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Do setbacks save energy without compromising comfort?

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cathodeRay
(@cathoderay)
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The latest last 24 hours of data:

image

 

Pretty much as before, steady and mild OAT at around 10 degrees, heat pump cycling around twice an hour to modulate output, clearly visible savings during overnight setback, and a small but visible increase in energy input at the beginning of the three hour recovery period. The 1400-1500 spike yesterday was the DHW heating.

For the avoidance of doubt, this is my system in my house in the current conditions. Other systems in other settings, as well as my system in different conditions, may vary.    

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


   
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(@newhouse87)
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@cathoderay would you consider extending setback ray? Currently im off from 9pm until midday on coldest days. Yesterday was warm so switched on heating at 5-9pm just to make sure house warm this morning which it was @21. Fixed flow temp @30 im liking so far, on coldest days@5 to 7deg i just switched it on at midday 3 hours earlier then mild days and off again@9pm, house was still warm following day.


   
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(@derek-m)
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@cathoderay

I think that I may have said so at the time, but if not, "I'm sorry that you have a system designed by freedom heat pumps".

How low did the IAT fall when you performed a setback during the recent cold spell?


   
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(@kev-m)
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Posted by: @cathoderay

The latest last 24 hours of data:

image

 

Pretty much as before, steady and mild OAT at around 10 degrees, heat pump cycling around twice an hour to modulate output, clearly visible savings during overnight setback, and a small but visible increase in energy input at the beginning of the three hour recovery period. The 1400-1500 spike yesterday was the DHW heating.

For the avoidance of doubt, this is my system in my house in the current conditions. Other systems in other settings, as well as my system in different conditions, may vary.    

@cathoderay, this is mine for the same period.  My setback is longer and my IAT higher but you can see the savings by comparing the setback with the peak before 9am when the house is back up to target IAT. My saving overnight is about 9kWh and the morning peak is about 3.75kWh so a total saving of 5kWh give or take. This is Mitsubishi AA. 

Screenshot
This post was modified 6 months ago 2 times by Kev M

   
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(@newhouse87)
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No way running constantly is most cost efficient. Even in cold weather if you can bear mornings time being a bit below desired the savings are very note worthy.


   
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(@newhouse87)
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@kev-m Running constantly would also have used some of the peak usage so i think your savings are even greater.


   
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(@kev-m)
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Posted by: @newhouse87

@kev-m Running constantly would also have used some of the peak usage so i think your savings are even greater.

I measured the 'peak' from the constant running level, not zero.  At least I meant to ...

 


   
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(@derek-m)
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@kev-m

Hi Kev,

Once again excellent data. Thank you.

I have recently developed a WC version of the modeling tool for the 14kW Ecodan, so it would be highly useful to test your real World data against the predictions from the spreadsheet.

The spreadsheet does indeed show that savings can be made by means of overnight setback. The actual quantity of electrical energy reduction that can be achieved being dependent upon four main factors.

The DT between IAT and OAT.

The length of the setback period.

The thermal capacity (Heat Mass) of the property.

How quickly recovery to desired IAT needs to be made.

I can provide further details if you wish.


   
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cathodeRay
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Posted by: @newhouse87

would you consider extending setback

I would, but at the moment I have what I think is the right balance between getting a saving and comfort. I have a bit of a cliff edge as the IAT falls at around 16-17 degrees, when I both perceive I feel a bit chilly - OK I can put on another layer - and get symptoms (chilblains -happens as we get older!) - OK I can put on fingerless gloves but I end up feeling a bit like old man Steptoe. I'm also not sure what will happen when we get sustained lower temps and defrost cycles - historically, my system (without my auto-adaption) has been very slow to recover, over 12 hours, when the house gets cold, and it remains to be seen how well my auto-adaption script deals with that. 

I have by the way managed to fix the script b*ggering up the DHW heating problem. The script now checks the position of the three port two way valve, the most reliable indicator of whether the system is in space or DHW heating mode, and doesn't run if it is in DHW mode. As this is generally between 1300 and 1400, the script probably wouldn't have made any changes anyway, so nothing lost, if it would have done, it just does it an hour later.

Posted by: @derek-m

How low did the IAT fall when you performed a setback during the recent cold spell?

We haven't really had a cold spell round here, just a few cooler nights with the OAT briefly down to 3 degrees, not even enough to trigger defrost cycles. I think the lowest the IAT got to during a setback was around 16.4 degrees.

Posted by: @kev-m

My setback is longer and my IAT higher but you can see the savings by comparing the setback with the peak before 9am when the house is back up to target IAT.

Thanks, very useful to have similar data from another setup, and I agree, it shows the same thing, setbacks do save energy (based on Mk 1 eyeball assesssed areas under the curve, as in the recovery extra bulge (over and above what you would have used anyway) is smaller that what would have been used during the setback period, based on pre-setback usage) but I am not sure what the units are, eg I doubt very much you were using 35 kWh at 0600 this morning! Maybe it is Wh (Watt hours)? I think it also helps to have energy in (and out) shown as bars, the bar for each hour being the energy in (and out) for the preceding hour. Strictly speaking these should be grouped (placed side by side), bit personally I find it works better for me if they are overlaid (and crucially that is pointed out, they are not stacked), as both then fall on the hour, and I get an instant visual sense of the COP. It is also easier to quantify the savings and recovery extra usage and get a value for net saving.  

Posted by: @kev-m

I measured the 'peak' from the constant running level, not zero.  At least I meant to ...

Yes, this is essential, we want to know how much extra energy the recovery used, over and above what would have been used anyway. We then subtract that from the setback savings to get the net savings.

Posted by: @derek-m

The spreadsheet does indeed show that savings can be made by means of overnight setback. The actual quantity of electrical energy reduction that can be achieved being dependent upon four main factors.

The DT between IAT and OAT.

The length of the setback period.

The thermal capacity (Heat Mass) of the property.

How quickly recovery to desired IAT needs to be made.

It seems that the model has now caught up with the real world - not so long ago you were regretting bursting my mythological bubble. I think it is worth quoting the lance thrown at my bubble in full, because it shows just how dangerous modelling can be. At that point, your position, based on nothing less than the Laws of of the Universe, otherwise known as the basic Laws of Physics and Thermodynamics, was that what I proposed (using setbacks to save energy) was doomed, and worse than that, actually detrimental:  

@cathoderay

I hate to burst your bubble, but what you propose will not be possible to achieve, since it would defy the basic Laws of Physics and Thermodynamics.

From the modelling (sorry simulations) that I have carried out, the test data showed that at a constant OAT of 10C, to maintain an IAT of 20C would require an electrical energy input of approximately 1.62kW, giving a thermal energy output of 6.76kW at a COP of 4.17. The calculated LWT is 42.3C.

Stopping the heat pump for 1 hour would save 1.62kWh of electrical energy, but the building would lose 6.76kWh of thermal energy, and hence cause the indoor temperature to fall to 19.73C.

If the heat pump is restarted and the LWT setting raised to 46.5C, the electrical energy input will increase to 2.2kW, giving a thermal energy output of 8.8kW at a COP of 4. Under these operating conditions the thermal energy supplied by the heat pump will exceed the building heat loss by approximately 2.2kW, so it will take approximately 3 hours for the heat pump to replace the lost energy and bring the IAT back to 20C. Total electrical energy consumption is 130W higher throughout the 24 hour period.

During a 2 hour shutdown, the building loses approximately 13.3kWh of thermal energy with the IAT reducing to 19.47C, and takes 6 hours of higher output heat pump operation to restore the iAT to 20C. Total electrical energy consumption is 200W higher throughout the 24 hour period.

During a 3 hour shutdown, the building loses approximately 19.7kWh of thermal energy with the IAT reducing to 19.21C, and takes 9 hours of higher output heat pump operation to restore the iAT to 20C. Total electrical energy consumption is 250W higher throughout the 24 hour period.

So the conclusion would be that under constant OAT conditions, using an overnight setback would actually be detrimental, and use slightly more electrical energy.

Note the last paragraph ('So the conclusion would be that under constant OAT conditions, using an overnight setback would actually be detrimental, and use slightly more electrical energy.') and compare it to your statement in this thread ('The spreadsheet does indeed show that savings can be made by means of overnight setback.').

You are, and I mean this, due full applause for adopting the sensible response to the old saw of uncertain origin, 'When the facts change, I change my mind. What do you do, sir?'. The reason I quoted the above at length is to emphasise that models can very easily get things wrong, and should never be relied on as the final answer. The last time we did that on a national and global scale was during the pandemic, and look where that got us. By and large, observations on the real world are always far far better than any model, the trouble during the pandemic was that we had dishonest, ill-disposed and delusional scientists directing terrifying whatiffery at scientifically incompetent politicians, as sure fire a recipe for disaster as can be.

Yes, observations are always better...

But what if the observations, and/or their processing before presentation, are wrong?

What if the observations are only good as far as the go?

That is why we should always be very careful about being dogmatic, and why I am very careful to point out that my data is data for my system in my house in the current conditions, nothing more and nothing less (and yes it may still be wrong; and that is @kev-m's observations are so useful, because they replicate my findings: different system in a different house but similar results; but it is still only two swallows, and two swallows do not a summer make). I'm just going to have to wait and see what the effect of sustained lower OATs is.

God helps us all when the world goes full AI...      

 

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


   
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(@kev-m)
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@derek-m 

yes I would be interested in seeing how my data compares with the model.

 

Kev 


   
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(@kev-m)
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Thanks, very useful to have similar data from another setup, and I agree, it shows the same thing, setbacks do save energy (based on Mk 1 eyeball assesssed areas under the curve, as in the recovery extra bulge (over and above what you would have used anyway) is smaller that what would have been used during the setback period, based on pre-setback usage) but I am not sure what the units are, eg I doubt very much you were using 35 kWh at 0600 this morning! Maybe it is Wh (Watt hours)? I think it also helps to have energy in (and out) shown as bars, the bar for each hour being the energy in (and out) for the preceding hour. Strictly speaking these should be grouped (placed side by side), bit personally I find it works better for me if they are overlaid (and crucially that is pointed out, they are not stacked), as both then fall on the hour, and I get an instant visual sense of the COP. It is also easier to quantify the savings and recovery extra usage and get a value for net saving.  

@cathodray,

I have all the data points behind the charts so I could work it out more precisely. 

The monitoring system creates a record for each minute.  For energy, it logs 1 pulse per watt hour and counts how many of them there are in each minute. So 35 means 35 watt hours per minute. Which is 60x35=2100 watt hours per hour, which means that for the minute in question, it's running at 2.1kW.  Maybe there is an easier way of explaining this!  Just multiply by 60 and that's watts. 

I can get the delivered trace also but it's a bit spiky. Some more work required.

 

Screenshot 2023 11 15 22.25.23

 

This post was modified 6 months ago by Kev M

   
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(@derek-m)
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Posted by: @kev-m

@derek-m 

yes I would be interested in seeing how my data compares with the model.

 

Kev 

If you would care to provide some data from your system covering particular periods I can run it through the model to see how the results match.

Data if possible should include LWT, RWT, IAT, OAT, setpoints, any setback periods, flowrates if you know them, energy in and energy out, and anything else you think may be useful.

Also detail any results that may be of use.

 

This post was modified 6 months ago by Derek M

   
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