Do setbacks save en...
 
Notifications
Clear all

Do setbacks save energy without compromising comfort?

843 Posts
24 Users
100 Likes
18 K Views
(@derek-m)
Illustrious Member Moderator
13481 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4134
 

@jamespa

I can now provide some figures that may be useful in understanding what may be occurring within a hypothetical home during an overnight setback.

The data is based upon a 14kW Ecodan, operating at a home with a calculated heat loss of 12kW, maximum heat emitter capacity of 24kW, and an assumed medium to high thermal mass of 250kW at an IAT of 20C. The system is operating in WC mode with curve settings of 53.57C @ -5C and 25.7C @ 20C.

With the system balanced at an OAT of 10C, the following values will be probable.

IAT = 20C, COP = 4.46, LWT = 37.46C, Energy In = 1120W, Energy Out = 4999W with a Heat Loss of 5000W.

Attached are the results from a 6 hour setback test, which shows the probable effects within the system. Table 1 being with no recovery boost and Table 2 with sufficient boost to return the IAT to 20C by hour 24.

Please feel free to make constructive criticism.

 


   
ReplyQuote
(@william1066)
Reputable Member Member
1333 kWhs
Joined: 1 year ago
Posts: 206
 

@derek-m It seems that without a boost, it did not quite get back to the starting temperature.  This is only 0.6 degC but if [no boost] was repeated for 5 days in a row, would the temperature then be 3 below 20?


   
ReplyQuote
(@derek-m)
Illustrious Member Moderator
13481 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4134
 

Posted by: @william1066

@derek-m It seems that without a boost, it did not quite get back to the starting temperature.  This is only 0.6 degC but if [no boost] was repeated for 5 days in a row, would the temperature then be 3 below 20?

No. If there were no external factors the prediction indicates that the IAT would stabilise at 19.31C, falling to 17.32C by the end of each setback period, but then recovering back to 19.31C.

Obviously in the real World, factors such as solar gain, wind chill, rain effect and human activity may cause changes which cannot easily be quantified.

 


   
ReplyQuote



(@kev-m)
Famed Member Moderator
5550 kWhs
Joined: 3 years ago
Posts: 1299
 

Posted by: @derek-m

@jamespa

I can now provide some figures that may be useful in understanding what may be occurring within a hypothetical home during an overnight setback.

The data is based upon a 14kW Ecodan, operating at a home with a calculated heat loss of 12kW, maximum heat emitter capacity of 24kW, and an assumed medium to high thermal mass of 250kW at an IAT of 20C. The system is operating in WC mode with curve settings of 53.57C @ -5C and 25.7C @ 20C.

With the system balanced at an OAT of 10C, the following values will be probable.

IAT = 20C, COP = 4.46, LWT = 37.46C, Energy In = 1120W, Energy Out = 4999W with a Heat Loss of 5000W.

Attached are the results from a 6 hour setback test, which shows the probable effects within the system. Table 1 being with no recovery boost and Table 2 with sufficient boost to return the IAT to 20C by hour 24.

Please feel free to make constructive criticism.

 

Looks interesting. I'll see if I can get some real data from mine in a few days.

 


   
ReplyQuote
cathodeRay
(@cathoderay)
Famed Member Moderator
6758 kWhs
Joined: 2 years ago
Posts: 1381
Topic starter  

Posted by: @derek-m

Please feel free to make constructive criticism.

A bit rum restricting comment. What I might consider constructive you might consider destructive. Never forget that free speech includes the right to offend.

That said, I am afraid that 'spreadsheet' fails even the most basic transparency tests. Although it has a xlsx extension, suggesting it is a spreadsheet, it in fact just a printout, with no explanation, no working, no methods, no formulae. It could just as well be a text file with a txt extension, though you would lose the traffic light crayon box, though that wouldn't be a bad thing, as using traffic light colour schemes is an almost certain marker that a know-it-all wants to patronise the rest of us. In particular, there is no explanation of how you determined the building had "an assumed medium to high thermal mass of 250kW at an IAT of 20C" or what this actually means (who assumed? why did they assume what they did?), while in the printout it doesn't even appear, instead, there is a row labelled "Thermal Capacity @ 20C" with an initial entry of 250000 which then varies over time for no apparent reason. 

I've said it before and I'll say it again: publishing results without methods (how you got those results) is never science, it's propaganda. If you insist on a constructive comment, I suggest you construct a proper spreadsheet, that shows your methods and workings.

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


   
ReplyQuote
(@derek-m)
Illustrious Member Moderator
13481 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4134
 

Posted by: @cathoderay

Posted by: @derek-m

Please feel free to make constructive criticism.

A bit rum restricting comment. What I might consider constructive you might consider destructive. Never forget that free speech includes the right to offend.

That said, I am afraid that 'spreadsheet' fails even the most basic transparency tests. Although it has a xlsx extension, suggesting it is a spreadsheet, it in fact just a printout, with no explanation, no working, no methods, no formulae. It could just as well be a text file with a txt extension, though you would lose the traffic light crayon box, though that wouldn't be a bad thing, as using traffic light colour schemes is an almost certain marker that a know-it-all wants to patronise the rest of us. In particular, there is no explanation of how you determined the building had "an assumed medium to high thermal mass of 250kW at an IAT of 20C" or what this actually means (who assumed? why did they assume what they did?), while in the printout it doesn't even appear, instead, there is a row labelled "Thermal Capacity @ 20C" with an initial entry of 250000 which then varies over time for no apparent reason. 

I've said it before and I'll say it again: publishing results without methods (how you got those results) is never science, it's propaganda. If you insist on a constructive comment, I suggest you construct a proper spreadsheet, that shows your methods and workings.

Please find attached the underlying spreadsheet for you to criticise.

 


   
ReplyQuote
cathodeRay
(@cathoderay)
Famed Member Moderator
6758 kWhs
Joined: 2 years ago
Posts: 1381
Topic starter  

@derek-m - thanks.

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


   
ReplyQuote
(@derek-m)
Illustrious Member Moderator
13481 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4134
 

Posted by: @cathoderay

@derek-m - thanks.

You're welcome.

Please feel free to question anything that you do not fully understand.

 


   
ReplyQuote
cathodeRay
(@cathoderay)
Famed Member Moderator
6758 kWhs
Joined: 2 years ago
Posts: 1381
Topic starter  

@derek-m - I can't even begin to imagine how you put it all together. Thousands of cells, hundreds of formulae, some of which make my eyes pop:

=IF((($I$5-$I$6)/($K$6-$K$5)*($K$6-D$16)+$I$6+D11)>=$I5,$I5,IF((($I$5-$I$6)/($K$6-$K$5)*($K$6-D$16)+$I$6+D11)<=$I6,$I6,($I$5-$I$6)/($K$6-$K$5)*($K$6-D$16)+$I$6+D11))

With this degree of complexity, the chance of random errors increases substantially.

A few first observations (please note that I opened the spreadsheet in LibreOffice Calc):

(1) There is still no explanation of the derivation of thermal mass/capacity, and, beyond the visible formulae, no explanation why it is implemented as it is. My hunch is that it is used as a sort of thermal store, but I shouldn't have to rely on hunches. There is still no explanation of how or why the initial Building Thermal Capacity (W) is set to 250000 (sheet Initial Data, cell AF1). 250000 is a lot of Watts...

(2) In the Play Area:

(a) changing the setpoint temp (eg setting it to 16 for the first six hours to simulate a setback) has no effect

(b) the LWT stays constant, to two decimal places, this is not what heat pumps do in the real world

(c) cell AB19 is the sum of the power input over the 24 hour period, cell AC19 has a hard-coded unexplained number (26893), and, between them they end up determining the % gain, which sounds important, but isn't fully explained.

This all represents another common problem with models: they are just too damned complicated. Almost anyone (there will always be exceptions...) with a degree in a science subject should be able to understand a model that can be contained with a spreadsheet after taking the necessary time to study it, but I am struggling to make sense of this spreadsheet, and I have no doubt other will struggle too, and that creates a serious problem: lack of verification. The spreadsheet in effect becomes a black box, and we have no way of knowing whether that black box contains an Aladdin's Cave or a Chamber of Horrors.   

This is nothing personal, I have made exactly the same sort of remarks about other models in different contexts. Instead, I see it as a general malaise that has crept up on us, and we are the worse for it.

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


   
ReplyQuote



(@bob77)
Trusted Member Member
292 kWhs
Joined: 1 year ago
Posts: 36
 

Well, having had the heat pump running continually yesterday (on weather compensation, with the house at a comfortable 20-21C downstairs and 19C upstairs) we hit a new record high consumption for the year of 40 kWh! (Normally 25-30 when heating is on)

I will reserve judgement, as three people had baths, there was laundry going on, and the weather has turned colder, so not quite a fair test.


   
ReplyQuote
(@derek-m)
Illustrious Member Moderator
13481 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4134
 

@cathoderay

There is indeed a degree of complexity within the spreadsheet, but I think that by now most forum members will have come to the conclusion that a home heating system is a quite complex subject.

Each formula was carefully developed, and where possible tested, to perform a part of the overall functionality. If you would care to provide details of the parts you don't understand, I will be happy to explain the structure and the underlying laws of Physics and/or Thermodynamics.

1) There is indeed no explanation about the Thermal Mass calculations, though I would have thought that they are fairly self explanatory. The cell AF1 on the Initial Data sheet is highlighted in green, indicating that it is an input, and can be changed by the user.

A value of 250kWh was chosen, as stated previously, to represent a medium to high thermal mass building, though as you stated previously there does not appear to be a standard method to quantify a building's thermal mass. Please feel free to change the value if you are not happy with 250kWh.

If thermal energy is dissipated within a building it will have an effect upon the IAT. If thermal energy is added at a rate greater than the heat loss, then the IAT will increase, along with the quantity of thermal energy stored within the fabric of the building.

Is that explanation adequate?

2a) The heat pump is deemed to be operating in WC mode, which on the Ecodan does not have the setpoint available, instead a WC offset can be applied. Cells D1 to AA1 perform the function of switching the heat pump on and off, as has been clearly indicated.

2b) It is indeed true that heat pumps do vary their LWT in the real World, possibly due to variations in OAT, or maybe because the heat pump is cycling, though on some charts I have seen recently there have been periods when the LWT has been fairly constant. How many decimal places would you prefer.

2c) 26893 is the electrical energy input before setback was implemented, it is the reference value, since if you wish to know how much the values have changed, you have to know the starting value. I would have thought that was obvious.

I think that by now most forum members are fully aware of your love of models, along with HA. I am not certain why you denigrate anyone who is trying to answer the question that you initially posed.

In an attempt to answer your original question. It would appear the some energy can be saved by using a setback period, but whether or not a setback will or will not compromise comfort depends not only upon a number of factors, but also upon the homeowners tolerance to being cold.

If there are further aspects that you do not fully understand then please feel free to ask. That also applies to other forum members.


   
ReplyQuote
(@jamespa)
Noble Member Member
4065 kWhs
Joined: 1 year ago
Posts: 683
 

@derek-m can I ask a couple of questions to check my understanding of the the 'results' sheet.

 

  • It gives figures for energy in reduction and energy out reduction but I cant see where these come from in the tables above.  Also they say they are in watts (=power), are they actually watt-hours (=energy)

 

  • The first one ends up colder than you started, so hasn't restored the house to its starting position.  In a post above you say that this becomes the new 'normal' IAT.  if so then should a part of the reduction in energy consumption be attributed to the fact that you have reduced the IAT rather than to setback (obviously you can also reduce the IAT without setback to save energy)?

Posted by: @cathoderay

This all represents another common problem with models: they are just too damned complicated. Almost anyone (there will always be exceptions...) with a degree in a science subject should be able to understand a model that can be contained with a spreadsheet after taking the necessary time to study it, but I am struggling to make sense of this spreadsheet, and I have no doubt other will struggle too, and that creates a serious problem: lack of verification. The spreadsheet in effect becomes a black box, and we have no way of knowing whether that black box contains an Aladdin's Cave or a Chamber of Horrors.   

Perhaps a little harsh for a totally voluntary, amateur effort (if you don't mind me saying so - which you may well do).  The real world problem is inherently complex, and this complexity must feature somewhere in any model which purports to approximate to the real world.

However I do see the point you are trying to make, and by way of positive contribution have been thinking about how it could be made more transparent.  I think it would benefit greatly from custom formulae for 1, 2 and 3 d interpolation and for extracting COP and max power directly (or more or less directly) from the manufacturers tables. 

Excel custom functions for 1, 2 d interpolation are readily available on the internet (and fairly easy to write - in fact I have one in one of the spreadsheets I have posted on this forum), but not 3d so far as I know.  Combining this with the second is challenging and, whilst I can see in principle how it might be done I cant currently figure out even an outline algorithm to do it neatly.    

However this is all academic unless @derek-m is open to collaboration on the model, which I can readily understand he may not be.  Also, unfortunately, I have only excel 2003, which does not do iteration to resolve circular references (a feature which is incorporated and unfortunately necessary to calculate emitter load).  I do also have libre office which does do iteration, but the syntax for custom functions is not exactly equivalent (although it is usually possible to write custom functions which will work with both).  Thus, while I would be willing to write some custom functions if @derek-m is willing to accept them (and the significant restructuring that would follow), there would be some limitations.  @derek-m please let me know if you are interested in embarking on a restructuring based on some custom functions to increase transparency.  I'm not offering guarantees but reckon it could be cracked with a bot of thought.

 

 


   
ReplyQuote
Page 9 / 71



Share:

Join Us!

Latest Posts

x  Powerful Protection for WordPress, from Shield Security
This Site Is Protected By
Shield Security