money is involved we all know what rises to the top!

Posted by: @mikef

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Correct me if i am wrong, but i believe the figure shown in the pumps data is one generated by a algorithm produced by the manufacture, using variables like the flow, outside temp,target temp, compressor speed, delta T and who know what else. If this is slightly out at some temperatures(ie in defrost or after setback) this could change the overall output enough to make the differences between the science and the real life observations.

Hi Mike. I think you make a valid point. I would assume the post defrost algorithms are part of a pre determined reheat to provide  some additional heat that would be an expected requirement. And as you say it would be likely they are not exact. At some point the normal weather curve setting would take over. 

Posted by: @mikef

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men in white coats were involved with the Dieselgate emissions scandal

I agree, I have even been known to call some of the 'optimistic' energy output claims 'compressorgate' claims. The thing I was trying to get across is that I believe the data in those tables has to come, at least in part, from real measurements, they can't just be done from 'modelling', or as I often call it, 'whatiffery'. By now, it should I hope be clear that I share your scepticism. 

Posted by: @sunandair

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I would assume the post defrost algorithms are part of a pre determined reheat to provide  some additional heat that would be an expected requirement

The link @old_scientist provided recently goes to a very detailed analysis of a Samsung defrost cycle algorithm, and I imagine your Mitsubishi and my Midea heat pumps have something similar. At least with a defrost setback, the heat pump presumably has some data on how the defrost affected energy flows, and thereby how much energy needs to be put back during the recovery.

Posted by: @sunandair

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Posted by: @sunandair

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I would assume the post defrost algorithms are part of a pre determined reheat to provide  some additional heat that would be an expected requirement

 

 

The link @old_scientist provided recently goes to a very detailed analysis of a Samsung defrost cycle algorithm, and I imagine your Mitsubishi and my Midea heat pumps have something similar. At least with a defrost setback, the heat pump presumably has some data on how the defrost affected energy flows, and thereby how much energy needs to be put back during the recovery.

Thanks @cathoderay, I did read it earlier when it was posted. I wouldn’t pretend to have understood most of it but there were a few good bits of information.

1 that there are a few separate algorithms at play that deal with the defrost. Eg. Defrost Recovery algorithm and Normal Operation algorithm. As listed.
2 that the targeted recovery energy that is put into the water circuit would appear to make reference to the WCC set temperature and probably referencing the outside temperature to make an appropriate energy upward adjustment.

item 2 above would would ensure only enough energy was added at the recovery stage to allow the heat pump to resume normal operation and  merge in with the target flow temperature of the WCC (also called ‘Water Law’ in Samsung-speak.

So as a relative benchmark to this discussion thread - That would mean a low temperature operating system such as @robs ‘s system operating at 29C at ambient 3c would have a reduced recovery requirement than say my system which would need 39c at ambient 3c.

It would follow that *Other harder working systems and also other colder ambients in the defrost zone will need far greater recovery input from the recovery algorithm. 
EEK

Posted by: @old_scientist

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There is a detailed analysis of a defrost cycle for a Samsung heat pump here that may be of interest.

Here is that Samsung link posted by @old_scientist 

Posted by: @sunandair

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@robs Your Heat Pump setup is quite exceptional. I can clearly see your heat pump is delivering the heat you require at a flow temperature of 29c when the outside ambient is as low as 3c. Likewise the detail of your monitoring setup is exemplary. Given more time I would love to understand it more.

Hi @sunandair 

Thanks, if you or anyone would like to know more I'm happy to describe our setup in more detail.

Posted by: @sunandair

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In terms of this topic, you might be able to come up with a cost saving if you avoided a night of  defrosts using something like a setback. I didn’t quite understand the data terms to make use of them. But with your understanding, Even your efficient system may benefit and it looks like the information is already there at your fingertips. 

So in terms of energy saving I believe, as a Retrofit installation the defrost temperature zone is a particularly inefficient and costly period which becomes only more costly the colder the temperature gets. (In other words the true defrost burden should really reflect a mix of colder temperatures) 

@cathoderay (who has higher flow temperatures and consumption levels) and my data both show that there is very little extra consumption from the defrosts, it is just the lower OAT that drives the increased consumption. 

Posted by: @sunandair

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Finally there is the recovery process and getting the property up to 21c by a certain time of day. This was discussed earlier upthread and is very much down to the thermal properties if the external walls of the property. We have fully insulated the inside faces of all external walls, with a small exception of a brick fireplace. We have some internal brick walls which don’t cool down much in the setback period. Nonetheless we still have created a schedule which starts to reheat at either 4am or 6.30am depending on how cold the temp drop is. 

Recovery in time for the occupants needs, I think is a key factor in the setback question. Most "working households" will need the IAT to be restored by 5 or 6am five days at week and IAT to still be comfortable at 10 or 11pm. With reheat durations of 4-6 hours and a setback starting around 1 hour before bedtime, that results in a short setback period of only a few hours. Also, a setback a few hours around midnight when it is typically not the coldest period of a night (so COP isn't too bad) and a reheat during what is typically the coldest period of a night in the hours before dawn (so COP is quite poor even before needing to run the heat pump harder to reheat the house after a setback) won't be good for saving energy/money. 

Another factor is that as your house has internal insulation, and hence a lower thermal mass, you have a house that is probably easier and quicker to reheat than most.

In my opinion insulated walls are not a “nice to have” option, rather it is necessity.

Insulation of older housing stock must be seen as an absolute priority. It improves not just thermal efficiency for heat-pump use but it also transforms wall temperatures to directly eliminate condensation, mould growth and all the health hazards associated with it. 

here is a simple comparison on a 9inch brick wall. One left without insulation and one lined with 10cm PIR vapour barrier and battened plasterboard.

the next graph shows the internally insulated wall. This doesn’t cheat the physics of heat loss in a building. It changes the the physics of the building to one of low conduction.

It’s true that external insulated cladding also works to slow down the loss of heat in a building. But the physics changes in a slightly different way in that the thermal mass of the external walls remain facing the room space within the thermal envelope. It therefore will store heat in a warm room and absorb any radiant or air heat in a colder room until it reaches a balance.

it would be also possible to insulate with a thinner layer of PIR eg 75 mm or 50 mm (so to reduce the overall thickness of the lining) this would also reduce the thermal improvement a little bit but still provide a wall with considerably lower heat conduction.

These comments are meant as principles of design and not intended as guidelines.