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Do setbacks save energy without compromising comfort?

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Toodles
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@cathoderay Speaking on behalf of my Daikin EDLA08 HP, the OAT sensor is positioned a few cms. away from the ‘back’ of the unit with the inflow of air passing over it; this would suggest that it is sampling the air as it enters the unit and should truly represent the OAT at the site that the HP occupies. The sensor is almost level with the top face of the pump by the way. Regards, Toodles.

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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cathodeRay
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Posted by: @jamespa

but surely the actual unit adjusts its output at any point between

But based on what? At any one point in time, the OAT is fixed, the Set LWT is fixed (by the WCC or because the heat pump is Fixed LWT mode), but what then decides what the 'capacity level', be it one of three, or in a range, at any one time? That's what I can't see (along with not being able to see the current capacity). The modbus register tables do have an entry for 'Ability of the hydraulic module', units kW, but it is always 14, and I think this is just the nominal 'ability' of the unit. There is an undocumented value that might just be the current 'capacity' ie what it currently producing. This also seems to appear on the Wired Controller Operational Parameter pages as 'Heat Pump Capacity' and it is currently running at around 7 kW, which seems on the low side for the prevailing conditions. But there is still no indication of whether that is max norm or min. The engineering data for the current conditions (OAT 5, LWT 50) shows for these conditions 13798, 10500 and 7865 (Watts) respectively. Perhaps it is running on 'min' - but why?      

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


   
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cathodeRay
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Posted by: @toodles

this would suggest that it is sampling the air as it enters the unit and should truly represent the OAT at the site that the HP occupies

This is the point that I failed to get across to @derek-m, whether we like it or not, as far as the heat pump is concerned the OAT, is what it is where it is. It doesn't know about or respond to the OAT elsewhere. 

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


   
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(@jamespa)
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Posted by: @newhouse87

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.

Well for certain not running 24 hrs can be more expensive:

Consider a house with very high thermal mass, so high that it barely responds to setbacks of 12hours (yours is actually close which is why I put out the warning).  So the amount of energy it loses is not affected by setback, which means that the amount of energy that must be provided in any 24 hr period to make up the amount lost in that 24 hr period is also not materially affected by set back. 

The most efficient way to provide this (because of the characteristics of heat pumps - ie they work most efficiently at lower flow temperatures) is at the lowest possible flow temperature for 24 hours, not a higher flow temperature for a shorter amount of time.  Such a house will cost more to run (at costant OAT) by setting back because it will require a higher flow temperature to deliver the same amount of energy.  

 

Another way to look at it is this.  With reference to your figures, if the normal on consumption is 2kW (2kWh/h), then you are emphatically not saving 2kW  by switching it off temporarily.  What is happening is that the house is continuing to lose energy at almost the same rate (which ultimately you must replenish), but because there is lots of energy stored in the fabric you don't yet experience a temperature drop.

Perhaps an analogy would help.  Consider a lake  (house) with an inflow (heat pump) and an outflow (walls etc).  The amount of water passing through the outflow (heat loss) will depend on the height of the water in the lake (indoor temperature) above whatever is the threshold over which the water must pass (bar at the end of the lake, etc - the outdoor temperature).

Now suppose that, during the daytime, I feed water in at a rate which just balances the rate at which water is lost, then the height of the lake (temperature) stays the same.  Lets suppose this is 4 cubic metres per hour.  Now further suppose that I stop feeding water in for six hours.  I have apparently 'saved' 24 cubic metres of water during setback.  Lets also suppose, like your house, that the lake is sufficiently large that its level falls by only a small fraction.  So during the 6 hours the lake will lose just a bit less than 4 cu m per hour, lets suppose the average loss is 3.5cu m per hour.  Thus during the 6 hours it will lose 21cu m instead of 24cu m.

Although the lake has only dropped a bit, it has dropped, and to return the lake to its original position I have to put back 21cu m in addition to whatever it continues to lose during the recovery time.  If I dont then I've depleted the lake, and I cant go on doing this.  So rather than a saving of 24cu m the saving is only in fact 3 cu m.

 

 

 


   
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Posted by: @cathoderay

Posted by: @jamespa

but surely the actual unit adjusts its output at any point between

But based on what? At any one point in time, the OAT is fixed, the Set LWT is fixed (by the WCC or because the heat pump is Fixed LWT mode), but what then decides what the 'capacity level', be it one of three, or in a range, at any one time?     

The demand, which is set by the emitters.  The heat pump is a slave to the house, not the other way round!

 


   
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(@newhouse87)
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@jamespa I take it you mean my house will be yoyoing in a sense with regard IAT so by having setback. My counter point is it doesn't affect our current lifestyle as we are warm when we are there. I get what you mean theoretically but i suppose in my case its theoretical as its not having any real world impact on our comfort. The lost energy is generally going to be replaced when OAT is higher so heatpump cop will be better. Im just trying to articulate that running 24/7 may not be a universal fit.


   
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cathodeRay
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Posted by: @jamespa

The demand, which is set by the emitters.

But how does it measure, and respond to, the demand? And why is it currently (if my above assessment of what the data represents) choosing to run at 'min' level, even perhaps a bit below that?

@toodles - I should have perhaps added that the WCC curve can be and usually is set to the OAT as measured by the heat pump, and so in effect factors in the fact that that local OAT isn't necessarily what it might be elsewhere. The WCC in effect says when the OAT, including any heat pump effects, is this, then the LWT should be that.

@derek-m - instead of straying off into what looks to me like ad hominem attacks, could you please explain why my hypothesis that the primary cause of cycling is over-reaction, ie the heat pump raises the LWT too fast, over-shoots, and that is what turns it off, causing the cycling.

The problem I have with your hypothesis is the OAT as measured by the heat pump over the last 24 hours has really been rather stable. I don't see the drop in OAT from exhaust air triggering a rise in the Set LWT that you say is the cause of cycling. Here's a zoomed in section from last night's data:

image

 

The Set LWT is constant for hours on end, the OAT shows small upticks (as expected) when the compressor stops, yet we still have very evident cycling, with nonetheless the average LWT being a close approximation to the Set LWT ie it is delivering an average that is close to that intended. And furthermore, perhaps this is by design: given the heat pump can't do a slower rise, it will over-shoot, and the by design bit is to design in cycling to deal with this over-shoot. 

     

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: @toodles

this would suggest that it is sampling the air as it enters the unit and should truly represent the OAT at the site that the HP occupies

This is the point that I failed to get across to @derek-m, whether we like it or not, as far as the heat pump is concerned the OAT, is what it is where it is. It doesn't know about or respond to the OAT elsewhere. 

I will make one final attempt.

But your heat pump controller does know the true OAT shortly after it has stopped running, the problem being that it does not know the true OAT whilst it is running.

Do you agree that the OAT sensor reading, and therefore the actual air going into the heat pump, drops several degrees below the true OAT after the heat pump starts operating? Since the heat pump extracts most of its thermal energy from the air flowing through the evaporator, your heat pump is therefore operating as if it were in an OAT several degrees lower than actual. The lower temperature air going into the heat pump means that there is less thermal energy for it to absorb, it therefore needs to work harder to extract the required quantity of thermal energy, which along with additional electrical energy, because the heat pump is working harder, combines to produce the thermal energy output.

When the heat pump restarts, the controller is informed by the OAT sensor that the OAT has suddenly fallen several degrees quite rapidly, the controller therefore panics, and pushes up the LWT setting in anticipation of the heat loss from your home also rising rapidly. Of course the true OAT does not rise rapidly and neither does the heat loss from your home. The heat pump is therefore producing more thermal energy than is required, so the LWT increases about the the Set LWT value until it reaches the upper deviation setting, which you have set to either +4C or +5C. When this setting is reach the heat pump stops operating, and does not restart until the LWT falls to the lower deviation setting, which I think is -9C.

These are the reasons why your heat pump cycles almost continuously.

 


   
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(@derek-m)
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@cathoderay

What was the operating mode of the heat pump?


   
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cathodeRay
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Well, I will have one last attempt. Most of your theory doesn't fit the data. In the most recent chart I posted, which clearly shows cycling, the OAT doesn't drop by 'several degrees', it drops by a degree (or two) when the compressor starts. The controller doesn't push up the Set LWT, is stays constant throughout, apart from a small drop and then rise between 1800 and 1900. 

Posted by: @derek-m

When the heat pump restarts, the controller is informed by the OAT sensor that the OAT has suddenly fallen several degrees quite rapidly, the controller therefore panics, and pushes up the LWT setting in anticipation of the heat loss from your home also rising rapidly.

This quote is the core of your theory, but none of it survives an examination of the data, for the reasons given above: there is no sudden rapid decline in the OAT, there is no pushing up of the Set LWT. 

I do accept there may be minor effects along the lines you describe, but I cannot see that they are the primary driver of the cycling.

Here is the chart again, for ease of reference:

image

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


   
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Toodles
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@derek-m The inlet of air to the heat pump (at the back of the unit) would surely be close to the OAT unless the pump or another nearby device is causing air that is not at the OAT to be mixed with the OAT air that is entering the device. Under ideal circumstances, the ‘exhaust’ (colder than OAT) air flows away from the area and does not re-enter the device and mix with the true OAT air. I have no idea what percentage of exhaust air might leave the front of my pump and be deflected around to the back - but I would hope it is minimal! Regards, Toodles.

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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cathodeRay
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Posted by: @derek-m

What was the operating mode of the heat pump?

Last night it was on Weather Compensation, no setback.

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


   
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