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Do setbacks save energy without compromising comfort?

cathodeRay · 2023-11-05T10:16:26Z

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. But here is a documented record showing that I saved around 5-6 kWhs last night by applying a 26 degree to 12 degree setback between the hours of 2100 and 0300. Result (chart shows the last 48 hours, the first 24 hour no setback period for comparison with the second 24 hour setback period; I have also done the long overdue conversion of the energy in/out plots to bars rather than lines, with each bar showing the preceding hour's energy in/out value; note the bars are overlaid not stacked, ie each bar starts at zero): There is zero energy use for heating during the setback. As the current normal energy use is roughly 1 kWh per hour, that means I saved 6 kWh of energy. Now comes the 64 million dollar question: how much extra energy did I use during the recovery (which happened fairly promptly, the room temp was back to the desired room temp of 19 degrees by 0700, having fallen to a nadir of 18 degrees during the setback, not a lot I agree, a consequence of a large thermal mass and mild but by no means unusual temperatures)? I emphasise extra energy because in continuous running (no setback), the heat pump would, in the prevailing conditions, use about 1 kWh per hour, what I want to know is how much extra energy I used during the recovery period. The answer is, in the prevailing conditions, very little. The recovery boost is managed by my python auto-adaption script (see below for details), which increases the weather compensation curve endpoints (and so the LWT) when the room temp is below where it should be, as it was at the end of the setback. If you look at the first hour after the setback, the energy out (red bar) is visibly higher (as is the LWT in the upper chart) but the increase in energy in is minimal, 1.24 kWh for that hour, compared to 0.84 kWh in the hour before the setback, and 1.1 kWh in the second hour after the setback. That gives around an extra 0.24 kWh used, over and above the around 1 kWh I would have used anyway. I therefore saved 6 - 0.24 = 5.76 kWh last night by using a setback, and comfort was not compromised, the room temp was where it should be when I got up at 0700 this morning (and yes it was a bit cooler in the middle of the night, I know because I have to do what most middle aged and over men have to do during the night). A result, I hope you will agree. Some things to bear in mind. 1. As noted above, and emphasised again here, this is one building over one night in the weather prevailing at the time. I will add more data once I have it, and it has something to show. 2. I call it a setback, because it is a setback of the room stat from 26 to 12 degrees, but its effect is to act as an on/off timer, because in normal circs the house will never fall to 12 degrees, meaning the heat pump will never come on during the setback. A heat pump on/off timer could used to do the same thing. 3. The parameter monitoring is pretty tight, data collected over modbus at one minute intervals, meaning we can really see what is going on. All the data bar the room temperature comes from the Midea controller, and so far as I have been able to verify its accuracy, it is not far out. The room temp (IAT) comes from a third party sensor in centre of the living area of the house. 4. I am currently running my DIY auto-adaption. This uses a python script that checks the difference between the actual room temp and desired room temp every hour and adjusts the end points of the weather compensation curve according to the difference. The current logic moves the ends of the curve one degree up or down from the baseline endpoints for every degree the actual room temp is below of above the desired room temp respectively, eg if the room temp is one degree below the desired room temp, the ends of the weather compensation curve go up by one degree. The maximum increase in the endpoints is limited to a three degree rise, to stop the heat pump melting itself down. One effect of this auto-adaption is that it can boost the output when the room temp is below where it should be, eg at the end of a setback period, and provide a so called setback recovery boost, meaning the heat pump doesn't take forever to get the room temp back to where it should be. The auto-adaption can also reduce output eg if there is solar gain, increase it if there is significant wind chill, for example from draughts from leaky window frames.

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Air Source Heat Pumps (ASHPs)

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Derek M

(@derek-m)

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Joined: 5 years ago

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21/12/2023 2:14 pm

@cathoderay

Please find attached the results for the 24 hour period from 21:00 on 12th Dec. 2023 to 21:00 on 13th Dec. 2023.

The first table uses LWT's averaged from the whole of the minute data during any 1 hour period, the second table averages the LWT only when the compressor was producing thermal energy. It would be interesting to know which method may give the more aligned result.

The time shown is at the end of each 1 hour period.

The OAT value used has been averaged over each 1 hour period from the raw data, no adjustment has been made for deviation from the true ambient temperature.

If anyone has any questions then please ask.

Midea 14kW CR Test Results for 12th Dec 2023.xlsx

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cathodeRay

(@cathoderay)

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Topic starter 21/12/2023 2:27 pm

@derek-m - thanks. I've got various other things to be getting on with today, but later on I will plot the two predictions against the observed values and post the result.

The period in this spreadsheet is as you note in the tables a no setback period. Are you also able to do a prediction covering one or more of the setback periods (16/17 Dec onward) (a) with a 6 hour 2100 to 0300 setback and (b) without a setback?

Midea 14kW (for now...) ASHP heating both building and DHW

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Derek M

(@derek-m)

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21/12/2023 2:45 pm

Posted by: @cathoderay

↑

@derek-m - thanks. I've got various other things to be getting on with today, but later on I will plot the two predictions against the observed values and post the result.

The period in this spreadsheet is as you note in the tables a no setback period. Are you also able to do a prediction covering one or more of the setback periods (16/17 Dec onward) (a) with a 6 hour 2100 to 0300 setback and (b) without a setback?

Yes. No Problem.

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cathodeRay

(@cathoderay)

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Topic starter 21/12/2023 7:14 pm

Here are charts of @derek-m's model predictions vs my observed (based on amps x volts) values for energy in (PI in @derek-m's spreadsheet), first scatter plots and then bar charts. They are hourly energy in values over a 24 hour period, 2100 12 Dec to 2100 13 Dec 2023, no setback and a moderate relatively stable OAT (range 7.1 - 8.9). The text box on the left hand side of the scatter plot gives the 24 hour totals, 'Midea' is the value based on the Midea total (cumulative) values (current value - value one hour ago). I have checked for errors typos etc but E&OE as ever, I am only human.

Edit: bar charts temporarily removed because there was an error, correct version will be posted shortly

Edit: corrected bar charts now added

Midea 14kW (for now...) ASHP heating both building and DHW

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Derek M

(@derek-m)

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Joined: 5 years ago

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21/12/2023 7:15 pm

@cathoderay

Please find attached the previous results with those from 13th to 16th Dec. added.

Please also note that as yet the spreadsheet makes no correction for DHW heating, since, as yet, I am undecided as to how much DHW heating may contribute to overall building heating.

Midea 14kW CR Test Results for 12th to 16th Dec 2023.xlsx

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Derek M

(@derek-m)

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21/12/2023 7:20 pm

@cathoderay

Thank you, it will be interesting to see how other days and weather conditions compare.

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cathodeRay

(@cathoderay)

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Topic starter 21/12/2023 7:59 pm

@derek-m - sorry, I did make an error in one of the bar charts (second lower chart had same data as first), I have now corrected that. I have also done the same thing, this time using my combined OAT vs energy in regression line equation (the basis I used to calculate predicted values in my earlier observed vs predicted charts):

I will shortly post all the charts in one image, to make direct comparison easier.

Midea 14kW (for now...) ASHP heating both building and DHW

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cathodeRay

(@cathoderay)

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Topic starter 21/12/2023 8:19 pm

Here are all the charts in one image, to (I hope - you need a large monitor) make direct comparison easier:

This is a comparison, albeit only for one rather unexceptional but not atypical day, of predictions based on observed data (using my regression line) and a model worked up from scratch. I think the comparison is interesting: despite having the worst R squared value, my regression line equation makes the best 24 hour value prediction. If we take the 'true' value to be 35.32 kWh (with the Midea 36 being a rounding error), then my regression over-estimates by 2.6 kWh, while your closest model prediction under-estimates by 3.05 kWh. @jamespa will no doubt correct me, but what I think is going on here is a sort of regression to the mean: the hourly values are a bit out, but over a 24 hour period the 'outness' cancels itself out, if that makes sense. I'm also if I admit it a bit suspicious of the third R squared value...it looks a bit low to me. All thoughts and comments welcome. Please bear in mind the goal is to be able to predict accurately what the 24 hour energy in would have been, had a setback not been applied, which can then be compared to the actual use with a setback applied.

Midea 14kW (for now...) ASHP heating both building and DHW

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cathodeRay

(@cathoderay)

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Topic starter 21/12/2023 8:34 pm

Posted by: @derek-m

↑

Please find attached the previous results with those from 13th to 16th Dec. added.

Please also note that as yet the spreadsheet makes no correction for DHW heating, since, as yet, I am undecided as to how much DHW heating may contribute to overall building heating.

Thanks, I will do what I can as soon as I can. The really interesting 24 periods will be the setback periods, but the non-setback ones are also very useful.

Midea 14kW (for now...) ASHP heating both building and DHW

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Derek M

(@derek-m)

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21/12/2023 8:58 pm

@cathoderay

Please find attached the results file with the remaining data added up to the 18th December, which is all the data provided.

Could you now post the un-redacted raw data so that meaningful comparison can be made.

Midea 14kW CR Test Results for 12th to 18th Dec 2023.xlsx

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cathodeRay

(@cathoderay)

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Topic starter 21/12/2023 9:04 pm

Posted by: @derek-m

↑

Could you now post the un-redacted raw data so that meaningful comparison can be made.

Will do, but juggling many things at the moment for all the usual reasons, so may be a delay. But will do as soon as I can.

PS what is unmeaningful about the charts I have already posted?

Midea 14kW (for now...) ASHP heating both building and DHW

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Derek M

(@derek-m)

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Posts: 4425

21/12/2023 9:31 pm

Posted by: @cathoderay

↑

Posted by: @derek-m

↑

Could you now post the un-redacted raw data so that meaningful comparison can be made.

Will do, but juggling many things at the moment for all the usual reasons, so may be a delay. But will do as soon as I can.

PS what is unmeaningful about the charts I have already posted?