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Do setbacks save energy without compromising comfort?

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cathodeRay
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Posted by: @derek-m

What is the 'source' of the Set LWT reading? Is it read from the Midea controller?

Yes, it is in one of the modbus registers, and I just collect it as I would collect any other register value. I'm guessing it is set by an algorithm that checks the current OAT, and then sets the Set LWT according to the weather compensation curve. My worked example of doing this manually the other day suggests the guess is correct, the Set LWT appears to correspond to that determined by the WCC, plus or minus any adjustment done by my auto adaption script. I am not sure how often the algorithm runs, but it appears to be fairly frequently, given the Set LWT is normally fairly responsive to any changes in the OAT, see for example the beginning of the setback in the chart above.

Thinking about it, the tendency to cycle rather than run in a steady state is an example of an over active system. The same applies to triggering defrosts. In each case the rise in the LWT happens too fast and too vigorously. In normal running it then overshoots, and has to turn itself off to recover; in cold conditions, the vigorous rise then triggers icing, which in turn then triggers a defrost. I wonder if there is any way of dampening the behaviour, to avoid the overshoots and the icing caused by working too hard, the idea being to get a slower initial rise in LWT, but when it gets to the Set LWT it stays there.  

     

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


   
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(@derek-m)
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Posted by: @cathoderay

Posted by: @derek-m

What is the 'source' of the Set LWT reading? Is it read from the Midea controller?

Yes, it is in one of the modbus registers, and I just collect it as I would collect any other register value. I'm guessing it is set by an algorithm that checks the current OAT, and then sets the Set LWT according to the weather compensation curve. My worked example of doing this manually the other day suggests the guess is correct, the Set LWT appears to correspond to that determined by the WCC, plus or minus any adjustment done by my auto adaption script. I am not sure how often the algorithm runs, but it appears to be fairly frequently, given the Set LWT is normally fairly responsive to any changes in the OAT, see for example the beginning of the setback in the chart above.

Thinking about it, the tendency to cycle rather than run in a steady state is an example of an over active system. The same applies to triggering defrosts. In each case the rise in the LWT happens too fast and too vigorously. In normal running it then overshoots, and has to turn itself off to recover; in cold conditions, the vigorous rise then triggers icing, which in turn then triggers a defrost. I wonder if there is any way of dampening the behaviour, to avoid the overshoots and the icing caused by working too hard, the idea being to get a slower initial rise in LWT, but when it gets to the Set LWT it stays there.  

     

That is the reason why I suggested putting the system in fixed LWT mode for a period of time, it is similar to putting an auto control system into manual control to stop swings.

The problem is that the heat pump, when it is operating, is lowering the OAT sensor reading, which in turn increases the Set LWT value, which then causes the LWT to be pushed higher than required, to the point where it reaches the upper limit and stops the heat pump.

The correct solution is to stop the cooler air being drawn into the heat pump intake.

 


   
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(@newhouse87)
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Last night was first below freezing temps. Heating went off at 9pm, kitchen was21.5 and bedroom21, both lost 1 degree overnight. I think thats reasonable, house still warm enough for the morning rush. I have however the time for heating to come on today at 12pm-9pm, 2 hours earlier then normal as not much sun these days. Heatpump seems to run flat out at full flow 33/34l/min and zero cycling as far as i can tell. LWT has been fixed@32 for few weeks now and seems to be warming house nicely. I have my usage from nov-January last year and i will compare then but i have a hunch im going to save approx 20% energy compared to last winter. Most people i know run on/off stat on weather compensation. I think when using stats or setbacks fixed lwt may be the way to go.


   
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(@iaack)
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@cathoderay I'd be interested to know if your controller is set to max power, intermediate, or quiet mode.  It would be enlightening to understand how switching power modes affects your problem of frequent defrosts.


   
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cathodeRay
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Posted by: @derek-m

That is the reason why I suggested putting the system in fixed LWT mode for a period of time, it is similar to putting an auto control system into manual control to stop swings.

I am still unclear how practically to do this. The OAT has been falling overnight, and is currently 4 degrees, ie approaching the defrost cycle range. From the original heat loss calculations, and the design minimum OAT, and the empirically derived WCC endpoints, I know the fixed LWT needed in these conditions is going to need to be around 50 degrees, and above if it gets colder, and this combination of low OAT and high LWT is exactly where capacity (max output) and COP get seriously compromised, and, I am pretty sure, it will also mean defrost cycles, which will only make matters worse. To complicate things even more, I am going to have to set the room stat to 19 degrees, so that if by some miracle the system gets the IAT above 19 degrees, the heat pump stops adding yet more heat. 

Posted by: @derek-m

The problem is that the heat pump, when it is operating, is lowering the OAT sensor reading, which in turn increases the Set LWT value, which then causes the LWT to be pushed higher than required, to the point where it reaches the upper limit and stops the heat pump.

The correct solution is to stop the cooler air being drawn into the heat pump intake.

The problem I have with this is that my installation is far from unique. All Midea and Midea mono block clones have same set up (an internal OAT sensor), and I suspect it is the norm for many other manufacturers. Almost all heat pumps have their backs against the wall (the colloquial meaning of the phrase  being not entirely lost on me) and as I pointed out before that means they will be inclined to draw their intake air from air that contains exhaust air, and is therefore cooler than the 'true' ambient air. I use quotes because in fact it is the true ambient air as far as the heat pump is concerned, it is only we humans who have a notion of some other true ambient temp, as in what it would be if the heat pump wasn't running, but that's an abstraction, because the heat pump is running. I have also seen photos of installations where the heat pump has been placed in far more of a cold well than mine is.

I haven't fully thought this through, but I also wonder if a WCC already does, or could be made to, incorporate the fact that a running heat pump will lower the OAT? 

I'm also not sure what 'the upper limit' is (as in 'it reaches the upper limit'). In normal not cold conditions, the LWT/RWT saw tooth about the Set LWT, with their average being about the Set LWT. Perhaps the problem is the one I effectively alluded to earlier: the heat pump can't lower the gradient of the rise in LWT, and that is why it over shoots? It's the old car analogy: achieving 30mph by putting your foot down and then taking it off, rather than applying a steady gentle pressure.   

In short, I suspect that many heat pumps operate in more or less the same circumstances as mine, and might even hazard a guess that it is the norm. To determine whether that is true or not, we need more data.    

Here is my data for the last two days. In the last 24 hours, the OAT has had two periods of relative stability, with the LWT/RWT saw toothing about the Set LWT. I don't actually see how or where the heat pump's running is lowering the OAT so as to cause the Set LWT to be set too high (yes, the heat pump's running is lowering the OAT, but I don't see the alleged consequences, instead I see a shary rise in the LWT that overshoots, by that may be by design, under the limitation that the gradient of the rise can be reduced, to get the average LWT/RWT over time to approximate the Set LWT)...

image

   

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


   
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Posted by: @newhouse87

Last night was first below freezing temps. Heating went off at 9pm, kitchen was21.5 and bedroom21, both lost 1 degree overnight. I think thats reasonable, house still warm enough for the morning rush. I have however the time for heating to come on today at 12pm-9pm, 2 hours earlier then normal as not much sun these days. Heatpump seems to run flat out at full flow 33/34l/min and zero cycling as far as i can tell. LWT has been fixed@32 for few weeks now and seems to be warming house nicely. I have my usage from nov-January last year and i will compare then but i have a hunch im going to save approx 20% energy compared to last winter. Most people i know run on/off stat on weather compensation. I think when using stats or setbacks fixed lwt may be the way to go.

Id be a bit careful with the interpretation of this.  If the house only cools by 1C peak (ie an average of 0.5C) for 12 hrs when it's 0 outside then at most you will reduce your daily energy consumption by 1.25%, unless it's quite a bit warmer outside when the heating is on than when it's off, so that the loss of efficiency whilst the heat pump is forced to work harder to catch up is counteracted by the gain because the OAT is higher. 

At this time of year that maybe the case, but in the depths of winter much less likely (the temperature varies less through the day in winter than it does in autumn & spring).  If, on the other hand,  you don't benefit from the oat during recovery being higher, its quite possible you will lose overall.

In summary please bear in mind that, to the best of current understanding (and rooted firmly in thermodynamics) the potential saving if the house doesn't cool much is quite small, and its entirely possible that in some circumstances it will cost.  I'm absolutely not saying 'don't do it', but I would look out for any contraindications (eg you feel cold so have to increase the wc or thermostat setting, however marginally, to compensate)

 


   
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cathodeRay
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Posted by: @newhouse87

LWT has been fixed @32 for few weeks now and seems to be warming house nicely.

Unfortunately there is no way I can run with such a low LWT and have a warm house. This was known about at design time, having a small listed building with limited ceiling and window sill heights in many rooms places severe constraints on the sizes of the radiators. As it was, I went for the largest K3s that I could, without turning the house into a radiator emporium, but they still need a high LWT to stand any chance of delivering enough heat to keep the house warm. 

Posted by: @iaack

I'd be interested to know if your controller is set to max power, intermediate, or quiet mode.  It would be enlightening to understand how switching power modes affects your problem of frequent defrosts.

I too wondered about this. The engineering data output capacity tables clearly show what Midea call 'max' 'norm' and 'min' 'capacity levels', but I have never found a way to determine what level is currently set, or how and indeed whether it can be set.

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


   
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(@derek-m)
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@cathoderay

No problem.


   
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cathodeRay
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@derek-m - let's not start rowing again, but what exactly does 'no problem' mean or contribute?

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


   
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(@jamespa)
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Posted by: @cathoderay

I too wondered about this. The engineering data output capacity tables clearly show what Midea call 'max' 'norm' and 'min' 'capacity levels', but I have never found a way to determine what level is currently set, or how and indeed whether it can be set.

I have always assumed that they modulate continuously between max and min.  Obviously you can only provide the engineering data at a limited number of points, but surely the actual unit adjusts its output at any point between


   
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(@newhouse87)
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Posted by: @jamespa

Posted by: @newhouse87

Last night was first below freezing temps. Heating went off at 9pm, kitchen was21.5 and bedroom21, both lost 1 degree overnight. I think thats reasonable, house still warm enough for the morning rush. I have however the time for heating to come on today at 12pm-9pm, 2 hours earlier then normal as not much sun these days. Heatpump seems to run flat out at full flow 33/34l/min and zero cycling as far as i can tell. LWT has been fixed@32 for few weeks now and seems to be warming house nicely. I have my usage from nov-January last year and i will compare then but i have a hunch im going to save approx 20% energy compared to last winter. Most people i know run on/off stat on weather compensation. I think when using stats or setbacks fixed lwt may be the way to go.

Id be a bit careful with the interpretation of this.  If the house only cools by 1C peak (ie an average of 0.5C) for 12 hrs when it's 0 outside then at most you will reduce your daily energy consumption by 1.25%, unless it's quite a bit warmer outside when the heating is on than when it's off, so that the loss of efficiency whilst the heat pump is forced to work harder to catch up is counteracted by the gain because the OAT is higher. 

At this time of year that maybe the case, but in the depths of winter much less likely (the temperature varies less through the day in winter than it does in autumn & spring).  If, on the other hand,  you don't benefit from the oat during recovery being higher, its quite possible you will lose overall.

In summary please bear in mind that, to the best of current understanding (and rooted firmly in thermodynamics) the potential saving if the house doesn't cool much is quite small, and its entirely possible that in some circumstances it will cost.  I'm absolutely not saying 'don't do it', but I would look out for any contraindications (eg you feel cold so have to increase the wc or thermostat setting, however marginally, to compensate)

 

Fair enough, but i really dont get how running 24hours could not be more expensive, generally up at 2kwh/hr usage when running at fixed@32 full flow. I know from trials i cant run lower then27 due to excessive cycling and also house wont reach set22. Also think the curve i would create to run24/7 wouldnt heat house to setpoint at certain oat as i can only set points on my curve no slopes. All i can do is compare to last winter where somedays when i had stat on/off using installers curves i used 70kwh in day. All houses are different and some are more suited to running 24/7 then others. My heating will come on @12 today and by the time herself gets home @5pm it should be up to 22 and like that until midnight then.

Most days it will be a good few deg warmer at12pm then in the middle of the night, we have few days where it is freezing all day.

Also think real life impact as in peoples situations are different can be taken into account and in my case i think its much cheaper and would be for lots in my scenario to run like this. Also heat pump keeps me awake so running 24.7 not really an option. I think if i ran 24/7 using lowest curve possible for a day i would use up to15kwh more in a 24hour period. My house temp would be constant but its not needed to be like this.

 


   
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(@derek-m)
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Posted by: @cathoderay

@derek-m - let's not start rowing again, but what exactly does 'no problem' mean or contribute?

It means that I don't have a problem if you wish to keep going round in circles.

 


   
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