Posted by: @cathoderay

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Posted by: @derek-m

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But what happens if the OAT reading varies from 0C to 3C deviation with heat pump loading?

Counter-intuitive perhaps, but it still doesn't matter. I suggest you are trying to get an exact answer to the wrong question, instead, what matters is can the OAT as measured by the heat pump accurately predict the energy in, and the answer, visible in the scatter plots and the regression statistics, is that it can, at an accuracy that is 'good enough'. We have a good enough answer to the right question. It might be interesting to know how it does that, but we don't have to know, the fact it can do so makes it good enough for the intended purpose, predicting the expected energy in for a given OATp.      

 

If I may comment, from a pure physics standpoint, I think its pretty much certain that the temperature of the intake air (which I think is what @cathoderay 's sensor is actually measuring) will be a good predictor of energy consumption in the circumstances that

• the heat pump is operated with a fixed WC curve and fixed flow rate
• the energy output from the heat pump is determined solely (or pretty much solely) by the heat pump - ie the emitters are well oversized relative to the house demand and are not otherwise throttled by roomstats, balancing or TRVs (or if there are TRVs, they are set at a temperature well above IAT)  and thus simply emit more or less everything thrown at them
• the variation in IAT is small compared to the difference between IAT and flow temperature.

From the data I have seen these conditions appear to be met in the experiment that has been conducted, and may well be met in other heat pump set ups. 

Departing from the physics to the more human, I'm rather less certain that this scenario is one which is the one which should be applied when trying to answer the general question posed in the thread.  In this case it is surely sensible to apply the constraint that the house is maintained at a more or less constant temperature except for defined hours during and shortly following setback, ie assume tolerably well balanced emitters and a WC curve which has first been optimised to establish a position where the house temperature just meets the desired temperature and no more.  If we start with a more relaxed condition, where either the house demand is well over satisfied and clamped materially by another mechanism outside the heat pump, or not in practice clamped at all but allowed to set its own level without reference to a design temperature, then I think we may be answering a different question altogether about optimising heat pump set ups.

Perhaps this indicates that the question isn't sufficiently well defined!

Posted by: @derek-m

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Hi James.

The raw data is the hourly records provided by CathodeRay recently, I think that you have been working with a copy.

I took the data on face value, assuming that it had been correctly averaged from the minute data.

The two rows of numbers above each table are the IAT values for that particular day extracted from the raw data, and the offset value required to produce the correct IAT value in the spreadsheet, to match that from the raw data. I included these values with the tables so that I could easily replicate the results by entering these values back into the relevant cells.

I made several initial attempts, but then realised that the results where being compromised by the PHE effect, in that the spreadsheet was producing the wrong Energy Supply value from the heat emitters, because it was using a LWT value that would be higher than that in the real World, so I improved the spreadsheet by adding a PHE DT value in the calculation of Energy Supply. I set the PHE DT value to 5C, which I think is a common value used by PHE manufacturers. I also added a Other Heating Source (OHS) value to the Energy Supply calculation, this is to take account of the other electrical devices, and humans, that generate thermal energy within the building. I set the OHS at 8kWh of thermal energy for each day, spread evenly across all 24 hours.

I have arranged for both of these two parameters to be changed if necessary, or removed from the calculation by setting their value to zero.

The settings used in the Initial Data sheet were 11kW Heat Loss, 23kW Heat Emitter Capacity and 246kWh of Thermal Mass/Capacity.

For each day the results are contained in three tables, the lower one is from the raw data values for a Setback with Boost, this is used as the reference. The middle table is with a Setback but no boost with the requirement that the specified IAT value is obtained by 9pm. The upper table of the three is with the Setback removed, so 24 hour operation, again with the specified IAT being achieved by 9pm.

I was too busy producing the results so did not look at them in close detail. 

As expected, the Setback without boost always produces an energy saving when compared to the reference Setback with Boost. The highest energy saving predicted being 2.64kWh on the night of 8th to 9th of November. Of course the extra energy saving being achieved by having lower IAT's.

Running without Setback was a bit of a mixed bag, with a prediction of an energy saving of 1.84kWh by running continuously on the night of 6th to 7th November, but then an energy saving of 2.1kWh by adopting a Setback on the night of the 7th to 8th November. So no clearly defined results.

As expected, in all cases continuous running put more thermal energy into the building, with as much as 12.5kWh on the night of 10th to 11th of November. Whilst not increasing the final IAT measured at 9pm, it is used to keep the IAT more even throughout the 24 hour period, sometimes with a slight increase in Energy In, but also sometimes with a slight reduction.

One thing that I did note is that the spreadsheet consistently predicted higher COP values than those in the raw data. There could be several possible reasons, but I would not care to speculate as to the reason why.

 

Thanks, I will take some time to study them.

Posted by: @jamespa

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Posted by: @cathoderay

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Posted by: @derek-m

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But what happens if the OAT reading varies from 0C to 3C deviation with heat pump loading?

Counter-intuitive perhaps, but it still doesn't matter. I suggest you are trying to get an exact answer to the wrong question, instead, what matters is can the OAT as measured by the heat pump accurately predict the energy in, and the answer, visible in the scatter plots and the regression statistics, is that it can, at an accuracy that is 'good enough'. We have a good enough answer to the right question. It might be interesting to know how it does that, but we don't have to know, the fact it can do so makes it good enough for the intended purpose, predicting the expected energy in for a given OATp.      

 

If I may comment, from a pure physics standpoint, I think its pretty much certain that the temperature of the intake air (which I think is what @cathoderay 's sensor is actually measuring) will be a good predictor of energy consumption in the circumstances that

• the heat pump is operated with a fixed WC curve and fixed flow rate
• the energy output from the heat pump is determined solely (or pretty much solely) by the heat pump - ie the emitters are well oversized relative to the house demand and are not otherwise throttled by roomstats, balancing or TRVs (or if there are TRVs, they are set at a temperature well above IAT)  and thus simply emit more or less everything thrown at them
• the variation in IAT is small compared to the difference between IAT and flow temperature.

From the data I have seen these conditions appear to be met in the experiment that has been conducted, and may well be met in other heat pump set ups. 

Departing from the physics to the more human, I'm rather less certain that this scenario is one which is the one which should be applied when trying to answer the general question posed in the thread.  In this case it is surely sensible to apply the constraint that the house is maintained at a more or less constant temperature except for defined hours during and shortly following setback, ie assume tolerably well balanced emitters and a WC curve which has first been optimised to establish a position where the house temperature just meets the desired temperature and no more.  If we start with a more relaxed condition, where either the house demand is well over satisfied and clamped materially by another mechanism outside the heat pump, or not in practice clamped at all but allowed to set its own level without reference to a design temperature, then setback may well save energy, in the first case a marginal saving if at all because the (reduced) output of the heat pump more closely matches the demand, and in the second case in proportion to the setback period because reductions in the output of the heat pump do not result in corrective measures being taken either by the automatic or manual control system.

 

Perhaps this indicates that the question isnt sufficiently well defined!

 

James, I think that you pointed out some time ago that it is the heat emitters, along with the heat loss, that dictate the required thermal energy output, rather than the heat pump itself. The WC curve sets the required LWT which the heat pump tries to produce, and the heat emitters will emit the quantity of thermal energy dictated by the LWT and their heating capacity. If the thermal energy being emitted is greater than the heat loss of the building, then the IAT will increase until demand matches supply.

If the heat pump is producing more thermal energy than can be emitted by the heat emitters at the specified LWT, the heat pump controller should throttle back by lowering the flowrate or allowing the DT to reduce. It will also lower the compressor speed to produce less thermal energy. This in turn will lower the electrical energy input, without any change in OAT.

The building heat loss is set by the prevailing weather conditions, not by the heat pump OAT sensor, this just sets the required LWT. If the OAT sensor is reading lower than the true ambient temperature, then the required LWT may be set too high, which means the heat pump will probably be operating at lower efficiency, which will in turn increase the electrical energy input. But conversely, if the LWT is now too high, the heat pump will probably throttle back, once more lowering the electrical energy input.

I don't dispute that OAT is one of the major factors with regard to how much thermal energy the heat pump needs to produce, which also determines, though not directly proportionally, how much electrical energy the heat pump will use in the process. Calculated Heat Loss and Heat Emitter Capacity all have a part to play, without even mentioning weather conditions.

I cannot see how it would be possibly to say "Oh it is 0C outside, so the heat pump must be using x kW's". A prime example is how the heat pump operates at the end of the Setback period where it possibly goes to close on full power output, all without the true ambient air temperature having changed. Some time later the power output could be much lower, all at the same ambient air temperature.

I appreciate under stable running conditions there will be better correlation between OAT and Energy In, but I very much doubt it will be 'good enough' to answer the original question that was posed, particularly since there would appear to be some doubt as to the reliability of some of the data being used.

Posted by: @jamespa

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There is nothing troublesome about the results or analysis per se if the only interest is in your system in the particular circumstances of the experiment.  However the title of this thread is not 'Do setbacks save energy without compromising comfort on the system owned by @cathoderay', it is 'Do setbacks save energy without compromising comfort?'.  

There does seem to be a lot of hindsight present in this thread these days. When I wrote the thread title, I had no idea how the thread and analysis would develop. Nonetheless, my opening sentences were:

An answer appears to be yes. I say 'an answer', rather than 'the answer', because this is one night in one building, with the weather conditions that prevailed at the time. Other buildings and/or other prevailing weather conditions may - very probably will - give different results. 

I have never claimed I have a universal answer, but that doesn't stop me asking the universal question, and providing the data and analysis my setup can provide.

Posted by: @jamespa

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From the data I have seen these conditions appear to be met in the experiment that has been conducted

Yes, though the rads aren't 'well oversized' (I might be well pleased if they were), just a bit oversized most of the time. It depends on the rad/room delta t, at design temps (OAT -2, IAT 19, LWT 55, room/rad delta t ~33), on paper they are oversized by about 7%.      

Posted by: @derek-m

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James, I think that you pointed out some time ago that it is the heat emitters, along with the heat loss, that dictate the required thermal energy output, rather than the heat pump itself. The WC curve sets the required LWT which the heat pump tries to produce, and the heat emitters will emit the quantity of thermal energy dictated by the LWT and their heating capacity. If the thermal energy being emitted is greater than the heat loss of the building, then the IAT will increase until demand matches supply.

I did indeed, and its still my view, but I had in mind a reasonably well adjusted system.  In retrospect my thinking was a little muddled (perhaps I was trying too hard to explain the reported experimental results) but nevertheless, if

• IAT is reasonably stable (which appears to be the case, albeit for reasons as yet unexplained),
• the flow temp driven solely by the WC curve, which in turn is driven by the 'OAT sensor' (actually in the @cathoderay case an intake air sensor) and nothing else,
• the flow rate constant (which we are told is the case) and the rads oversized at the flow rate/IAT in question

then there will be strong relationship between the temperature measured by the intake air (OAT) sensor and the energy demand from the rads = the energy supplied by the Heat pump.  Im not saying it will be perfect but, with only small IAT excursions relative to the difference in temp between IAT and the intake sensor, it will be strong, which is what is observed.  

For me the question (to answer to help explain the observations) becomes therefore - why is the IAT apparently more stable than perhaps would be expected?  I need to get back to looking at your simulation to see whether in fact it is more stable than perhaps would be expected.

Posted by: @derek-m

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I appreciate under stable running conditions there will be better correlation between OAT and Energy In, but I very much doubt it will be 'good enough' to answer the original question that was posed, particularly since there would appear to be some doubt as to the reliability of some of the data being used.

I entirely agree it wont be.  Even if the above is a more or less correct analysis it doesn't apply the boundary conditions which we need to apply to answer the question, which must surely include achieving the design IAT for at least a reasonable proportion of the day irrespective of OAT or whether setback is applied.

Posted by: @cathoderay

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I have never claimed I have a universal answer, but that doesn't stop me asking the universal question, and providing the data and analysis my setup can provide.

I never said it did or that you did.  All I did was comment on the utility of the data in relation to the question in the prevailing circumstance, namely that it is unreconciled with the laws of physics.

Unfortunately, rather than exploring possible explanations which might increase the utility of the data, recent discussion seems to focus on other matters, and in some cases appears actively to avoid looking for explanations. 

My interest remains in reconciling the observed behaviour with the laws of physics because only then has it actually told us anything of any use other than in a very specific situation.  Until we can do that a model is the best tool we have to predict the answer to a more general question.

Posted by: @derek-m

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The two rows of numbers above each table are the IAT values for that particular day extracted from the raw data, and the offset value required to produce the correct IAT value in the spreadsheet, to match that from the raw data

Can you elaborate on the second of these, Im not sure what the 'offset' is and where it is applied and what you define as 'correct'?

Also what determines the starting thermal capacity and IAT each day, is it the results from the previous day or the actual from @cathoderay? (with a subtext - will these results tell us about predicted trends over several days or is the model 'reset' each day)

Posted by: @jamespa

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which must surely include achieving the design IAT for at least a reasonable proportion of the day irrespective of OAT or whether setback is applied.

Whether this is the case or not is in the data. Generally it is within a degree or so of the desired IAT. It has periods of minor over and undershoot because I am not a control freak, and only tweak the WCC manually from time to time (the auto adapt script on the other hand will do it briefly and automatically). The only clear exceptions are both expected, during and after setbacks, and when there is a sustained cold spell, and the heat loss exceeds the capacity of the heat pump to replace it. The combination of both factors produces the lowest IATs (see 26 Nov below - in my system, setbacks in cold weather do compromise comfort). Here is the last month's minute data, you can see exactly what is going on: 

Posted by: @jamespa

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All I did was comment on the utility of the data in relation to the question in the prevailing circumstance, namely that it is unreconciled with the laws of physics.

I know this may appear to be an unnecessary question, but I think it is important: which part of my tool methodology is unreconciled with the laws of physics?

Edit: quote sentence got chopped, edited to include full sentence 

Posted by: @cathoderay

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Posted by: @jamespa

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All I did was comment on the utility of the data in relation to the question in the prevailing circumstance, namely that it is unreconciled with the laws of physics.

I know this may appear to be an unnecessary question, but I think it is important: which part of my tool methodology is unreconciled with the laws of physics?

 

Edit: quote sentence got chopped, edited to include full sentence 

Its not the methodology, its the observations.  There is as yet no explanation of how, during setback, you can (apparently) put ~20% less energy into the house yet it (apparently) remains at (very nearly) the same temperature in otherwise (apparently) similar conditions to those which prevail during the control period. 

Until this is explained to at least a reasonable degree we don't actually know whats going on, we just have some measurements, and since we don't know whats going on we cant apply it to other circumstances.  It is, in effect, just a black box with some inputs and some outputs in some specific circumstances, but no known set of rules to link the two.

The physics is simple enough that its reasonable to expect we can explain it if we have the relevant facts.  Since we can't explain, its clear that we don't yet have the relevant facts.

Posted by: @jamespa

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Its not the methodology, its the observations.

Sorry, I didn't word the question well, I meant the whole package, data and methods.

Posted by: @jamespa

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There is as yet no explanation of how, during setback, you can (apparently) put ~20% less energy into the house yet it (apparently) remains at (very nearly) the same temperature in otherwise (apparently) similar conditions to those which prevail during the control period. 

But it doesn't (stay at very nearly the same temp, it drops at times, sometimes by several degrees for periods) and the general rule of thumb is I believe 10% energy difference for every 1 degree of IAT difference. I think we need to look closely at what the actual IATs are actually telling us. There is also the thermal mass question (perhaps it is relatively huge? But it is difficult to measure...).

Have just done some PHE input/output readings (not many), will post details tomorrow.

Posted by: @jamespa

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Posted by: @derek-m

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The two rows of numbers above each table are the IAT values for that particular day extracted from the raw data, and the offset value required to produce the correct IAT value in the spreadsheet, to match that from the raw data

Can you elaborate on the second of these, Im not sure what the 'offset' is and where it is applied and what you define as 'correct'?

 

Also what determines the starting thermal capacity and IAT each day, is it the results from the previous day or the actual from @cathoderay? (with a subtext - will these results tell us about predicted trends over several days or is the model 'reset' each day)

CathodeRay's data provides the OAT and the IAT at hourly intervals. The model needs to adjust the energy output to represent what the heat pump controller would be doing, so that the IAT achieved by the model at the end of each 1 hour period matches the IAT value from the raw data. This would therefore provide a measure of how much thermal energy the heat pump must provide in that 1 hour period to achieve the required IAT. In an actual heat pump operating in WC mode, I believe that although the required LWT is set by the WC curve parameters and the OAT value, the actual thermal energy output is derived by a combination of the LWT-RWT DT, and the flowrate, the feedback being in the form of changes to RWT. This capability is not built into the spreadsheet, so to simulate the loading effect as energy is absorbed by the heat emitters, the WC offset parameters basically tells the simulation to output more energy. By varying the WC offset, the Energy Supply to the heat emitters is varied which in turn changes the calculated finish LWT value. I appreciate that it is a little complex, but it was the best way at the time that I could thing of how to vary the heat pump loading. If anyone has a better idea I am always open to suggestions.

So in the Setback with Boost scenario, when the heat pump restarts at 03:00, I have no idea how hard it is likely to work, but I know that by 04:00 it will have raised, or lowered, the IAT to the specified value. The WC offset is therefore adjusted until this required IAT is obtained.

I forgot to mention, the time shown in the tables is at the end of the 1 hour period, there is of course a start and finish IAT value, OAT is assumed to be the average over the 1 hour period, LWT is derived from the OAT value in conjunction with the WC curve, but also has the WC offset applied as a means to raise or lower the Energy Supply value.

The starting thermal capacity is derived from the starting IAT value, in a similar manner to your lake analogy. It is from CathodeRay's data, since the objective was to test his data in the spreadsheet to see how closely it matched the real World, and could then possibly be used on other buildings and other types and sizes of heat pumps and to answer the burning questions around Setbacks.

Inspection of the tables will show that it continues from day to day, such that the end parameters of the previous day become the start parameters of the present day, though you will notice that it actually covers the period 9pm on one day to 9pm on the following day, to align best with the Setback routine.

If there is anything that you do not fully understand then please feel free to ask. That applies to everyone.